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125 lines
3.2 KiB
ArmAsm
125 lines
3.2 KiB
ArmAsm
; MEMORY LAYOUT
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; R = reserved, U = usable
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; --------------------------------------------------------------------
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; R | 0x000000 - 0x000400: real-mode interrupt vector table
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; R | 0x000400 - 0x000500: bios data area
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; U | 0x000500 - 0x007c00: stack
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; U | 0x007000 - 0x007c00: big stage0 initial stack frame
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; | 0x007000 - 0x007001: boot drive number
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; | 0x007001 - 0x007002: boot drive sectors per track
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; | 0x007002 - 0x007004: boot drive number of heads
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; U | 0x007c00 - 0x007e00: boot sector
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; U | 0x007e00 - 0x080000: conventional usable memory
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; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
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; R | 0x0a0000 - 0x0c0000: video memory
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; R | 0x0c0000 - 0x0c8000: video bios
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; R | 0x0c8000 - 0x0f0000: bios expansiosn
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; R | 0x0f0000 - 0x100000: motherboard bios
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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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; Disable interrupts
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cli
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xor ax, ax
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mov ds, ax
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mov es, ax
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; We put stack base at 0x7c00 and give ourselves a big stack frame [0x7000,0x7c00) for spilling to.
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; Stack grows high->low, so we'll grow away from our program text at 0x7c00.
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mov ss, ax
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mov sp, 0x7c00
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mov bp, 0x7000
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; Segment for VGA (0xb800 * 16 = 0xb8000)
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mov ax, 0xb800
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mov fs, ax
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; Store boot drive number
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mov [bp], dl
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; Get drive geometry
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mov di, 0x00
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mov ah, 0x08
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int 0x13
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jc panic
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and cl, 0x3f ; sectors per track
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mov [bp + 1], cl
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movzx bx, dh
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inc bx ; number of heads
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mov [bp + 2], bx
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; Load LBA 1.
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cmp cl, 1 ; division of 1 by nonzero can be done with cmp
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sete al ; temp = LBA / (sectors per track)
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setne cl ; sector - 1 = LBA % (sectors per track)
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inc cl
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xor ah, ah ; zero-extend temp
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xor dx, dx ; TODO: explain why this is needed for div
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div bx ; ah = mod (head), al = div (cylinder)
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mov dh, ah ; head
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mov ch, al ; cylinder
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; we already have sector in cl
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mov ah, 2 ; read disk
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mov al, 1 ; load one sector
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mov bx, 0x7e00
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mov dl, [bp] ; drive number
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int 0x13
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jc panic
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; head = temp % number of heads
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; cylinder = temp / number of heads
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; Set VGA mode
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; https://mendelson.org/wpdos/videomodes.txt
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mov ax, 0x0003
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int 0x10
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; mov word fs:[0x0000], 0xc048
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; mov word fs:[0x0002], 0xc069
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mov al, [0x7e00]
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mov byte fs:[0x0000], al
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mov al, [0x7e01]
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mov byte fs:[0x0002], al
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mov al, [0x7e02]
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mov byte fs:[0x0004], al
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hlt
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panic:
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mov ax, 0x0003
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int 0x10
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mov ax, 0xb800
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mov ds, ax
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mov word [0x0000], 0x4f46
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mov word [0x0002], 0x4f41
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mov word [0x0004], 0x4f49
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mov word [0x0006], 0x4f4c
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hlt
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; TODO: enable A20
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; TODO: load a second stage
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; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
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; e.g. in a fixed memory location
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; TODO:
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; - Generate GPT in justfile
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; - Parse global parition table
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; - Load second stage from GPT partition with a particular UUID / name like GRUB does
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; (it's Hah!IdontNeedEFI in GRUB)
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; - https://en.wikipedia.org/wiki/BIOS_boot_partition
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; - Future work:
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; - Boot from UEFI
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; - Boot on non-GPT partitioned disk
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; MBR bootstrap field is 440 bytes long
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%if ($ - $$) > 440
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%error "exceeded mbr bootstrap field size"
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%endif
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