126 lines
1.7 KiB
NASM
126 lines
1.7 KiB
NASM
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section .bss
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clus resw 1
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section .text
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; Set cluster for next operations
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; IN dx cluster number
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setclus mov [clus], dx
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ret
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; get ptr to single byte in fat table
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; IN dx position byte in fat table
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; OUT bx ptr to byte in dskbuf
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_fat1 push dx
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mov ax, dx
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xor dx, dx
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mov cl, 9
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shr ax, cl
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add ax, [bpb+BPBRSC]
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adc dx, 0
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call map
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pop dx
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mov bx, dx
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and bx, 0x1FF
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add bx, dskbuf
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ret
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; get ptr to word in fat table
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_fat2 push dx
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mov ax, dx
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xor dx, dx
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mov cl, 8
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shr ax, cl
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add ax, [bpb+BPBRSC]
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adc dx, 0
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call map
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pop dx
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mov bl, dl
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mov bh, 0
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add bx, bx
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add bx, dskbuf
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ret
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; read value from fat table
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; index given with setclus
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; OUT bx value
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; carry set if EOF marker
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rdfat mov dx, [clus]
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mov ax, dx
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shr ax, 1
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add dx, ax
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call _fat1
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push word [bx]
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inc dx
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call _fat1
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mov si, bx
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pop bx
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mov bh, [si]
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test word [clus], 1
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jz l001
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mov cl, 4
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shr bx, cl
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l001 and bx, 0xFFF
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cmp bx, 0xFF0
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cmc
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ret
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; set value in FAT table
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; IN dx value
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wrfat and dh, 0x0F
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push dx
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; DX = index * 1.5
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mov dx, [clus]
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mov ax, dx
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shr dx, 1
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add dx, ax
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call _fat1
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pop ax
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test [clus], 1
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jnz l004
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; even, first byte full
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mov [bx], al
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push ax
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call dirty
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; DX = index * 1.5
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mov dx, [clus]
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mov ax, dx
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shr dx, 1
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add dx, ax
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inc dx
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call _fat1
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pop ax
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mov al, [bx]
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and al, 0xF0
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or al, ah
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mov [bx], al
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call dirty
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jmp l005
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; odd, second byte full
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l004: mov ch, [bx]
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; concat existing and our nib
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and ch, 0x0F
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mov cl, 4
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shl ax, cl
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or al, ch
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; write back lower 4 bits
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mov [bx], al
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call dirty
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push ax
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; re-calculate offset
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mov ax, dx
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shr dx, 1
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add dx, ax
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inc dx
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; map second byte
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call _fat1
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pop ax
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; write higher 8 bits
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mov [bx], ah
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call dirty
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l005: ret
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; wrfat from above, FAT16 flavour
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wf16: ret
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