Absolute Disk Read
;
; Synopsis: int getsec(drive,numsec,begsec,buffer)
; unsigned int drive; /* 0=A, 1=B, etc. */
; unsigned int numsec; /* Number of sectors to read */
; unsigned int begsec; /* Beginning logical sector */
; char *buffer; /* Transfer address */
;
; Function: The number of sectors specified are transferred 
; between the given drive and the transfer address. 
; LOGICAL SECTOR NUMBERS are obtained by numbering
; each sector sequentially starting from track 0, head 0,
; sector 1 (logical sector 0) and continuing along the
; same head, then to the next head until the last sector
; on the last head of the track is counted.  Thus, 
; logical sector 1 is track 0, head 0, sector 2,
; logical sector 2 is track 0, head 0, sector 3,  & so on.
;

; Returns: NULL if the operation is successful.
; otherwise, error codes are as follows:
;
; hex 80 Attachment failed to respond.
; hex 40 SEEK operation failed.
; hex 20 Controller failure.
; hex 10  Bad CRC on diskette read.
; hex 08 DMA overrun on operation.
; hex 04 Requested sector not found.
; hex 03 Write attempt on write-protected diskette.
; hex 02 Address mark not found.
; hex FF Unspecified (error other than those above).
;
code segment byte public ;segment registers remain intact
assume cs:code ;all other registers will be destroyed
public getsec

getsec: push bp ;save old frame pointer
mov bp,sp ;get new frame pointer
mov ax,4[bp] ;put drive number into AL
xor ah,ah
mov cx,6[bp] ;number of sectors to fetch
mov dx,8[bp] ;logical record number of 1st sector
mov     bx,10[bp] ;pointer to transfer address
int 25h ;BIOS call
jc error ;error has occurred if carry flag = 1
mov al,00H ;NULL to indicate sucessful completion
jmp done
error: cmp al,00H ;detect unspecified error code 00H
jne done ;...change to 0FFh if found to 
mov al,0FFH ;...differentiate it from success code
done: xor ah,ah ;return AL only
popf ;remove flags int 0x25 left on stack
pop bp ;restore original frame pointer
ret ;all done

code ends
end

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