162 lines
2.6 KiB
NASM
162 lines
2.6 KiB
NASM
fcb1 equ 0x5C
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org 0x100
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jmp main
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%include "scan.inc"
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%include "print.inc"
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drvnum db 0
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track dw 39
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main mov di, 0x81
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mov al, 0x20
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mov ch, 0
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mov cl, [0x80]
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repe scasb
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repne scasb
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mov si, di
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call lodnum
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mov dx, ax
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test dx, ~0x83
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jnz drverr
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mov byte [drvnum], dl
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test dl, 0x80
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jz floppy
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call drverr
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drverr call putsh
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db "Failed to access drive", 0x0A, 0x0D, 0
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int 0x20
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; read bios parameter block
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floppy mov ax, 0x0201
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mov cx, 0x0001
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mov dh, 0
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mov bx, prog
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int 0x13
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jc drverr
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; calculate start of cluster area
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mov ax, [prog+0x011] ; number of dir entries
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mov cl, 5
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shl ax, cl
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mov bx, [prog+0x0B] ; bytes per sector
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div bx
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; TODO: abort if dx is not zero
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mov cx, ax
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mov ax, [prog+0x016] ; logical sectors per fat
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; TODO: abort if ax is zero (=large FAT32)
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mov bl, [prog+0x010]
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mov bh, 0
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mul bx
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add ax, cx
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add ax, [prog+0x00E] ; reserved sectors
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mov si, ax
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; calculate sector offset of target track
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mov ax, [prog+0x018]
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mul word [prog+0x01A]
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mul word [track]
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; calculate cluster offset of target track
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sub ax, si
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mov bl, [prog+0x00D] ; sectors per cluster
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mov bh, 0
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call debug
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div bx
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call debug
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ret
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; TODO:
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; calculate cluster offset of target track
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; calculate cluster length of target track
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; calculate offset into fat table
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; load part of fat table spanning our track
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; check that all relevant clusters are 0 or 0xFFF7
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; set all relevant clusters to 0xFFF7
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; track is now available for direct writing
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; set up boot code
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mov di, prog
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mov ax, 0x7FEB
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stosw
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mov byte [di], 0x80
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add di, 0x7F - b_ld + b_rst
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mov si, b_rst
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mov cx, b_end - b_rst
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rep movsb
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; write back bootsector
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mov ax, 0x0301
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mov cx, 0x0001
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mov dh, 0
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mov bx, prog
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int 0x13
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jc drverr
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mov dx, fcb1
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mov ah, 0xF
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int 0x21
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test al, al
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jz .fok
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.ferr call putsh
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db "Failed to read input file", 0x0A, 0x0D, 0
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ret
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.fok mov si, prog
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; apply our target address
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.rloop mov dx, si
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mov ah, 0x1A
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int 0x21
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; increment address for next iteration
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add si, 0x80
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; do a read
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mov dx, fcb1
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mov ah, 0x14
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int 0x21
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; success: continue
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cmp al, 0
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jz .rloop
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; eof: break
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cmp al, 1
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jz .cok
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; segment wrap: fail
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cmp al, 2
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jz .ferr
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; we assume al=3: eof
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.cok nop
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mov ax, si
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sub ax, args
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add ax, 0x1FF
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mov cl, 9
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shr ax, cl
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call debug
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ret
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; code to copy into boot sector
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b_rst mov ah, 0
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int 0x13
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b_ld mov ax, 0x60
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mov ds, ax
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mov es, ax
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mov ss, ax
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xor sp, sp
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mov ax, 0x0208 ; read, 8 sectors
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b_len equ $ - 2
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mov cx, 0x2701 ; track 39, sector 1
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b_trck equ $ - 1
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b_strt equ $ - 2
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mov dh, 0 ; head 0
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xor bx, bx ; es:bx=0x60:0
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int 0x13
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jc b_rst
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mov byte [4], dl
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jmp 0x60:0x100
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b_end equ $
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align 2
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psp equ $
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args equ psp + 0x80
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prog equ psp + 0x100
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