71 lines
1.3 KiB
NASM
71 lines
1.3 KiB
NASM
; Load a sector from drive.
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; IN DX:AX sector number (lba)
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; BX ptr to FCB
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; OUT CL Sector number
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; CH Cylinder number
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; DH Head number
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drive_chs_calc:
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push bx ; backup fcb ptr
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mov bx, [bx+fcb_spt] ; nos is in BH now
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push bx ; backup nos
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xor bh, bh ; zero out nos so spt is left
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div word bx ; ax:temp = (lba / spt)
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inc dx ; dx:sector = (lba % spt) + 1
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mov cl, dl ; sector number
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pop bx ; restore nos
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xchg bl, bh ; put nos into bl
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xor bh, bh ; zero out spt
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div word bx ; ax:cylinder = (temp / nos)
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; dx:head = (temp % nos)
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mov ch, al ; cylinder number
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mov dh, dl ; head number
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pop bx ; restore fcb ptr
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ret
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; Load a sector from drive.
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; IN DX:AX sector number
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; BX ptr to FCB
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drive_read:
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push ax
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push cx
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push dx
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push bx
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push si
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mov cx, dx
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or cx, ax
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jz .fast
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call drive_chs_calc
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mov si, 5 ; retry count
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.try:
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mov dl, [bx+fcb_drv] ; drive number
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and dl, 0x7F
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dec dl
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mov ax, 0x0201
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mov bx, diskbuf
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; try read
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int 0x13
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jnc .end
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; reset disk
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mov ah, 0x00
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int 0x13
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; loop with si
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dec si
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test si, si
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jnz .try
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stc
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.end:
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pop si
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pop bx
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pop dx
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pop cx
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pop ax
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ret
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.fast: ; fast lane for boot sector
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inc cl
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jmp .try
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