Ain
abc7b563cd
New features: - ES:DI (potential PnP structure) is preserved - FAT detection now works by checking for BPB values - Uses booterr procedure
143 lines
1.9 KiB
NASM
143 lines
1.9 KiB
NASM
; FDC fields in VBR
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%define bps (bs + 0x0B)
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%define md (bs + 0x15)
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%define spt (bs + 0x18)
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%define nos (bs + 0x1A)
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%define po (bs + 0x1C)
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; blah dont judge me, i miss 8080
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%macro retnz 0
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jz short ($+3)
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ret
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%endmacro
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%macro callc 1
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jnc short ($+5)
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call %1
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%endmacro
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cpu 8086
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org 0x0600
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bs: equ 0x7C00
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init: cli
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xor ax, ax
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mov ss, ax
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xor sp, sp
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; save driver number
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push dx
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; save PnP ptr
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push es
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push di
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; Relocate away from bootsector loading area
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mov ds, ax
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mov es, ax
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mov si, bs
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mov di, $$
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mov cx, 0x100
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rep movsw
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jmp 0:main
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%include "inc/booterr.asm"
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; find active partition or bail out
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; result in si
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findp: mov si, part1
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mov cx, 4
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.loop: test BYTE [si], 0x80
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retnz
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add si, 0x10
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loop .loop
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call errcll
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main: ; find active partition
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call findp
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; transfer starting address into DAP
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push si
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add si, 0x08
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mov di, dap.num
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movsw
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movsw
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pop si
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; load sector
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push si
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mov si, dap
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mov ah, 0x42
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stc
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int 0x13
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callc errcll
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pop si
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; FAT bpb has "bytes per sector" at this place
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; we support only 512 byte sectors anyways
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cmp word [bps], 512
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jne chain
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; Media descriptor byte is never smaller than 0xF0
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cmp byte [md], 0xF0
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jc chain
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; ask BIOS what our disk geometry is
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mov ah, 0x08
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stc
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int 0x13
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jc chain
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; ES might be messed up from int 13h, fix it
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push cs
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pop es
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; update sectors per track
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xor ax, ax
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mov al, cl
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mov [spt], ax
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; update number of sides
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xor ax, ax
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mov al, dh
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mov [nos], ax
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; update partition offset
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push si
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add si, 0x08
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mov di, po
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movsw
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movsw
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pop si
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chain: ; restore PnP structure
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pop di
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pop es
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pop dx
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xor cx, cx
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mov bp, si
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; chain into partition boot sector
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jmp 0:bs
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dap:
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.size: db 0
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db 0
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.count: dw 1
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.buf: dw bs
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dw 0
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.num: dq 0
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times (0x1BE - ($-$$)) db 0
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part1: db 0x80
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db 0xFF, 0xFF, 0xFF
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db 0x01
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db 0xFF, 0xFF, 0xFF
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dd 1
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dd (FLOPPY * 2)
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times (0x1FE - ($-$$)) db 0
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; Boot signature
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dw 0xAA55
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