library-ify nasm helper

.gitignore

@@ -0,0 +1,4 @@
+*.bin
+.pc
+target/
+

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "seabios"]
+	path = seabios
+	url = https://github.com/coreboot/seabios.git

boot0.s

@@ -3,35 +3,25 @@
 ; --------------------------------------------------------------------
 ; R | 0x000000 - 0x000400: real-mode interrupt vector table
 ; R | 0x000400 - 0x000500: bios data area
-; U | 0x000500 - 0x007c00: stack
+; U | 0x000500 - 0x004000: main stack
+; U | 0x004000 - 0x006a00: globals
+; U | 0x006a00 - 0x007c00: memory map
 ; U | 0x007c00 - 0x007e00: boot sector
 ; U | 0x007e00 - 0x080000: conventional usable memory
 ; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
 ; R | 0x0a0000 - 0x0c0000: video memory
 ; R | 0x0c0000 - 0x0c8000: video bios
-; R | 0x0c8000 - 0x0f0000: bios expansiosn
+; R | 0x0c8000 - 0x0f0000: bios expansions
 ; R | 0x0f0000 - 0x100000: motherboard bios
 
-; BASE STACK FRAME
-; --------------------------------------------------------------------
-; bp - 0x02: drive number
-; bp - 0x04: sectors per track
-; bp - 0x06: number of heads
-; bp - 0x08: partition array starting LBA
-; bp - 0x0a: number of GPT partitions (saturated to 16-bits)
-; bp - 0x0c: sector stride when loading GPT entries
-; bp - 0x0e: byte stride when loading GPT entries
-; bp - 0x10: entries per sector when loding GPT entries
-; bp - 0x12: current entry index
-; bp - 0x14: current entry index within the current sector
-; bp - 0x16: number of sectors loaded
-; bp - 0x18: current LBA
+%include "defines.s"
 
 ; BIOS puts our boot sector at 0000:7c00
-org 0x7c00
+[org BOOT0_LOADPOINT]
 ; We're (probably) in real mode
-bits 16 
+[bits 16]
 
+main:
   ; Disable interrupts
   cli
 
@@ -40,12 +30,21 @@ bits 16
   mov ds, ax
   mov es, ax
 
-  ; Put the stack base at 0x7c00.
-  ; Stack grows high->low, so we'll grow away from our program text at 0x7c00.
+  ; Put the stack base at 0x4000.
+  ; Stack grows high->low, so we'll grow away from our globals and program text.
   mov ss, ax
-  mov bp, 0x7c00
+  mov bp, STACK_BASE
   mov sp, bp
 
+  ; Segment for VGA (0xb800 * 16 = 0xb8000)
+  mov ax, 0xb800
+  mov fs, ax
+
+  ; Set VGA mode
+  ; https://mendelson.org/wpdos/videomodes.txt
+  mov ax, 0x0003
+  int 0x10
+
   ; Store boot drive number
   xor dh, dh
   push dx
@@ -54,7 +53,7 @@ bits 16
   mov di, 0x00
   mov ah, 0x08
   int 0x13
-  jc .panic
+  jc panic
   ; Load sectors per track into cx & spill
   and cl, 0x3f
   xor ch, ch
@@ -65,32 +64,16 @@ bits 16
   push bx
 
   ; Load LBA 1.
-  cmp cl, 1  ; Division of 1 by nonzero can be done with cmp
-  sete al    ; temp = LBA / (sectors per track)
-  setne cl   ; sector - 1 = LBA % (sectors per track)
-  inc cl
-  xor ah, ah ; Zero-extend temp
-  xor dx, dx ; div by 16-bit register divides dx:ax, so we zero dx
-  div bx     ; dx = mod (head), ax = div (cylinder)
-  
-  mov dh, dl ; Head
-  mov ch, al ; Cylinder
-             ; We already have sector in cl
-  mov ah, 2  ; Read disk
-  mov al, 1  ; Load one sector
+  mov ax, 1
   mov bx, 0x7e00
-  mov dl, [bp - 0x2] ; Drive number
-  int 0x13
-  jc .panic
+  call read_lba
 
-  ; head = temp % number of heads
-  ; cylinder = temp / number of heads
-
-  mov cx, 8
-  mov si, .gpt_magic
+  ; Check the GPT header magic "EFI PART"
+  mov cx, GPT_MAGIC_LEN
+  mov si, gpt_magic
   mov di, 0x7e00
   repe cmpsb
-  jne .panic
+  jne panic
 
   ; Ensure the 8-byte GPT starting LBA fits in 16 bits
   mov di, 0x7e00 ; The rep increments di so we need to reset it
@@ -98,10 +81,9 @@ bits 16
   mov bx, [di + 0x4a]
   or ax, bx
   or eax, eax
-  jnz .panic
-  ; Load and spill the GPT starting LBA
-  mov ax, [di + 0x48]
-  push ax
+  jnz panic
+  ; Store the first 16 bits of the GPT starting LBA (we have made sure the remaining bits are 0)
+  push word [di + 0x48]
 
   ; Load number of partitions
   mov ax, [di + 0x50]
@@ -122,22 +104,22 @@ bits 16
   ; - If each entry is larger than a sector (512 bytes), they'll be sector-aligned.
   ; - If each entry is smaller than a sector, an integer number of them will fit into a sector.
   or eax, eax  ; Test size != 0 because 128 * 2^n != 0
-  jz .panic
-  and eax, 127 ; Test size is a multiple of 128
-  jnz .panic
-  mov eax, ebx
-  shr eax, 7   ; Divide by 128
-  mov ecx, eax
-  dec eax
-  and eax, ecx ; Test size/128 is a power of 2. popcnt gives an exception for some reason...
-  jnz .panic
-  mov eax, ebx
+  jz panic
+  test eax, 127 ; Test size is a multiple of 128
+  jnz panic
+  ; Use the (n & (n - 1)) == 0 trick to test if the entry size is a power of 2. Since we already
+  ; know it's a nonzero multiple of 128, if size is a power of 2 then size = 128 * 2^n holds.
+  ; Therefore we don't need to bother dividing by 128 first (shr 7), which saves a couple of bytes.
+  mov ecx, ebx
+  dec ecx
+  and ecx, eax
+  jnz panic
 
   ; Find the "sector stride", which is the number of sectors we increment by each time we want to
   ; load a new entry.
   shr eax, 9      ; Divide by sector size to get sectors per entry
   cmp eax, 0xffff ; Make sure sectors per entry fits in 16 bits
-  ja .panic
+  ja panic
   or ax, ax
   jnz .gpt_sector_stride_loaded
   ; Sector stride must be at least one or we'll load the same sector each time!
@@ -164,109 +146,125 @@ bits 16
   xor ax, ax
   push ax                                ; Current entry
   push ax                                ; Current entry within the current sector
-  push ax             ; First LBA loaded?
-  mov ax, [bp - 0x08] ; Starting LBA
-  push ax             ; Current LBA
+  push ax                                ; Number of sectors loaded
+  push word [bp - GPT_ENTRIES_START_LBA] ; Current LBA
 
   ; Search for the partition storing our second stage.
 .loop_find_stage2:
-  mov dx, [bp - 0x12]
-  cmp [bp - 0x0a], dx
+  mov dx, [bp - GPT_CURRENT_ENTRY_IDX]
+  cmp [bp - GPT_N_ENTRIES_16], dx
   ; Panic if we've run out of partitions and haven't found the second stage yet.
-  jbe .panic
+  jbe panic
 
   ; If we haven't loaded any sectors yet, load the first one.
-  cmp word [bp - 0x16], 0   ; Load number of sectors loaded
+  cmp word [bp - GPT_SECTORS_LOADED], 0
   je .load_first_lba
   ; If there's still more entries in the current sector, skip loading a new sector
-  mov ax, [bp - 0x18]       ; Load current entry index within the current sector
-  cmp [bp - 0x10], ax       ; Compare to entries per sector
+  mov ax, [bp - GPT_SECTOR_ENTRY_IDX]   ; Load current entry index within the current sector
+  cmp [bp - GPT_ENTRIES_PER_SECTOR], ax ; Compare to entries per sector
   ja .process_current_entry
   
-  mov ax, [bp - 0x0c]      ; Load sector stride
-  add word [bp - 0x18], ax ; Increment current LBA by sector stride
-  mov word [bp - 0x14], 0  ; Reset the current entry index within the current sector
+  mov ax, [bp - GPT_SECTOR_STRIDE]        ; Load sector stride
+  add word [bp - GPT_CURRENT_LBA], ax     ; Increment current LBA by sector stride
+  mov word [bp - GPT_SECTOR_ENTRY_IDX], 0 ; Reset the current entry index within the current sector
 .load_first_lba:
-  mov ax, [bp - 0x18]      ; Load current LBA
-  xor dx, dx
-  div word [bp - 0x04]     ; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
-  mov cx, dx
-  inc cx                   ; Put sector into cx (bios will read sector from cl)
-  xor dx, dx
-  div word [bp - 0x06]     ; Divide by number of heads. dx = mod (head), ax = div (cylinder)
-  mov dh, dl
-  mov ch, al
-  mov dl, byte [bp - 0x02] ; Load drive number
-  mov ah, 0x02
-  mov al, 1
+  ; Read the current LBA to 0x8000 (just past the end of the GPT header)
+  mov ax, [bp - GPT_CURRENT_LBA]
   mov bx, 0x8000
-  int 0x13                 ; Read sector
-  jc .panic
-  inc word [bp - 0x16]     ; Increment number of sectors loaded
+  call read_lba
+  ; Increment number of sectors loaded
+  inc word [bp - GPT_SECTORS_LOADED]
 
 .process_current_entry:
-  jmp .debug
-  ; TODO
-  ; TODO: remember to read offset by current entry index within the current sector
+  ; Calculate the address of the current GPT entry.
+  mov ax, [bp - GPT_SECTOR_ENTRY_IDX] ; Load current entry index within current sector
+  xor dx, dx
+  mul word [bp - GPT_BYTE_STRIDE]     ; Get the byte offset in the current sector of the current entry
+  add ax, 0x8000                      ; Convert offset to address (we loaded the sector at 0x8000)
 
-  inc word [bp - 0x12]  ; Increment current entry index
-  inc word [bp - 0x14]  ; Increment current entry index within the current sector
+  ; Compare entry GUID to our stage 2 partition GUID.
+  mov cx, GUID_LEN
+  mov si, guid_stage2
+  mov di, ax
+  repe cmpsb
+  je .found_stage2
+
+  ; Next iteration
+  inc word [bp - GPT_CURRENT_ENTRY_IDX] ; Increment current entry index
+  inc word [bp - GPT_SECTOR_ENTRY_IDX]  ; Increment current entry index within the current sector
   jmp .loop_find_stage2
 
-.debug:
-  ; Segment for VGA (0xb800 * 16 = 0xb8000)
-  mov ax, 0xb800
-  mov fs, ax
+.found_stage2:
+  push ax ; Address of the GPT entry for stage 2
+  mov si, ax
 
-  ; Set VGA mode
-  ; https://mendelson.org/wpdos/videomodes.txt
+  ; Load partition LBA start.
+  mov eax, [si + 0x20]
+  mov ebx, [si + 0x24]
+  ; Ensure it fits in 16 bits.
+  or ebx, ebx
+  jnz panic
+  cmp ebx, 0xffff
+  ja panic
+  ; Load partition LBA end.
+  mov ecx, [si + 0x28]
+  mov edx, [si + 0x2c]
+  ; Assert that the end LBA is greater than or equal to the start LBA, so we have at least one
+  ; sector to load (end LBA is inclusive).
+  or edx, edx
+  jnz .stage2_end_lba_ok
+  cmp eax, ecx
+  ja panic
+.stage2_end_lba_ok:
+
+  mov bx, BOOT1_LOADPOINT
+  call read_lba
+  jmp bx
+
+  ; Load a single boot disk sector. Panic on failure.
+  ; Inputs:
+  ; - ax: LBA to load
+  ; - bx: address to read sector to
+  ; Clobber: ax, cx, dx
+read_lba:
+  ; sector - 1 = LBA  % sectors_per_track
+  ; temp       = LBA  / sectors_per_track
+  ; head       = temp % n_heads
+  ; cylinder   = temp / n_heads
+  xor dx, dx
+  ; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
+  div word [bp - SECTORS_PER_TRACK]
+  ; Put the sector into cx (the bios call will use cl)
+  mov cx, dx
+  inc cx
+  xor dx, dx
+  ; Divide by number of heads. dx = mod (head), ax = div (cylinder)
+  div word [bp - N_HEADS]
+  mov dh, dl
+  mov ch, al
+  mov dl, byte [bp - BOOT_DRIVE]
+  mov ah, 0x02
+  mov al, 1
+  ; Read sector
+  int 0x13
+  jc panic
+  ret
+
+panic:
   mov ax, 0x0003
   int 0x10
-
-  mov al, [0x7e00]
-  mov byte fs:[0x0000], al
-
-  mov al, [0x8000]
-  mov byte fs:[0x0002], al
-
-  ; mov word fs:[0x0000], 0xc048
-  ; mov word fs:[0x0002], 0xc069
-  
+  mov word fs:[0x0000], 0x4f21
   hlt
 
-.panic:
-  mov ax, 0x0003
-  int 0x10
-  mov ax, 0xb800
-  mov ds, ax
-  mov word [0x0000], 0x4f21
-  hlt
+gpt_magic     db "EFI PART"
+GPT_MAGIC_LEN equ $ - gpt_magic
 
-  ; TODO: enable A20
+; Our stage2 guid: fdffea69-3651-442f-a11d-88a09bf372dd
+guid_stage2 db 0x69, 0xea, 0xff, 0xfd, 0x51, 0x36, 0x2f, 0x44, \
+               0xa1, 0x1d, 0x88, 0xa0, 0x9b, 0xf3, 0x72, 0xdd
+GUID_LEN   equ $ - guid_stage2
 
-  ; TODO: load a second stage
-
-  ; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
-  ; e.g. in a fixed memory location
-
-  ; TODO:
-  ; - Generate GPT in justfile
-  ; - Parse global parition table
-  ; - Load second stage from GPT partition with a particular UUID / name like GRUB does
-  ;   (it's Hah!IdontNeedEFI in GRUB)
-  ;   - https://en.wikipedia.org/wiki/BIOS_boot_partition
-  ; - Future work:
-  ;   - Boot from UEFI
-  ;   - Boot on non-GPT partitioned disk
-
-.gpt_magic:
-  db "EFI PART"
-
-.guid_linux:
-  db 0xaf, 0x3d, 0xc6, 0x0f, 0x83, 0x84, 0x72, 0x47, \
-     0x8e, 0x79, 0x3d, 0x69, 0xd8, 0x47, 0x7d, 0xe4
-
-  ; MBR bootstrap field is 440 bytes long
-  %if ($ - $$) > 440
-  %error "exceeded mbr bootstrap field size"
-  %endif
+; MBR bootstrap field is 440 bytes long
+%if ($ - $$) > 440
+%error "exceeded mbr bootstrap field size"
+%endif

boot1/.cargo/config.toml

@@ -0,0 +1,7 @@
+[build]
+target = "target_protected.json"
+
+[unstable]
+build-std = [ "core", "compiler_builtins" ]
+build-std-features = [ "compiler-builtins-mem" ]
+

boot1/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "boot1"
+version = "0.1.0"

boot1/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "boot1"
+version = "0.1.0"
+edition = "2021"
+
+[profile.dev]
+opt-level = "s"
+debug = 0
+
+[profile.release]
+opt-level = "s"
+debug = 0
+
+[dependencies]

boot1/asm/bios_call.s

@@ -0,0 +1,186 @@
+%include "defines.s"
+
+extern gdt_flat_slice
+extern gdt_flat
+extern GDT_FLAT_IDX_CODE_32
+extern GDT_FLAT_IDX_CODE_16
+extern GDT_FLAT_IDX_DATA
+
+
+section .text
+
+[bits 32]
+
+; FIXME: use a separate stack for real mode
+_bios_call:
+  .PARAMS_START             equ 36
+  .PARAM_BIOS_INTR_NUM      equ .PARAMS_START
+  .PARAM_BIOS_INTR_ARGS_PTR equ .PARAMS_START + 4
+
+  pushad
+  mov ebp, esp
+  push ds
+  push es
+  push fs
+  push gs
+  push ss
+
+  ; Push cr0, the GDT and the IDT to the stack so we can restore them once we're finished with real mode.
+  mov eax, cr0
+  push eax
+  sub esp, 8
+  sidt [esp]
+  sub esp, 8
+  sgdt [esp]
+
+  ; Load the address of the `int` instruction we want to run, and store it on the stack for later.
+  mov eax, [ebp + .PARAM_BIOS_INTR_NUM]
+  lea eax, [int_fns + (3 * eax)]
+  push eax
+
+  ; Copy the BIOS call arguments provided by the caller onto the stack, for us to pop later.
+  sub esp, 28
+  mov ecx, 28
+  mov esi, [ebp + .PARAM_BIOS_INTR_ARGS_PTR]
+  mov edi, esp
+  rep movsb
+
+  ; Switch to a 16-bit code segment, but remain in protected mode.
+  jmp GDT_FLAT_IDX_CODE_16:.protected_mode_16
+
+[bits 16]
+.protected_mode_16:
+
+  ; Disable paging and protected mode.
+  mov eax, cr0
+  and eax, 0x7ffffffe
+  mov cr0, eax
+
+  jmp 0:.real_mode
+
+.real_mode:
+  
+  xor ax, ax
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+  mov ss, ax
+
+  ; Load the caller-provided register values off the stack.
+  pop eax
+  pop ebx
+  pop ecx
+  pop edx
+  pop esi
+  pop edi
+  pop ds
+  pop es
+
+  ; Call the function which runs the correct `int` instruction.
+  push bp
+  mov bp, sp
+  call [bp + 2]
+  pop bp
+
+  ; Copy the resulting register values back into the `BiosIntr` struct provided by the caller.
+  pushfd
+  push es
+  push ds
+  push edi
+  push esi
+  push edx
+  push ecx
+  push ebx
+  push eax
+  mov cx, 32
+  mov si, sp
+  mov di, [bp + .PARAM_BIOS_INTR_ARGS_PTR]
+  rep movsb
+  add sp, 32
+
+  ; Reset the segment registers that were changed to the caller-provided values.
+  xor ax, ax
+  mov ds, ax
+  mov es, ax
+
+  ; Pop the `int` function off the stack.
+  add sp, 4
+
+  ; Reload the GDT in case the BIOS call changed it.
+  mov bx, sp
+  lgdt [bx]
+  add sp, 8
+
+  mov bx, sp
+  lidt [bx]
+  add sp, 8
+
+  ; Restore cr0, re-enabling protected mode.
+  pop eax
+  mov cr0, eax
+
+  jmp GDT_FLAT_IDX_CODE_32:.protected_mode_32
+
+[bits 32]
+.protected_mode_32:
+
+  pop ss
+  pop gs
+  pop fs
+  pop es
+  pop ds
+  popad
+  ret
+
+global _bios_call
+
+
+%macro int_fn 1
+  int %1
+  ret
+%endmacro
+
+
+int_fns:
+  int_fn 0x00
+  int_fn 0x01
+  int_fn 0x02
+  int_fn 0x03
+  int_fn 0x04
+  int_fn 0x05
+  int_fn 0x06
+  int_fn 0x07
+  int_fn 0x08
+  int_fn 0x09
+  int_fn 0x0a
+  int_fn 0x0b
+  int_fn 0x0c
+  int_fn 0x0d
+  int_fn 0x0e
+  int_fn 0x0f
+  int_fn 0x10
+  int_fn 0x11
+  int_fn 0x12
+  int_fn 0x13
+  int_fn 0x14
+  int_fn 0x15
+  int_fn 0x16
+  int_fn 0x17
+  int_fn 0x18
+  int_fn 0x19
+  int_fn 0x1a
+  int_fn 0x1b
+  int_fn 0x1c
+  int_fn 0x1d
+  int_fn 0x1e
+  int_fn 0x1f
+
+
+section .rodata
+
+idt_real:
+  dw 0x3ff
+  dd 0
+
+

boot1/asm/gdt.s

@@ -0,0 +1,95 @@
+section .rodata
+
+gdt_flat_slice:
+  dw GDT_FLAT_LEN
+  dd gdt_flat
+
+global gdt_flat_slice
+
+; Segment descriptor layout
+; | Range (bits) | Field         |
+; |--------------|---------------|
+; |         0-16 | limit         |
+; |        16-32 | base          |
+; |        32-40 | base cont.    |
+; |        40-48 | access        |
+; |        48-52 | limit cont.   |
+; |        52-56 | flags         |
+; |        56-64 | base cont.    |
+;
+; Flags
+; - 0: reserved
+; - 1: long-mode code segment
+; - 2: size
+;   - unset: 16-bit
+;   - set: 32-bit
+; - 3: granularity
+;   - unset: limit is measured in bytes
+;   - set: limit is measured in 4KiB pages
+;
+; Access
+; - 0: accessed
+;   - unset: CPU will set it when the segment is accessed
+; - 1: readable / writable
+;   - data segments: is segment writable (data segments are always readable)
+;   - code segments: is segment readable (code segments are never writable)
+; - 2: direction / conforming
+;   - data segments: whether segment grows down
+;   - code segments: whether this can be executed from a lower-privilege ring
+; - 3: executable
+;   - unset: this is a data segment
+;   - set: this is a code segment
+; - 4: descriptor type
+;   - unset: this is a task state segment
+;   - set: this is a data or code segment
+; - 5-6: privilege level (ring number)
+; - 7: present (must be set)
+;
+align 8
+gdt_flat:
+  ; First GDT entry must be 0.
+  dq 0
+
+; 32-bit code segment.
+; Bytes 0x0000 - 0xffff.
+.segment_code_32:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10011011b,  \
+     01000000b,  \
+     0x00
+
+; 16-bit code segment, to use if we want to switch back to real mode.
+; Bytes 0x0000 - 0xffff.
+.segment_code_16:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10011011b,  \
+     00000000b,  \
+     0x00
+
+; Data segment.
+; Pages 0x000000 - 0x0fffff, which covers the entire 32-bit address space (start of 0xfffff-th page
+; is 0xfffff * 4096 = 0xfffff000, end of page exclusive is 0xfffff000 + 4096 = 0x100000000).
+.segment_data:
+  db 0xff, 0xff, \
+     0x00, 0x00, \
+     0x00,       \
+     10010011b,  \
+     11001111b,  \
+     0x00
+
+global gdt_flat
+
+GDT_FLAT_LEN equ ($ - gdt_flat)
+
+GDT_FLAT_IDX_CODE_32 equ (gdt_flat.segment_code_32 - gdt_flat)
+global GDT_FLAT_IDX_CODE_32
+
+GDT_FLAT_IDX_CODE_16 equ (gdt_flat.segment_code_16 - gdt_flat)
+global GDT_FLAT_IDX_CODE_16
+
+GDT_FLAT_IDX_DATA equ (gdt_flat.segment_data - gdt_flat)
+global GDT_FLAT_IDX_DATA

boot1/asm/prelude.s

@@ -0,0 +1,377 @@
+%include "defines.s"
+%include "ps2.s"
+
+[bits 16]
+
+extern boot1_bin_len
+extern boot1_bin_sectors
+extern boot1_magic
+extern _start
+
+extern gdt_flat_slice
+extern gdt_flat
+extern GDT_FLAT_IDX_CODE_32
+extern GDT_FLAT_IDX_CODE_16
+extern GDT_FLAT_IDX_DATA
+
+section .prelude
+
+%macro copy_stack_var_to_globals 2
+  mov %1, [bp - %2]
+  mov [GLOBALS + %2], %1
+%endmacro
+
+; boot0 loads only our first sector into memory. We must load the rest.
+self_load:
+  ; Now that we're not doing instruction byte golf like we were in boot0, we can afford to move
+  ; the various boot0 stack variables to the globals section.
+  copy_stack_var_to_globals ax, BOOT_DRIVE
+  copy_stack_var_to_globals ax, SECTORS_PER_TRACK
+  copy_stack_var_to_globals ax, N_HEADS
+  copy_stack_var_to_globals ax, GPT_ENTRIES_START_LBA
+  copy_stack_var_to_globals ax, GPT_N_ENTRIES_16
+  copy_stack_var_to_globals ax, GPT_SECTOR_STRIDE
+  copy_stack_var_to_globals ax, GPT_BYTE_STRIDE
+  copy_stack_var_to_globals ax, GPT_ENTRIES_PER_SECTOR
+  copy_stack_var_to_globals ax, GPT_CURRENT_ENTRY_IDX
+  copy_stack_var_to_globals ax, GPT_SECTOR_ENTRY_IDX
+  copy_stack_var_to_globals ax, GPT_SECTORS_LOADED
+  copy_stack_var_to_globals ax, GPT_CURRENT_LBA
+  copy_stack_var_to_globals ax, BOOT1_GPT_ENTRY_ADDR
+
+  ; Reset the stack, now we've got everything we need from it.
+  mov sp, bp
+
+  mov si, [GLOBALS + BOOT1_GPT_ENTRY_ADDR]
+  mov eax, [si + 0x20] ; Partition / boot1 start LBA lower
+  mov ebx, [si + 0x24] ; Partition / boot1 start LBA upper
+  mov ecx, [si + 0x28] ; Partition end LBA lower
+  mov edx, [si + 0x32] ; Partition LBA upper
+
+  ; Panic if the partition / boot1 starting LBA overflows 16 bits.
+  or ebx, ebx
+  jnz panic_simple
+  ror eax, 16
+  or ax, ax
+  jnz panic_simple
+  ror eax, 16
+
+  ; Calculate the boot1 end LBA and panic if it overflows 16 bits.
+  ; n.b. ebx is zero before this so both bx and ebx can be used as the boot1 end LBA.
+  mov bx, ax
+  add bx, boot1_bin_sectors
+  jc panic_simple
+
+  ; Panic if the boot1 end LBA is after the partition end LBA.
+  ; If the upper 32 bits of the partition end LBA are nonzero, then it must be greater than our
+  ; 16-bit boot1 end LBA.
+  or edx, edx
+  jnz .end_lba_ok
+  ; Compare the boot1 end LBA to the lower 32 bits of the partition end LBA.
+  cmp ebx, ecx
+  ja panic_simple
+
+.end_lba_ok:
+
+  ; The first sector has already been loaded (we're running it right now!) so increment the
+  ; current LBA.
+  inc ax
+  push ax                        ; Current LBA
+  push bx                        ; boot1 end LBA
+  mov ebx, BOOT1_LOADPOINT + 512 ; Current sector load address
+
+.self_load_loop:
+  mov ax, [bp - 0x02]      ; Load current LBA
+  cmp word [bp - 0x04], ax ; Compare to boot1 end LBA
+  jb .self_load_done
+
+  mov ecx, ebx
+  call read_sector
+  jc panic_simple
+  
+  add ebx, 512
+  inc word [bp - 0x02]
+  jmp .self_load_loop
+
+.self_load_done:
+
+  ; Check the magic bytes at the end of boot1.
+  push es
+  mov ebx, boot1_magic
+  call addr32_to_addr16
+  cmp dword es:[bx], BOOT1_MAGIC
+  pop es
+  jne panic_simple
+  
+  jmp prelude_main
+
+
+; Converts a 32-bit address to a 16-bit sector and offset.
+; Arguments:
+; - ebx: 32-bit address
+; Return:
+; - es: 16-bit address segment (unchanged on failure)
+; - ebx: 16-bit address offset
+; - cf: unset on success, set on failure
+; Clobber: none
+addr32_to_addr16:
+  fnstart
+  push es
+  push eax
+
+  mov eax, ebx
+  ; Divide addr by 16 and saturate to 16 bits to get the segment.
+  shr eax, 4
+  ror eax, 16
+  or ax, ax
+  jz .segment_ok
+  mov eax, 0xffff0000
+.segment_ok:
+  ror eax, 16
+  mov es, ax
+
+  ; Calculate offset = addr - (16 * segment), failing if the offset doesn't fit in 16 bits.
+  shl eax, 4
+  sub ebx, eax
+  ror ebx, 16
+  or bx, bx
+  jnz .fail
+  ror ebx, 16
+  
+  pop eax
+  add sp, 2 ; Discard the original es from the stack
+  pop bp
+  clc
+  ret
+
+.fail:
+  pop eax
+  pop es
+  stc
+  fnret
+  
+
+; Reads a single sector at the given LBA into memory.
+; Arguments:
+; - ax: start LBA
+; - ecx: address to read sector to
+; Return:
+; - cf: unset on success, set on failure
+; Clobber: eax, ecx, edx
+read_sector:
+  ; sector - 1 = LBA  % sectors_per_track
+  ; temp       = LBA  / sectors_per_track
+  ; head       = temp % n_heads
+  ; cylinder   = temp / n_heads
+
+  fnstart
+  push es
+  push ebx
+  
+  mov ebx, ecx
+  call addr32_to_addr16
+  jc .return
+  
+  ; Calculate sector and temp
+  xor dx, dx
+  ; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
+  div word [GLOBALS + SECTORS_PER_TRACK]
+  ; Put the sector into cx (the bios call will use cl)
+  mov cx, dx
+  inc cx
+  
+  ; Calculate head and cylinder
+  xor dx, dx
+  ; Divide by number of heads. dx = mod (head), ax = div (cylinder)
+  div word [GLOBALS + N_HEADS]
+  mov dh, dl
+  mov ch, al
+
+  mov dl, byte [GLOBALS + BOOT_DRIVE]
+  mov ah, 0x02
+  mov al, 1
+  ; Read sector
+  int 0x13
+
+.return:
+  pop ebx
+  pop es
+  fnret
+
+
+panic_simple:
+  mov ax, 0x0003
+  int 0x10
+  mov word fs:[0x0000], 0x4f21
+.halt:
+  hlt
+  jmp .halt
+
+
+%if ($ - $$) > 512
+%error "boot1 self-loader exceeded sector size"
+%endif
+
+
+; Check whether the A20 line is enabled. Writes to the boot sector identifier.
+; Arguments: none
+; Return:
+; - ax: 0 if A20 disabled, nonzero if A20 enabled
+; Clobber: none
+test_a20:
+  push bp
+  mov bp, sp
+  push gs
+  
+  ; Restore the boot sector identifier in case it was overwritten by anything.
+  mov word [0x7dfe], 0xaa55
+
+  mov ax, 0xffff
+  mov gs, ax
+  xor ax, ax
+
+  ; If the word at 0x107dfe (1 MiB after the boot sector identifier) is different to the boot
+  ; sector identifier, than A20 must be enabled.
+  cmp word gs:[0x7e0e], 0xaa55
+  setne al
+  jne .return
+
+  ; Even if A20 was enabled, the two words may have been equal by chance, so we temporarily swap
+  ; the boot sector identifier bytes and test again.
+  ror word [0x7dfe], 8
+  cmp word gs:[0x7e0e], 0x55aa
+  setne al
+  ror word [0x7dfe], 8
+  jmp .return
+
+.return:
+  pop gs
+  pop bp
+  ret
+
+
+; Wait for the Intel 8042 input buffer to become empty, so we can write.
+; Arguments: none
+; Return: none
+; Clobber: al
+intel_8042_wait_write:
+.loop:
+  ; Read the 8042 status register.
+  in al, INTEL_8042_IN_STATUS
+  ; Input buffer status flag set means the input buffer is full, so loop in this case.
+  test al, INTEL_8042_STATUS_MASK_IBUF
+  jnz .loop
+  ret
+
+
+; Wait for the Intel 8042 output buffer to become filled, so we can read.
+; Arguments: none
+; Return: none
+; Clobber: al
+intel_8042_wait_read:
+.loop:
+  ; Read the 8042 status register.
+  in al, INTEL_8042_IN_STATUS
+  ; Output buffer status flag unset means output buffer is empty, so loop in this case.
+  test al, INTEL_8042_STATUS_MASK_OBUF
+  jz .loop
+  ret
+
+
+; Try to enable A20 using the Intel 8042 PS/2 keyboard controller.
+; Arguments: none
+; Return: none
+; Clobber: ax, cx, dx
+enable_a20_intel_8042:
+  ; Temporarily disable the keyboard.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_PS2_1_DISABLE
+  out INTEL_8042_OUT_CMD, al
+
+  ; Read the controller output port.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_CONTROLLER_OUT_PORT_READ
+  out INTEL_8042_OUT_CMD, al
+  call intel_8042_wait_read
+  in al, INTEL_8042_IO_DATA
+
+  ; The second bit is "A20 enabled", so set it.
+  mov cl, al
+  or cl, 2
+
+  ; Write the modified byte back to the controller output port.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_CONTROLLER_OUT_PORT_WRITE
+  out INTEL_8042_OUT_CMD, al
+  call intel_8042_wait_write
+  mov al, cl
+  out INTEL_8042_IO_DATA, al
+
+  ; Re-enable the keyboard.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_PS2_1_ENABLE
+  out INTEL_8042_OUT_CMD, al
+
+  ; Wait for writes to finish.
+  call intel_8042_wait_write
+
+  ret
+
+
+prelude_main:
+  call test_a20
+  test al, al
+  jnz .a20_enabled
+
+  ; Try to enable A20 using the Intel 8042 PS/2 keyboard controller.
+  call enable_a20_intel_8042
+  call test_a20
+  test al, al
+  jnz .a20_enabled
+
+  ; TODO: try other methods first before we panic:
+  ; - [ ] BIOS interrupt
+  ; - [ ] Fast A20 enable
+  jmp panic_simple
+
+.a20_enabled:
+  mov ax, 0x0003
+  int 0x10
+
+  ; Disable cursor
+  mov ax, 0x0100
+  mov cx, 0x3f00
+  int 0x10
+
+  ; Ensure interrupts are definitely disabled.
+  cli
+
+  ; Load our flat-address-space GDT.
+  lgdt [gdt_flat_slice]
+
+  ; Set the protected-mode bit in cr0.
+  mov eax, cr0
+  or al, 0x01
+  mov cr0, eax
+
+  ; Long jump to set the code segment to gdt_flat.segment_code, and to clear the instruction
+  ; pipeline.
+  jmp GDT_FLAT_IDX_CODE_32:.protected_mode_32
+
+[bits 32]
+.protected_mode_32:
+
+  ; Set the data segments to gdt_flat.segment_data.
+  mov eax, GDT_FLAT_IDX_DATA
+  mov ds, eax
+  mov es, eax
+  mov fs, eax
+  mov gs, eax
+  mov ss, eax
+
+  ; Reset the stack.
+  mov ebp, STACK_BASE
+  mov esp, ebp
+
+  jmp _start
+  jmp panic_simple

boot1/build.rs

@@ -0,0 +1,213 @@
+use std::{env, ffi::{OsStr, OsString}, fmt, fs, io, path::{Path, PathBuf}, process::{self, Command}};
+
+fn main() {
+  let out_dir = env::var_os("OUT_DIR")
+    .expect("OUT_DIR not set");
+  let out_dir = PathBuf::from(out_dir);
+
+  let search_path = SearchPath::load();
+  
+  let nasm_path = search_path.search("nasm")
+    .next()
+    .expect("failed to find nasm in PATH");
+  let nasm = Nasm::new(nasm_path);
+
+  build_asm(&nasm, &out_dir);
+  
+  emit_link_args();  
+
+  rerun_if_changed("build.rs".as_ref());
+}
+
+fn emit_link_args() {
+  let linker_script_path = "link.ld";
+  println!("cargo::rustc-link-arg=-T{}", linker_script_path);
+  rerun_if_changed(linker_script_path.as_ref());
+}
+
+fn rerun_if_changed(path: &Path) {
+  let path_str = path.to_str()
+    .expect("expected path to be valid utf8");
+  println!("cargo::rerun-if-changed={}", path_str);
+}
+
+fn link_obj(path: &Path) {
+  let path_str = path.to_str()
+    .expect("expected path to be valid utf8");
+  println!("cargo::rustc-link-arg={}", path_str);
+}
+
+fn build_asm(nasm: &Nasm, out_dir: &Path) {
+  let asm_srcs = fs::read_dir("asm")
+    .expect("failed to get asm sources");
+
+  for asm_src in asm_srcs {
+    let asm_src = asm_src.expect("failed to get asm source");
+    
+    let ty = asm_src.file_type().expect("failed to get file type");
+    if !ty.is_file() {
+      continue;
+    }
+
+    let src_path = asm_src.path();
+
+    let is_asm = src_path
+      .extension()
+      .map(|ext| ext == "s")
+      .unwrap_or(false);
+    if !is_asm {
+      continue;
+    }
+
+    let out_filename = src_path
+      .file_name()
+      .unwrap()
+      .apply(PathBuf::from)
+      .with_extension("o");
+    let out_path = out_dir.join(out_filename);
+
+    nasm.assemble(
+      &out_path,
+      &[&src_path],
+      &["../include".as_ref()]
+    ).expect("failed to assemble");
+
+    link_obj(&out_path);
+  }
+}
+
+struct Nasm {
+  bin_path: PathBuf,
+}
+
+impl Nasm {
+  fn new(bin_path: PathBuf) -> Self {
+    Self { bin_path }
+  }
+
+  fn assemble(&self, output: &Path, sources: &[&Path], includes: &[&Path]) -> Result<(), CmdError>
+  {
+    for source in sources {
+      rerun_if_changed(source);
+    }
+
+    let mut cmd = Command::new(&self.bin_path);
+    cmd
+      .arg("-Werror")
+      .arg("-f")
+      .arg("elf");
+
+    for include in includes {
+      let mut buf = OsString::new();
+      buf.push("-I");
+      buf.push(include);
+      cmd.arg(buf);
+    }
+
+    cmd
+      .arg("-o")
+      .arg(output)
+      .args(sources);
+
+    run_cmd(&mut cmd)?;
+
+    Ok(())
+  }
+}
+
+struct SearchPath {
+  paths: Vec<PathBuf>
+}
+
+impl SearchPath {
+  fn load() -> Self {
+    let path_var = env::var_os("PATH").unwrap_or_default();
+    let paths = env::split_paths(&path_var).collect();
+    Self { paths }
+  }
+  
+  fn search<'a, 'b, 'c, T>(&'a self, bin: &'b T) -> impl Iterator<Item = PathBuf> + 'c
+  where
+    'a: 'c,
+    'b: 'c,
+    T: AsRef<OsStr> + ?Sized,
+  {
+    let bin = bin.as_ref();
+    self.paths
+      .iter()
+      .filter_map(move |path| {
+        let path = path.join(bin);
+        fs::metadata(&path)
+          .ok()
+          .and_then(|meta| if meta.is_file() || meta.is_symlink() {
+            Some(path)
+          } else {
+            None
+          })
+      })
+  }
+}
+
+fn run_cmd(cmd: &mut Command) -> Result<process::Output, CmdError> {
+  use fmt::Write;
+
+  cmd
+      .output()
+      .map_err(CmdErrorKind::Io)
+      .and_then(|out| if out.status.success() {
+        Ok(out)
+      } else {
+        Err(CmdErrorKind::Status(out))
+      })
+      .map_err(|err| {
+        let mut cmd_buf = String::new();
+        write!(&mut cmd_buf, "{:?}", cmd).ok();
+        CmdError::new(cmd_buf, err)
+      })
+}
+
+#[derive(Debug)]
+struct CmdError {
+  cmd: String,
+  kind: CmdErrorKind,
+}
+
+impl CmdError {
+  fn new(cmd: String, kind: CmdErrorKind) -> Self {
+    Self { cmd, kind }
+  }
+}
+
+impl fmt::Display for CmdError {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    write!(f, "`{}` failed: ", self.cmd)?;
+    match &self.kind {
+      CmdErrorKind::Io(err) => err.fmt(f),
+      CmdErrorKind::Status(out) => write!(f, "exited with status {}", out.status),
+    }
+  }
+}
+
+#[derive(Debug)]
+enum CmdErrorKind {
+  Io(io::Error),
+  Status(process::Output),
+}
+
+trait Apply: Sized {
+  fn apply<T, F>(self, f: F) -> T
+  where
+    F: FnOnce(Self) -> T;
+}
+
+impl<U> Apply for U
+where
+  U: Sized,
+{
+  fn apply<T, F>(self, f: F) -> T
+  where
+    F: FnOnce(Self) -> T
+  {
+    f(self)
+  }
+}

boot1/link.ld

@@ -0,0 +1,43 @@
+OUTPUT_FORMAT("binary")
+
+. = 0x8200;
+
+SECTIONS {
+  .prelude : {
+    KEEP(*(.prelude))
+    *(.prelude)
+  }
+  
+  .text : {
+    *(.text)
+    *(.text.*)
+  }
+  
+  .data : {
+    *(.data)
+    *(.data.*)
+  }
+  
+  .bss : {
+    *(.bss)
+    *(.bss.*)
+  }
+  
+  .rodata : {
+    *(.rodata)
+    *(.rodata.*)
+  }
+  
+  .magic : {
+    /* Magic bytes the prelude uses to make sure it's loaded the subsequent sectors correctly. */
+    LONG(0x544e4150)
+  }
+
+  boot1_magic = ADDR(.magic);
+
+  /* Define a symbol for the total length of the binary, so the prelude knows how many blocks to
+   * load from disk.
+   */
+  boot1_bin_len = . - 0x8200;
+  boot1_bin_sectors = (boot1_bin_len + 511) / 512;
+}

boot1/src/main.rs

@@ -0,0 +1,72 @@
+#![no_std]
+#![no_main]
+
+mod spin;
+mod vga;
+
+use core::{arch::asm, panic::PanicInfo, fmt::Write};
+
+#[repr(C)]
+#[derive(Clone, Debug)]
+struct BiosIntr {
+  eax: u32,
+  ebx: u32,
+  ecx: u32,
+  edx: u32,
+  esi: u32,
+  edi: u32,
+  ds: u16,
+  es: u16,
+  eflags: u32,
+}
+
+unsafe extern "C" {
+  unsafe fn _bios_call(intr: u32, args: *mut BiosIntr);
+}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+  vga_println!("panic!");
+  if let Some(location) = info.location() {
+    vga_println!("{}", location);
+  }
+  vga_println!("{}", info.message());
+  hlt()
+}
+
+#[no_mangle]
+pub extern "C" fn _start() -> ! {
+  vga::vga_init();
+
+  vga_println!("hello from rust");
+
+  let mut args = BiosIntr {
+    eax: 0xe820,
+    ebx: 0,
+    ecx: 24,
+    edx: 0x534d4150,
+    esi: 0,
+    edi: 0x6a00,
+    ds: 0,
+    es: 0,
+    eflags: 0,
+  };
+
+  unsafe {
+    _bios_call(0x15, &raw mut args);
+  }
+
+  vga_println!("eax = {:x}", args.eax);
+
+  hlt()
+}
+
+#[inline]
+fn hlt() -> ! {
+  loop {
+    unsafe {
+      asm!("hlt");
+    }
+  }
+}
+

boot1/src/spin.rs

@@ -0,0 +1,101 @@
+use core::{cell::UnsafeCell, hint, ops::{Deref, DerefMut}, sync::atomic::{AtomicBool, Ordering}};
+
+pub struct Spinlock<T> {
+  data: UnsafeCell<T>,
+  locked: AtomicBool,
+}
+
+impl<T> Spinlock<T> {
+  pub const fn new(data: T) -> Self {
+    Self {
+      data: UnsafeCell::new(data),
+      locked: AtomicBool::new(false),
+    }
+  }
+
+  pub fn lock(&self) -> SpinlockGuard<T> {
+    // If we observe `locked` was `false`, then:
+    // - Use acquire ordering, so nothing inside the critical section gets reordered before we
+    //   observed the `false` value.
+    // - Store `true`, so nothing else can enter the critical section until we exit it. 
+    // Otherwise, spin until we observe a `false` value.
+    while self.locked
+      .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+      .is_err()
+    {
+      hint::spin_loop();
+    }
+
+    SpinlockGuard { lock: self }
+  }
+
+  /// # Safety
+  /// There must be no "active" `SpinlockGuards` for this lock, i.e. a `SpinlockGuard` which can be
+  /// used to obtain a reference to the spinlock-protected data.
+  unsafe fn unlock(&self) {
+    // Unset `locked` with release ordering so that nothing inside the critical section gets
+    // reordered to after we stored `false`.
+    self.locked.store(false, Ordering::Release);
+  }
+
+  /// # Safety
+  /// No mutable references to the spinlock-protected data may exist.
+  unsafe fn get<'s, 'a>(&'s self) -> &'a T
+  where
+    's: 'a,
+  {
+    unsafe { &*self.data.get() }
+  }
+
+  /// # Safety
+  /// No references to the spinlock-protected data may exist.
+  unsafe fn get_mut<'s, 'a>(&'s self) -> &'a mut T
+  where
+    's: 'a,
+  {
+    unsafe { &mut *self.data.get() }
+  }
+}
+
+unsafe impl<T> Sync for Spinlock<T>
+where
+  T: Send,
+{}
+
+pub struct SpinlockGuard<'a, T> {
+  lock: &'a Spinlock<T>,
+}
+
+impl<'a, T> Deref for SpinlockGuard<'a, T> {
+  type Target = T;
+
+  #[inline]
+  fn deref(&self) -> &Self::Target {
+    // SAFETY:
+    // For the entire lifetime of the `SpinlockGuard`, `locked` remains `true`, so we have
+    // exclusive access to `data`, so no mutable references to `data` can exist.
+    unsafe { self.lock.get() }
+  }
+}
+
+impl<'a, T> DerefMut for SpinlockGuard<'a, T> {
+  #[inline]
+  fn deref_mut(&mut self) -> &mut Self::Target {
+    // SAFETY:
+    // For the entire lifetime of the `SpinlockGuard`, `locked` remains `true`, so we have
+    // exclusive access to `data`, so no other references to `data` can exist.
+    unsafe { self.lock.get_mut() }
+  }
+}
+
+impl<'a, T> Drop for SpinlockGuard<'a, T> {
+  fn drop(&mut self) {
+    // SAFETY:
+    // Only one `SpinlockGuard` can exist at a time for a particular lock, since we set `locked`
+    // to true before creating a guard and refuse to create any new ones until it is `false` again.
+    // Therefore, we are the only `SpinlockGuard` for the lock. Since this is the destructor, and
+    // we don't access the spinlock-protected data here, there are therefore no "active"
+    // `SpinlockGuard`s remaining for the lock.
+    unsafe { self.lock.unlock() }
+  }
+}

boot1/src/vga.rs

@@ -0,0 +1,128 @@
+use core::{fmt, ptr};
+
+use crate::spin::Spinlock;
+
+const VGA_WIDTH: usize = 80;
+const VGA_HEIGHT: usize = 25;
+
+const COLOUR_MASK: u16 = 0x0a00;
+
+pub static VGA: Spinlock<Option<VgaBuf>> = Spinlock::new(None);
+
+#[macro_export]
+macro_rules! vga_print {
+  ($($args:tt)*) => ({
+    crate::vga::with_vga_buf(|vga_buf| {
+      ::core::write!(vga_buf, $($args)*).ok();
+    });
+  })
+}
+
+#[macro_export]
+macro_rules! vga_println {
+  ($($args:tt)*) => ({
+    crate::vga::with_vga_buf(|vga_buf| {
+      ::core::writeln!(vga_buf, $($args)*).ok();
+    });
+  })
+}
+
+pub fn vga_init() {
+  let vga_buf = unsafe { VgaBuf::new(0xb8000 as *mut u16) };
+  let mut guard = VGA.lock();
+  *guard = Some(vga_buf);
+}
+
+pub fn with_vga_buf<F>(f: F)
+where
+  F: FnOnce(&mut VgaBuf),
+{
+  let mut guard = crate::vga::VGA.lock();
+  let vga = guard.as_mut().expect("vga not initialised");
+  f(vga);
+}
+
+pub struct VgaBuf {
+  buf: *mut u16,
+  col: usize,
+  row: usize,
+}
+
+impl VgaBuf {
+  pub unsafe fn new(buf: *mut u16) -> Self {
+    Self { buf, col: 0, row: 0 }
+  }
+
+  pub fn vga_write_str(&mut self, s: &str) {
+    for c in s.chars() {
+      self.vga_write_char(c);
+    }
+  }
+
+  pub fn vga_write_char(&mut self, c: char) {
+    let newline = c == '\n';
+
+    if newline || self.col >= VGA_WIDTH {
+      self.col = 0;
+      self.row += 1;
+    }
+
+    if self.row >= VGA_HEIGHT {
+      self.scroll();
+      self.row -= 1;
+    }
+
+    if !newline {
+      self.vga_write_char_at(c, self.col, self.row);
+      self.col += 1;
+    }
+  }
+
+  pub fn vga_write_ascii_char_at(&mut self, c: u8, col: usize, row: usize) {
+    let vga_val = COLOUR_MASK | u16::from(c);
+
+    if col < VGA_WIDTH && row < VGA_HEIGHT {
+      unsafe {
+        self.coord_ptr(col, row)
+          .write_volatile(vga_val);
+      }
+    }
+  }
+
+  pub fn vga_write_char_at(&mut self, c: char, col: usize, row: usize) {
+    let c = u8::try_from(c).unwrap_or(0xfe);
+    self.vga_write_ascii_char_at(c, col, row);
+  }
+
+  fn scroll(&mut self) {
+    unsafe {
+      ptr::copy(
+        self.coord_ptr(0, 1),
+        self.coord_ptr(0, 0),
+        VGA_WIDTH * (VGA_HEIGHT - 1)
+      );
+      
+      for col in 0..VGA_WIDTH {
+        self.vga_write_ascii_char_at(0, col, VGA_HEIGHT - 1);
+      }
+    }
+  }
+
+  unsafe fn coord_ptr(&self, col: usize, row: usize) -> *mut u16 {
+    unsafe { self.buf.add((row * VGA_WIDTH) + col) }
+  }
+}
+
+impl fmt::Write for VgaBuf {
+  fn write_str(&mut self, s: &str) -> fmt::Result {
+    self.vga_write_str(s);
+    Ok(())
+  }
+  
+  fn write_char(&mut self, c: char) -> fmt::Result {
+    self.vga_write_char(c);
+    Ok(())
+  }
+}
+
+unsafe impl Send for VgaBuf {}

boot1/target_protected.json

@@ -0,0 +1,16 @@
+{
+  "arch": "x86",
+  "os": "none",
+  "llvm-target": "i686-unknown-none",
+  "data-layout": "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
+  "target-endian": "little",
+  "target-pointer-width": "32",
+  "target-c-int-width": "32",
+  "executables": true,
+  "linker-flavor": "ld.lld",
+  "linker": "rust-lld",
+  "panic-strategy": "abort",
+  "disable-redzone": true,
+  "features": "-mmx,-sse,+soft-float"
+}
+

buildtools/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "buildtools"
+version = "0.1.0"

buildtools/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "buildtools"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

buildtools/src/lib.rs

@@ -0,0 +1,109 @@
+use std::{ffi::OsString, path::{Path, PathBuf}, process::Command};
+
+#[derive(Clone)]
+pub struct Nasm {
+  bin_path: PathBuf,
+  includes: Vec<PathBuf>,
+  warnings: Vec<String>,
+  werror: bool,
+  out_format: Option<String>,
+}
+
+impl Nasm {
+  pub fn new(bin_path: PathBuf) -> Self {
+    Self {
+      bin_path,
+      includes: Vec::new(),
+      warnings: Vec::new(),
+      werror: true,
+      out_format: None,
+    }
+  }
+
+  pub fn include<T, I>(self, paths: I) -> Self
+  where
+    PathBuf: From<T>,
+    I: IntoIterator<Item = T>,
+  {
+    Self {
+      includes: {
+        let mut updated_includes = self.includes;
+        updated_includes.extend(paths.into_iter().map(From::from));
+        updated_includes
+      },
+      ..self
+    }
+  }
+
+  pub fn warnings<T, I>(self, warnings: I) -> Self
+  where
+    String: From<T>,
+    I: IntoIterator<Item = T>,
+  {
+    Self {
+      warnings: {
+        let mut updated_warnings = self.warnings;
+        updated_warnings.extend(warnings.into_iter().map(From::from));
+        updated_warnings
+      },
+      ..self
+    }
+  }
+
+  pub fn werror(self, enabled: bool) -> Self {
+    Self {
+      werror: enabled,
+      ..self
+    }
+  }
+
+  pub fn out_format<T>(self, format: T) -> Self
+  where
+    String: From<T>,
+  {
+    Self {
+      out_format: Some(From::from(format)),
+      ..self
+    }
+  }
+
+  // TODO: make this generic over sources & path types
+  pub fn to_cmd(&self, output: &Path, sources: &[&Path]) -> Command {
+    let mut cmd = self.to_common_cmd();
+
+    cmd
+      .arg("-o")
+      .arg(output)
+      .args(sources);
+
+    cmd
+  }
+
+  fn to_common_cmd(&self) -> Command {
+    let mut cmd = Command::new(&self.bin_path);
+
+    if let Some(format) = self.out_format.as_ref() {
+      cmd.arg("-f").arg(format);
+    }
+
+    for include in self.includes.iter() {
+      let mut buf = OsString::new();
+      buf.push("-I");
+      buf.push(include);
+      cmd.arg(buf);
+    }
+
+    for warning in self.warnings.iter() {
+      let mut buf = OsString::new();
+      buf.push("-w+");
+      buf.push(warning);
+      cmd.arg(buf);
+    }
+
+    if self.werror {
+      cmd.arg("-werror");
+    }
+
+    cmd
+  }
+}

elf/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "elf"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

elf/src/main.rs

@@ -0,0 +1,349 @@
+use core::fmt;
+use std::{char, env, fs};
+
+fn main() {
+  let mut args = env::args().skip(1);
+  let path = args.next().unwrap();
+
+  let bytes = fs::read(path).unwrap();
+
+  let elf = Elf::parse(&bytes).unwrap();
+}
+
+#[derive(Clone)]
+struct InputBytes<'a> {
+  bytes: &'a [u8],
+}
+
+impl<'a> InputBytes<'a> {
+  fn new(bytes: &'a [u8]) -> Self {
+    Self { bytes }
+  }
+
+  fn new_offset(bytes: &'a [u8], offset: usize) -> Self {
+    Self::new(bytes).advanced(offset)
+  }
+
+  fn prefix_len(&self, n: usize) -> usize {
+    self.bytes.len().min(n)
+  }
+
+  fn advanced(&self, n: usize) -> Self {
+    Self {
+      bytes: &self.bytes[self.prefix_len(n)..],
+    }
+  }
+
+  fn advance(&mut self, n: usize) -> &mut Self {
+    self.bytes = &self.bytes[self.prefix_len(n)..];
+    self
+  }
+
+  fn peek(&self, n: usize) -> Result<&'a [u8], ParseError> {
+    self.bytes.get(..n).ok_or(ParseError::UnexpectedEof)
+  }
+
+  fn pop(&mut self, n: usize) -> Result<&'a [u8], ParseError> {
+    let res = self.peek(n);
+    if res.is_ok() {
+      self.advance(n);
+    }
+    res
+  }
+
+  fn pop_arr<const N: usize>(&mut self) -> Result<[u8; N], ParseError> {
+    self.pop(N).map(|bs| <[u8; N]>::try_from(bs).unwrap())
+  }
+
+  fn pop_u8(&mut self) -> Result<u8, ParseError> {
+    self.pop_arr::<1>().map(|[x]| x)
+  }
+
+  fn pop_u16(&mut self, endianness: Endianness) -> Result<u16, ParseError> {
+    let bytes = self.pop_arr::<2>()?;
+    Ok(endianness.read_u16(bytes))
+  }
+
+  fn pop_u32(&mut self, endianness: Endianness) -> Result<u32, ParseError> {
+    let bytes = self.pop_arr::<4>()?;
+    Ok(endianness.read_u32(bytes))
+  }
+
+  fn pop_u64(&mut self, endianness: Endianness) -> Result<u64, ParseError> {
+    let bytes = self.pop_arr::<8>()?;
+    Ok(endianness.read_u64(bytes))
+  }
+
+  fn pop_word(&mut self, endianness: Endianness, width: Width) -> Result<u64, ParseError> {
+    match width {
+      Width::Bits32 => self.pop_u32(endianness).map(u64::from),
+      Width::Bits64 => self.pop_u64(endianness),
+    }
+  }
+
+  fn pop_word_usize(&mut self, endianness: Endianness, width: Width) -> Result<usize, ParseError> {
+    self.pop_word(endianness, width)
+      .and_then(|x| usize::try_from(x).map_err(|_| ParseError::OffsetOutOfBounds))
+  }
+
+  fn hexdump(&self, n: usize) {
+    print!("{: <8}", "");
+    for i in 0..16 {
+      print!("  {:x}", i);
+    }
+    println!();
+    for (i, chunk) in self.bytes[.. n.min(self.bytes.len())].chunks(16).enumerate() {
+      print!("{:08x}", i * 16);
+      for b in chunk {
+        print!(" {:02x}", b);
+      }
+      print!("{: <1$}", "", 4 + 3 * (16 - chunk.len()));
+      for b in chunk {
+        let c = match char::from(*b) {
+          c if c.is_ascii_graphic() => c,
+          _ => '.',
+        };
+        print!("{}", c);
+      }
+      println!();
+    }
+  }
+}
+
+struct Elf<'a> {
+  elf_bytes: &'a [u8],
+  bl: ByteLayout,
+  header_version: u8,
+  abi: u8,
+  ty: ElfType,
+  instruction_set: Option<InstructionSet>,
+  elf_version: u32,
+}
+
+impl<'a> Elf<'a> {
+  fn parse(bytes: &'a [u8]) -> Result<Self, ElfError> {
+    let mut hdr = InputBytes::new(bytes);
+    
+    hdr.hexdump(64);
+
+    let magic = hdr.pop(4)?;
+    if magic != &[0x7f, b'E', b'L', b'F'] {
+      return Err(ElfError::BadMagic);
+    }
+  
+    let width = match hdr.pop_u8()? {
+      1 => Width::Bits32,
+      2 => Width::Bits64,
+      _ => return Err(ElfError::UnsupportedValue),
+    };
+  
+    let endianness = match hdr.pop_u8()? {
+      1 => Endianness::Little,
+      2 => Endianness::Big,
+      _ => return Err(ElfError::UnsupportedValue),
+    };
+
+    let bl = ByteLayout {
+      endianness,
+      width,
+    };
+  
+    let header_version = hdr.pop_u8()?;
+
+    let abi = hdr.pop_u8()?;
+
+    hdr.pop(8)?;
+
+    let ty = match hdr.pop_u16(endianness)? {
+      1 => ElfType::Relocatable,
+      2 => ElfType::Executable,
+      3 => ElfType::Shared,
+      4 => ElfType::Core,
+      _ => return Err(ElfError::UnsupportedValue),
+    };
+    
+    let instruction_set = match hdr.pop_u16(endianness)? {
+      0 => None,
+      0x02 => Some(InstructionSet::Sparc),
+      0x03 => Some(InstructionSet::I386),
+      0x08 => Some(InstructionSet::Mips),
+      0x14 => Some(InstructionSet::PowerPc),
+      0x28 => Some(InstructionSet::Arm),
+      0x2a => Some(InstructionSet::SuperH),
+      0x32 => Some(InstructionSet::Ia64),
+      0x3e => Some(InstructionSet::Amd64),
+      0xb7 => Some(InstructionSet::Aarch64),
+      0xf3 => Some(InstructionSet::RiscV),
+      _ => return Err(ElfError::UnsupportedValue),
+    };
+
+    let elf_version = hdr.pop_u32(endianness)?;
+    let entry_offset = hdr.pop_word(endianness, width)?;
+    let pht_offset = hdr.pop_word_usize(endianness, width)?;
+    let sht_offset = hdr.pop_word_usize(endianness, width)?;
+    let flags = hdr.pop_u32(endianness)?;
+    let header_size = hdr.pop_u16(endianness)?;
+    let pht_entry_size = hdr.pop_u16(endianness)?;
+    let pht_len = hdr.pop_u16(endianness)?;
+    let sht_entry_size = hdr.pop_u16(endianness)?;
+    let sht_len = hdr.pop_u16(endianness)?;
+    let shstr_idx = hdr.pop_u16(endianness)?;
+
+    dbg!(entry_offset);
+    dbg!(pht_offset);
+    dbg!(sht_offset);
+    dbg!(flags);
+    dbg!(header_size);
+    dbg!(pht_entry_size);
+    dbg!(pht_len);
+    dbg!(sht_entry_size);
+    dbg!(sht_len);
+    dbg!(shstr_idx);
+
+    // TODO
+
+    let shstr = SectionHeader::parse(bl,
+      InputBytes::new_offset(bytes, sht_offset + usize::from(sht_entry_size * shstr_idx)))?;
+
+    let str_table = InputBytes::new_offset(bytes, shstr.offset);
+    str_table.hexdump(shstr.size);
+
+    Ok(Self {
+      elf_bytes: bytes,
+      bl,
+      header_version,
+      abi,
+      ty,
+      instruction_set,
+      elf_version,
+    })
+  }
+}
+
+struct SectionHeader {
+  name: u32,
+  // FIXME: parse type
+  ty: u32,
+  flags: u64,
+  addr: u64,
+  offset: usize,
+  size: usize,
+  link: u32,
+  info: u32,
+  align: u64,
+  ent_size: usize,
+}
+
+impl SectionHeader {
+  fn parse(bl: ByteLayout, mut bytes: InputBytes) -> Result<Self, ElfError> {
+    let name = bytes.pop_u32(bl.endianness)?;
+    let ty = bytes.pop_u32(bl.endianness)?;
+    let flags = bytes.pop_word(bl.endianness, bl.width)?;
+    let addr = bytes.pop_word(bl.endianness, bl.width)?;
+    let offset = bytes.pop_word_usize(bl.endianness, bl.width)?;
+    let size = bytes.pop_word_usize(bl.endianness, bl.width)?;
+    let link = bytes.pop_u32(bl.endianness)?;
+    let info = bytes.pop_u32(bl.endianness)?;
+    let align = bytes.pop_word(bl.endianness, bl.width)?;
+    let ent_size = bytes.pop_word_usize(bl.endianness, bl.width)?;
+
+    Ok(Self {
+      name,
+      ty,
+      flags,
+      addr,
+      offset,
+      size,
+      link,
+      info,
+      align,
+      ent_size,
+    })
+  }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+struct ByteLayout {
+  endianness: Endianness,
+  width: Width,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum Width {
+  Bits32,
+  Bits64,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum Endianness {
+  Little,
+  Big,
+}
+
+impl Endianness {
+  fn read_u16(self, bytes: [u8; 2]) -> u16 {
+    match self {
+      Self::Little => u16::from_le_bytes(bytes),
+      Self::Big => u16::from_be_bytes(bytes),
+    }
+  }
+
+  fn read_u32(self, bytes: [u8; 4]) -> u32 {
+    match self {
+      Self::Little => u32::from_le_bytes(bytes),
+      Self::Big => u32::from_be_bytes(bytes),
+    }
+  }
+
+  fn read_u64(self, bytes: [u8; 8]) -> u64 {
+    match self {
+      Self::Little => u64::from_le_bytes(bytes),
+      Self::Big => u64::from_be_bytes(bytes),
+    }
+  }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum ElfType {
+  Relocatable,
+  Executable,
+  Shared,
+  Core,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+enum InstructionSet {
+  Sparc,
+  I386,
+  Mips,
+  PowerPc,
+  Arm,
+  SuperH,
+  Ia64,
+  Amd64,
+  Aarch64,
+  RiscV,
+}
+
+#[derive(Debug)]
+enum ElfError {
+  BadMagic,
+  UnexpectedEof,
+  OffsetOutOfBounds,
+  UnsupportedValue,
+}
+
+impl From<ParseError> for ElfError {
+  fn from(value: ParseError) -> Self {
+    match value {
+      ParseError::UnexpectedEof => Self::UnexpectedEof,
+      ParseError::OffsetOutOfBounds => Self::OffsetOutOfBounds,
+    }
+  }
+}
+
+#[derive(Debug)]
+enum ParseError {
+  UnexpectedEof,
+  OffsetOutOfBounds,
+}

include/defines.s

@@ -0,0 +1,74 @@
+%define BOOT0_LOADPOINT 0x7c00
+%define BOOT1_LOADPOINT 0x8200
+
+%define MEMMAP                0x6a00
+%define MEMMAP_END            BOOT0_LOADPOINT
+%define MEMMAP_ENT_SIZE       32
+%define MEMMAP_CAP            ((MEMMAP_END - MEMMAP) / MEMMAP_ENT_SIZE)
+
+%define GLOBALS     0x4000
+%define GLOBALS_END MEMMAP
+%define STACK_BASE  GLOBALS
+
+%define EBDA_START 0x080000
+
+%define VGA_SEG 0xb800
+
+%define BOOT1_MAGIC 0x544e4150
+
+%define VGA_WIDTH  80
+%define VGA_HEIGHT 25
+
+; ASCII string "SMAP", which is used by e820 as a magic number argument and return value.
+%define E820_MAGIC 0x534d4150
+; Size of entry returned by e820. This is less than MEMMAP_ENT_SIZE because we add some fields of
+; our own.
+%define E820_ENTRY_SIZE 24
+
+%define MEMMAP_ENT_FIELD_BASE 0x00
+%define MEMMAP_ENT_FIELD_LEN  0x08
+%define MEMMAP_ENT_FIELD_TYPE 0x10
+%define MEMMAP_ENT_FIELD_EXT  0x14
+%define MEMMAP_ENT_FIELD_END  0x18
+
+%define GDT_DESCRIPTOR_SIZE 0x08
+
+; boot0 base stack frame variable offsets / globals
+; (we use the same offsets once we copy the variables to the globals section)
+; -------------------------------------------------------------------------------------------------
+; The boot drive number given to us by the BIOS.
+%define BOOT_DRIVE             0x02
+; Boot drive geometry
+%define SECTORS_PER_TRACK      0x04
+%define N_HEADS                0x06
+; Starting LBA of the GPT partition entries array.
+%define GPT_ENTRIES_START_LBA  0x08
+; Number of GPT entries, saturated to 16 bits.
+%define GPT_N_ENTRIES_16       0x0a
+; Number of sectors to advance by once we've read every GPT entry in the current sector.
+%define GPT_SECTOR_STRIDE      0x0c
+; Number of bytes to advance by in the current sector once we've read a GPT entry.
+%define GPT_BYTE_STRIDE        0x0e
+; Number of GPT entries which can fit in a single sector.
+%define GPT_ENTRIES_PER_SECTOR 0x10
+%define GPT_CURRENT_ENTRY_IDX  0x12
+%define GPT_SECTOR_ENTRY_IDX   0x14
+%define GPT_SECTORS_LOADED     0x16
+%define GPT_CURRENT_LBA        0x18
+%define BOOT1_GPT_ENTRY_ADDR   0x1a
+
+; Globals
+; ------------------------------------------------------------------------------------------------
+%define VGA_COL                0x1c
+%define TEXTBUF_LINE           0x1e
+%define MEMMAP_ENTRIES         0x20
+
+%macro fnstart 0
+  push bp
+  mov bp, sp
+%endmacro
+
+%macro fnret 0
+  pop bp
+  ret
+%endmacro

include/ps2.s

@@ -0,0 +1,18 @@
+%ifndef BOOT_PS2_H
+%define BOOT_PS2_H
+
+%define INTEL_8042_IO_DATA                        0x60
+%define INTEL_8042_IN_STATUS                      0x64
+%define INTEL_8042_OUT_CMD                        0x64
+
+%define INTEL_8042_STATUS_BIT_OBUF                0
+%define INTEL_8042_STATUS_BIT_IBUF                1
+%define INTEL_8042_STATUS_MASK_OBUF               (1 << INTEL_8042_STATUS_BIT_OBUF)
+%define INTEL_8042_STATUS_MASK_IBUF               (1 << INTEL_8042_STATUS_BIT_IBUF)
+
+%define INTEL_8042_CMD_PS2_1_DISABLE              0xad
+%define INTEL_8042_CMD_PS2_1_ENABLE               0xae
+%define INTEL_8042_CMD_CONTROLLER_OUT_PORT_READ   0xd0
+%define INTEL_8042_CMD_CONTROLLER_OUT_PORT_WRITE  0xd1
+
+%endif

justfile

@@ -1,17 +1,40 @@
 run:
-  qemu-system-x86_64 -drive format=raw,file=disk.bin
+  qemu-system-x86_64 \
+    -monitor stdio \
+    -no-reboot \
+    -bios seabios/out/bios.bin \
+    -m 512M \
+    -drive format=raw,file=disk.bin
+
 
 build:
-  nasm -f bin -o boot0.bin boot0.s
+  nasm -f bin -Iinclude -o boot0.bin boot0.s
+  cd boot1; cargo build --release
+  # nasm -f bin -Iinclude -o boot1.bin boot1.s
 
 zero_disk:
-  dd if=/dev/zero of=disk.bin bs=64K count=16
+  dd if=/dev/zero of=disk.bin bs=512 count=1000
 
 partition_disk:
   parted --script disk.bin mklabel gpt
-  parted --script disk.bin mkpart test 34s 50%
-  parted --script disk.bin type 1 0fc63daf-8483-4772-8e79-3d69d8477de4
+  parted --script disk.bin mkpart dummy1 34s 47s
+  parted --script disk.bin type 1 e3c9e316-0b5c-4db8-817d-f92df00215ae
+  parted --script disk.bin mkpart dummy2 64s 65s
+  parted --script disk.bin type 2 e3c9e316-0b5c-4db8-817d-f92df00215ae
+  parted --script disk.bin mkpart dummy3 66s 67s
+  parted --script disk.bin type 3 e3c9e316-0b5c-4db8-817d-f92df00215ae
+  parted --script disk.bin mkpart dummy4 68s 69s
+  parted --script disk.bin type 4 e3c9e316-0b5c-4db8-817d-f92df00215ae
+  parted --script disk.bin mkpart dummy5 48s 63s
+  parted --script disk.bin type 5 0fc63daf-8483-4772-8e79-3d69d8477de4
+  # parted --script disk.bin mkpart stage2 70s 200s
+  parted --script disk.bin mkpart stage2 70s 900s
+  parted --script disk.bin type 6 fdffea69-3651-442f-a11d-88a09bf372dd
 
-write_stage0:
+write_stage1:
   dd if=/dev/zero of=disk.bin bs=440 count=1 conv=notrunc
   dd if=boot0.bin of=disk.bin conv=notrunc
+
+write_stage2:
+  # dd if=boot1.bin of=disk.bin bs=512 seek=70 conv=notrunc
+  dd if=boot1/target/target_protected/release/boot1 of=disk.bin bs=512 seek=70 conv=notrunc

memregion_merge/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "memregion_merge"
+version = "0.1.0"

memregion_merge/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "memregion_merge"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

memregion_merge/src/main.rs

@@ -0,0 +1,282 @@
+// Notes:
+// - Once we've got a list of usable memory, we can use some of it to store the original e820 list
+//   for future use. We therefore don't have to worry about storing the unavailable regions for
+//   now, we can do that later.
+
+use std::array;
+
+struct MemRegions<const N: usize> {
+  buf: [MemRegion; N],
+  len: usize,
+}
+
+impl<const N: usize> MemRegions<N> {
+  fn new() -> Self {
+    Self { buf: array::from_fn(|_| Default::default()), len: 0 }
+  }
+
+  fn regions(&self) -> &[MemRegion] {
+    &self.buf[..self.len]
+  }
+
+  fn push_usable(&mut self, new: MemRegion) -> bool {
+    let mut merged = false;
+    for old in &mut self.buf[..self.len] {
+      if old.overlaps(&new) {
+        old.merge(&new);
+        merged = true;
+        break;
+      }
+    }
+
+    // If we merged the new region with an existing one, we may have filled in a gap that allows
+    // other regions to be merged, so check if any adjacent regions can be merged.
+    if merged {
+      let mut i = 1;
+      while i < self.len {
+        if self.buf[i - 1].overlaps(&self.buf[i]) {
+          let (left, right) = self.buf.split_at_mut(i);
+          left[i - 1].merge(&right[0]);
+          right.rotate_left(1);
+          self.len -= 1;
+        }
+        else {
+          i += 1;
+        }
+      }
+
+      return true;
+    }
+
+    // If we couldn't merge the new region with any others, then place it into its sorted position
+    // in the regions buffer.
+    else {
+      self.push_without_merge_at(new, 0)
+    }
+  }
+
+  fn push_without_merge_at(&mut self, new: MemRegion, start_idx: usize) -> bool {
+    // Fail if we don't have space for the new region.
+    if self.len >= N {
+      return false;
+    }
+
+    let mut i = start_idx;
+      while i < self.len {
+        if new.start < self.buf[i].start {
+          break;
+        }
+        i += 1;
+      }
+
+      self.buf[self.len] = new;
+      self.len += 1;
+      self.buf[i..self.len].rotate_right(1);
+
+      true
+  }
+
+  // n.b. expects to be run only after all `push_usable`s are done
+  fn push_unusable(&mut self, unusable: MemRegion) {
+    let mut i = 0;
+    while i < self.len {
+      if !unusable.overlaps(&self.buf[i]) {
+        i += 1;
+      }
+      else {
+        let left_region = MemRegion::new(self.buf[i].start, unusable.start);
+        let right_region = MemRegion::new(unusable.end, self.buf[i].end);
+  
+        // If the unusable region completely contains the usable region, remove the usable region
+        // entirely.
+        if left_region.empty() && right_region.empty() {
+          self.buf[i..self.len].rotate_left(1);
+          self.len -= 1;
+        }
+        // If the unusable region is completely contained within the usable region, then split the
+        // usable region in two.
+        else if !left_region.empty() && !right_region.empty() {
+          self.buf[i] = left_region;
+          self.push_without_merge_at(right_region, i + 1);
+          i += 1;
+        }
+        // Otherwise, trim the usable region.
+        else {
+          self.buf[i] = if !left_region.empty() { left_region } else { right_region };
+          i += 1;
+        }
+      }
+    }
+  }
+}
+
+#[derive(Clone, Default, PartialEq, Eq, Debug)]
+struct MemRegion {
+  start: u64,
+  end: u64,
+}
+
+impl MemRegion {
+  fn new(start: u64, end: u64) -> Self {
+    Self { start, end }
+  }
+
+  fn len(&self) -> u64 {
+    if self.end < self.start {
+      0
+    }
+    else {
+      self.end - self.start
+    }
+  }
+
+  fn empty(&self) -> bool {
+    self.len() == 0
+  }
+
+  fn overlaps(&self, other: &MemRegion) -> bool {
+    self.start <= other.end && other.start <= self.end
+  }
+
+  fn merge(&mut self, other: &MemRegion) {
+    debug_assert!(self.overlaps(other));
+
+    self.start = self.start.min(other.start);
+    self.end = self.end.max(other.end);
+  }
+}
+
+fn main() {
+
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+  
+  #[test]
+  fn test_push_usable() {
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 200),
+        MemRegion::new(250, 400),
+        MemRegion::new(500, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_usable(MemRegion::new(150, 300));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 400),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_usable(MemRegion::new(150, 300));
+      rs.push_usable(MemRegion::new(250, 400));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 400),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(150, 300));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 400),
+        MemRegion::new(500, 600),
+      ]);
+    }
+  }
+
+  #[test]
+  fn test_push_unusable() {
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_unusable(MemRegion::new(250, 400));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 200),
+        MemRegion::new(500, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_unusable(MemRegion::new(300, 400));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 200),
+        MemRegion::new(250, 300),
+        MemRegion::new(500, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_unusable(MemRegion::new(250, 300));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 200),
+        MemRegion::new(300, 400),
+        MemRegion::new(500, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<5>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_unusable(MemRegion::new(300, 350));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 200),
+        MemRegion::new(250, 300),
+        MemRegion::new(350, 400),
+        MemRegion::new(500, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+
+    {
+      let mut rs = MemRegions::<4>::new();
+      rs.push_usable(MemRegion::new(800, 900));
+      rs.push_usable(MemRegion::new(500, 600));
+      rs.push_usable(MemRegion::new(250, 400));
+      rs.push_usable(MemRegion::new(100, 200));
+      rs.push_unusable(MemRegion::new(150, 550));
+      assert_eq!(rs.regions(), &[
+        MemRegion::new(100, 150),
+        MemRegion::new(550, 600),
+        MemRegion::new(800, 900),
+      ]);
+    }
+  }
+}

patches/001_seabios_menuconfig.patch

@@ -0,0 +1,13 @@
+Index: boot/seabios/scripts/kconfig/lxdialog/check-lxdialog.sh
+===================================================================
+--- boot.orig/seabios/scripts/kconfig/lxdialog/check-lxdialog.sh
++++ boot/seabios/scripts/kconfig/lxdialog/check-lxdialog.sh
+@@ -43,7 +43,7 @@ trap "rm -f $tmp" 0 1 2 3 15
+ check() {
+         $cc -x c - -o $tmp 2>/dev/null <<'EOF'
+ #include CURSES_LOC
+-main() {}
++int main() { return 0; }
+ EOF
+ 	if [ $? != 0 ]; then
+ 	    echo " *** Unable to find the ncurses libraries or the"       1>&2

patches/series

@@ -0,0 +1 @@
+001_seabios_menuconfig.patch

readme.md

@@ -0,0 +1,22 @@
+## Overview
+Currently we boot from BIOS on a GPT-partitioned disk.
+
+`boot0.s` is the first-stage bootloader. It lives in the MBR bootstrap field, and must therefore
+fit into 440 bytes. Its tasks are:
+- [x] Set up a stack etc.
+- [x] Get the boot drive geometry
+- [x] Load and parse the GPT
+- [x] Find the partition of the second stage
+- [x] Load and jump to the first sector of the second stage
+
+`boot1.s` is the second-stage bootloader. Its tasks are:
+- [x] Load its own sectors not loaded by the first stage
+- [x] Enable A20
+- [ ] Detect available memory
+- [ ] Enter unreal mode
+- [ ] Read the kernel ELF from disk (don't have to parse the ELF just yet)
+- [ ] Get all the useful information we can from the BIOS before we leave unreal mode
+- [ ] Enter long mode (possibly via protected mode?)
+- [ ] Load the kernel ELF
+- [ ] Initialise stack etc. for the kernel
+- [ ] Jump to kernel

resources.md

@@ -2,18 +2,26 @@
 - <https://www.felixcloutier.com/x86/>
 - <http://www.unixwiz.net/techtips/x86-jumps.html>
 - <https://www.nasm.us/xdoc/2.13.01/nasmdoc.pdf>
+- <https://www.nasm.us/xdoc/2.13.02rc3/html/nasmdo10.html>
 - <https://wiki.osdev.org/Real_Mode>
 - <https://www.eecg.utoronto.ca/~amza/www.mindsec.com/files/x86regs.html>
 - <https://en.wikibooks.org/wiki/X86_Assembly/16,_32,_and_64_Bits>
 - <https://www.plantation-productions.com/Webster/www.artofasm.com/DOS/ch04/CH04-2.html>
+- <https://www.tortall.net/projects/yasm/manual/html/nasm-local-label.html>
 
 ## Boot sequence
 - <https://wiki.osdev.org/Boot_Sequence>
 - <https://wiki.osdev.org/Protected_Mode#Entering_Protected_Mode>
+- <https://wiki.osdev.org/A20_Line>
+
+## Unreal mode
+- <https://wiki.osdev.org/Unreal_Mode>
+- <http://www.rcollins.org/ddj/Aug98/Aug98.html>
 
 ## Memory
 - <https://wiki.osdev.org/Memory_Map_(x86)>
 - <https://wiki.osdev.org/Detecting_Memory_(x86)>
+- <http://www.uruk.org/orig-grub/mem64mb.html>
 
 ## BIOS interrupts
 - <https://en.wikipedia.org/wiki/BIOS_interrupt_call>
@@ -30,5 +38,22 @@
 - <https://en.wikipedia.org/wiki/Code_page_437>
 
 ## ELF
+- <http://www.skyfree.org/linux/references/ELF_Format.pdf>
 - <https://wiki.osdev.org/ELF>
 - <https://wiki.osdev.org/ELF_Tutorial>
+
+## Intel 8042 PS/2 controller
+- <https://wiki.osdev.org/%228042%22_PS/2_Controller>
+- <https://wiki.osdev.org/I/O_Ports>
+
+## Partitioning
+- <https://wiki.osdev.org/GPT>
+
+## Memory protection
+- <https://wiki.osdev.org/Global_Descriptor_Table>
+- <https://wiki.osdev.org/GDT_Tutorial>
+- <https://wiki.osdev.org/Segment_Limits>
+- <https://www.felixcloutier.com/x86/lgdt:lidt>
+
+## Qemu
+- <https://qemu-project.gitlab.io/qemu/system/monitor.html>