2023-08-06 21:35:05 -07:00
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; TODO implement boot via chs for old bioses (even ones
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; as new as 1996 lack support for lba via bios call)
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[bits 16]
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[org 0x7C00]
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jmp entry
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times 4 - ($-$$) db 0
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2023-10-27 21:54:33 -07:00
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; 16mb shown as example
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2023-08-06 21:35:05 -07:00
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db "MOTHFS",0
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db 0
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sz: dd 32768 ; 32768 * 512 = 16mb disk image
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off: dd 1 ; immediately follows mbr
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db "mkmothfs",0,0,0,0,0,0,0
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rsv: db 127 ; 64kb of boot code
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abm: dd 4096 ; 16mb/(512*8)
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entry:
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2023-10-27 21:54:33 -07:00
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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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mov fs, ax
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mov gs, ax
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mov ss, ax
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mov sp, 0x7BFE ; 30kb stack until 0x500
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mov bp, sp
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push ax
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2023-08-06 21:35:05 -07:00
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pop ds
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2023-10-27 21:54:33 -07:00
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mov dx, 0x80 ; change disk number to 80 because every bios is the worst
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2023-08-06 21:35:05 -07:00
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push dx
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; enable a20 a few different ways
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in al, 0xEE
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; bios call
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mov ax, 0x2401 ; l is real
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int 0x15
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; fast a20 gate
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in al, 0x92
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or al, 2
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out 0x92, al
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mov eax, [off]
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mov dword [dapw], eax
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; check for lba support
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mov ah, 0x41
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mov bx, 0x55AA
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int 0x13 ; dl is drive number - mbr should pass this value
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jc .nde
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shr cx, 2 ; wikipedia says bit 1 is disk packet support
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jnc .nde
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; load kernel flat binary into hma using lba
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pop dx
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mov ah, 0x42
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mov si, dap
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int 0x13
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; jump to FFFF:0010, the start of HMA. this limits the kernel
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; size to 65520 bytes but we are actually loading 496 bytes
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; less than that and even that should be more than enough for
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; foxdos and any more would probably blow up some garbage bios
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mov ax, 0xFFFF
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push ax
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mov ax, 16
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push ax
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retf
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.nde: mov eax, [sz]
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add eax, [off]
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cmp eax, 0xFB0400 ; max chs sectors
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jg $ ; too big to use chs
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jmp $ ; TODO load via chs
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dap: dw 0x10 ; size of disk packet
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dw 127 ; foxdos will always load 127 segments
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dw 16, 0xFFFF ; offset:segment to start of HMA - foxdos loads high
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dapw: dd 0, 0
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times 510 - ($-$$) db 0
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dw 0xAA55
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