113 lines
2.0 KiB
NASM
113 lines
2.0 KiB
NASM
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section .bss
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chsspt: equ 0 ; word sectors per track
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chsnos: equ 2 ; word number of sides
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chspo: equ 4 ; dword partition offset
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chssiz: equ 8
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chstab: resw (4*chssiz)
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section .text
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; Convert between drive number and BIOS dl
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; Bidirectional mapping
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; 0 <-> 0, 1 <-> 1, 2 <-> 0x80, 3 <-> 0x81
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dnconv: mov cx, 7
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ror dx, 1
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rol dl, 1
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dncl: rcl dl, 1
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rcr dh, 1
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loop dncl
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xchg dh, dl
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ret
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fndchs: ; find CHS table entry
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; IN dl DOS drive number (A=0, B=1, ...)
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; OUT cs:si ptr to chs table entry
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push ax
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mov ax, chssiz
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mul dl
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add ax, chstab
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mov si, ax
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pop ax
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ret
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; compare sector number in disk access packet with zero
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; sets zero flag accordingly
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iszero: push ax
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mov ax, [bx]
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or ax, [bx+2]
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pop ax
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ret
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; calculate CHS from dap and CHS table entry
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; IN ds:bx disk access packet
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; cs:si chstab table entry
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; OUT cx,dh chs data for int 13h
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calchs: push ax
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; load linear sector number
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mov dx, [bx+2]
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mov ax, [bx]
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; if any word of it is non-zero, we need to do calculation
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call iszero
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jz .zero
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; ax is track number (lba / spt)
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; dx is sector (lba % spt) + 1
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div word [cs:si+chsspt]
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inc dx
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; sector number
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mov cl, dl
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; ax is cylinder (track / heads)
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; dx is head (track % heads)
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xor dx, dx
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div word [cs:si+chsnos]
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; set up cylinder and head number
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mov ch, al
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mov dh, dl
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pop ax
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ret
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; set up CHS data for int 13h for reading sector zero
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.zero: mov cx, 1 ; C=0, S=1
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mov dh, 0 ; H=0
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pop ax
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ret
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; ABSOLUTE DISK READ / DISK WRITE
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int25h: mov ah, 2
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jmp adisk
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int26h: mov ah, 3
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adisk: push bp
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push ds
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push es
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mov bp, sp
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cmp cx, 0xFFFF
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je .islrg
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; build a disk access packet on the stack
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push ds ; dword transfer buffer
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push bx
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push cx ; word number of sectors
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push cs ; dword starting sector number (CS always 0)
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push dx
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; set our freshly created dap as DS:BX
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mov bx, sp
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push ss
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pop ds
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.islrg: mov dl, al
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call fndchs
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call calchs
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mov dl, al
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mov al, [bx+4]
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mov es, [bx+8]
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mov bx, [bx+6]
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int 0x13
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jc .ret
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.ret: mov sp, bp
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pop es
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pop ds
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pop bp
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retf
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