71 lines
1.0 KiB
NASM
71 lines
1.0 KiB
NASM
; First MCB block is always at 00540
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mcb_first:
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mov ax, 0x54
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mov ds, ax
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ret
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mcb_init:
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call mcb_first
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mov dx, ds
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inc dx
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xor si, si
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int 0x12
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mov cl, 6
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shl ax, cl
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sub ax, dx
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mov byte [si+0], 'Z'
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mov word [si+1], 0
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mov word [si+3], ax
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mov dx, cs
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call mcb_split
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mov word [si+1], 8
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ret
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; split mcb block into two
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; IN dx segment of second block
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mcb_split:
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call mcb_find
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dec dx
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push dx
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; Calculate size of first block
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; dx = dx - ds - 1
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stc
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mov ax, ds
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sbb dx, ax
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;
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push dx
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; swap old size with new size
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xchg dx, [si+3]
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; if current mcb is last, its not anymore
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mov cl, 'M'
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xchg cl, byte [si+0]
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; advance DS to begin of first block
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pop ax
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stc
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sbb dx, ax
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pop ax
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mov ds, ax
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; write MCB data
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mov byte [si+0], cl
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mov word [si+1], 0
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mov word [si+3], dx
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ret
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; IN dx segment we look for
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; OUT ds segment where mcb starts
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mcb_find:
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call mcb_first
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xor si, si
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.loop: stc
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adc ax, [si+3]
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cmp dx, ax
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jc .found
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cmp byte [si+0], 'Z'
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jz .fail
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mov ds, ax
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jmp .loop
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.fail: stc
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ret
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.found: clc
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ret
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