use labels for stack variables
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@@ -12,26 +12,34 @@
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; R | 0x0c8000 - 0x0f0000: bios expansiosn
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; R | 0x0f0000 - 0x100000: motherboard bios
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; BASE STACK FRAME
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; --------------------------------------------------------------------
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; bp - 0x02: drive number
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; bp - 0x04: sectors per track
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; bp - 0x06: number of heads
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; bp - 0x08: partition array starting LBA
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; bp - 0x0a: number of GPT partitions (saturated to 16-bits)
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; bp - 0x0c: sector stride when loading GPT entries
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; bp - 0x0e: byte stride when loading GPT entries
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; bp - 0x10: entries per sector when loding GPT entries
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; bp - 0x12: current entry index
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; bp - 0x14: current entry index within the current sector
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; bp - 0x16: number of sectors loaded
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; bp - 0x18: current LBA
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; BIOS puts our boot sector at 0000:7c00
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org 0x7c00
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; We're (probably) in real mode
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bits 16
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; BASE STACK FRAME VARIABLE OFFSETS
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; --------------------------------------------------------------------
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; The boot drive number given to us by the BIOS.
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BOOT_DRIVE equ 0x02
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; Boot drive geometry
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SECTORS_PER_TRACK equ 0x04
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N_HEADS equ 0x06
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; Starting LBA of the GPT partition entries array.
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GPT_ENTRIES_START_LBA equ 0x08
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; Number of GPT entries, saturated to 16 bits.
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GPT_N_ENTRIES_16 equ 0x0a
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; Number of sectors to advance by once we've read every GPT entry in the current sector.
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GPT_SECTOR_STRIDE equ 0x0c
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; Number of bytes to advance by in the current sector once we've read a GPT entry.
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GPT_BYTE_STRIDE equ 0x0e
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; Number of GPT entries which can fit in a single sector.
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GPT_ENTRIES_PER_SECTOR equ 0x10
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GPT_CURRENT_ENTRY_IDX equ 0x12
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GPT_SECTOR_ENTRY_IDX equ 0x14
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GPT_SECTORS_LOADED equ 0x16
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GPT_CURRENT_LBA equ 0x18
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STAGE2_GPT_ENTRY_ADDR equ 0x1a
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; Disable interrupts
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cli
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@@ -79,8 +87,8 @@ bits 16
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call .read_lba
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; Check the GPT header magic "EFI PART"
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mov cx, 8
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mov si, .gpt_magic
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mov cx, GPT_MAGIC_LEN
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mov si, gpt_magic
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mov di, 0x7e00
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repe cmpsb
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jne .panic
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@@ -155,55 +163,59 @@ bits 16
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; Set up stack variables for our second stage search loop.
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xor ax, ax
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push ax ; Current entry
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push ax ; Current entry within the current sector
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push ax ; First LBA loaded?
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mov ax, [bp - 0x08] ; Starting LBA
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push ax ; Current LBA
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push ax ; Current entry
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push ax ; Current entry within the current sector
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push ax ; Number of sectors loaded
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mov ax, [bp - GPT_ENTRIES_START_LBA]
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push ax ; Current LBA
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; Search for the partition storing our second stage.
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.loop_find_stage2:
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mov dx, [bp - 0x12]
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cmp [bp - 0x0a], dx
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mov dx, [bp - GPT_CURRENT_ENTRY_IDX]
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cmp [bp - GPT_N_ENTRIES_16], dx
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; Panic if we've run out of partitions and haven't found the second stage yet.
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jbe .panic
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; If we haven't loaded any sectors yet, load the first one.
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cmp word [bp - 0x16], 0 ; Load number of sectors loaded
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cmp word [bp - GPT_SECTORS_LOADED], 0
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je .load_first_lba
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; If there's still more entries in the current sector, skip loading a new sector
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mov ax, [bp - 0x14] ; Load current entry index within the current sector
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cmp [bp - 0x10], ax ; Compare to entries per sector
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mov ax, [bp - GPT_SECTOR_ENTRY_IDX] ; Load current entry index within the current sector
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cmp [bp - GPT_ENTRIES_PER_SECTOR], ax ; Compare to entries per sector
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ja .process_current_entry
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mov ax, [bp - 0x0c] ; Load sector stride
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add word [bp - 0x18], ax ; Increment current LBA by sector stride
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mov word [bp - 0x14], 0 ; Reset the current entry index within the current sector
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mov ax, [bp - GPT_SECTOR_STRIDE] ; Load sector stride
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add word [bp - GPT_CURRENT_LBA], ax ; Increment current LBA by sector stride
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mov word [bp - GPT_SECTOR_ENTRY_IDX], 0 ; Reset the current entry index within the current sector
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.load_first_lba:
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mov ax, [bp - 0x18] ; Load current LBA
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mov bx, 0x8000 ; Read sector just past end of the GPT header so we can keep it around
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; Read the current LBA to 0x8000 (just past the end of the GPT header)
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mov ax, [bp - GPT_CURRENT_LBA]
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mov bx, 0x8000
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call .read_lba
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inc word [bp - 0x16] ; Increment number of sectors loaded
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; Increment number of sectors loaded
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inc word [bp - GPT_SECTORS_LOADED]
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.process_current_entry:
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mov ax, [bp - 0x14] ; Load current entry index within current sector
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; Calculate the address of the current GPT entry.
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mov ax, [bp - GPT_SECTOR_ENTRY_IDX] ; Load current entry index within current sector
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xor dx, dx
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mul word [bp - 0x0e] ; Get the byte offset in the current sector of the current entry
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add ax, 0x8000 ; Convert offset to address (we loaded the sector at 0x8000)
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mul word [bp - GPT_BYTE_STRIDE] ; Get the byte offset in the current sector of the current entry
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add ax, 0x8000 ; Convert offset to address (we loaded the sector at 0x8000)
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mov cx, 16 ; GUID is 16 bytes
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mov si, .guid_linux
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; Compare entry GUID to our stage 2 partition GUID.
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mov cx, GUID_LEN
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mov si, guid_linux
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mov di, ax
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repe cmpsb
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je .found_stage2
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; Next iteration
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inc word [bp - 0x12] ; Increment current entry index
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inc word [bp - 0x14] ; Increment current entry index within the current sector
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inc word [bp - GPT_CURRENT_ENTRY_IDX] ; Increment current entry index
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inc word [bp - GPT_SECTOR_ENTRY_IDX] ; Increment current entry index within the current sector
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jmp .loop_find_stage2
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.found_stage2:
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push ax ; address of the GPT entry for stage 2
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push ax ; Address of the GPT entry for stage 2
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mov si, ax
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; Load partition LBA start.
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@@ -251,17 +263,21 @@ bits 16
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xor dx, dx
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; FIXME: we should probably get our globals from somewhere else, in case we want to change bp for
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; a function call
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div word [bp - 0x04] ; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
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; Divide by sectors per track. dx = mod (sector - 1), ax = div (temp)
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div word [bp - SECTORS_PER_TRACK]
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; Put the sector into cx (the bios call will use cl)
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mov cx, dx
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inc cx ; Put sector into cx (bios will read sector from cl)
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inc cx
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xor dx, dx
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div word [bp - 0x06] ; Divide by number of heads. dx = mod (head), ax = div (cylinder)
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; Divide by number of heads. dx = mod (head), ax = div (cylinder)
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div word [bp - N_HEADS]
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mov dh, dl
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mov ch, al
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mov dl, byte [bp - 0x02] ; Load drive number
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mov dl, byte [bp - BOOT_DRIVE]
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mov ah, 0x02
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mov al, 1
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int 0x13 ; Read sector
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; Read sector
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int 0x13
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jc .panic
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ret
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@@ -288,12 +304,12 @@ bits 16
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; - Boot from UEFI
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; - Boot on non-GPT partitioned disk
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.gpt_magic:
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db "EFI PART"
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gpt_magic db "EFI PART"
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GPT_MAGIC_LEN equ $ - gpt_magic
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.guid_linux:
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db 0xaf, 0x3d, 0xc6, 0x0f, 0x83, 0x84, 0x72, 0x47, \
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0x8e, 0x79, 0x3d, 0x69, 0xd8, 0x47, 0x7d, 0xe4
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guid_linux db 0xaf, 0x3d, 0xc6, 0x0f, 0x83, 0x84, 0x72, 0x47, \
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0x8e, 0x79, 0x3d, 0x69, 0xd8, 0x47, 0x7d, 0xe4
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GUID_LEN equ $ - guid_linux
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; MBR bootstrap field is 440 bytes long
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%if ($ - $$) > 440
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