INT 13h AH=00h: reset the disk system — 8086 assembly guide

Reset the disk controller for drive DL and recalibrate it. The standard step between retries of a failed floppy read or write.

INT 13h AH=00h resets the disk controller and recalibrates the drive: the heads go back to cylinder 0 so that the controller knows where they are again. Nothing on the disk changes. After a failed read, write or seek it brings the hardware back into a known state, and the next attempt often works. For a hard disk number (80h and up) most BIOSes reset the floppy controller as well.

The classic retry loop:

MOV     SI, 3      ; three tries
TRY1:
MOV     AX, 0201h  ; read one sector
INT     13h        ; (CX, DX, ES:BX set up before)
JNC     OK1        ; success
MOV     AH, 00h    ; failed: reset the drive...
INT     13h        ; (DL still holds the drive)
DEC     SI
JNZ     TRY1       ; ...and try again
; three failures: report the error
OK1:

The example resets the first hard disk and then reads the status of the last operation with AH=01h:

prints: reset 80h: ok, last status 00

MOV     AH, 00h       ; BIOS disk: reset
MOV     DL, 80h       ; the first hard disk
INT     13h
MOV     DX, OFFSET OK1
JNC     SHOW1         ; CF = 0: done
MOV     DX, OFFSET BAD1
SHOW1:
MOV     AH, 09h
INT     21h
MOV     AH, 01h       ; BIOS disk: status of the last operation
MOV     DL, 80h
INT     13h           ; AH = status, 00h = no error
MOV     AL, AH
CALL    PHEX2
 
OK1     DB      "reset 80h: ok, last status $"
BAD1    DB      "reset 80h: failed, last status $"
PHEX2   PROC    NEAR  ; print AL as two hex digits; keeps every register
PUSH    AX
PUSH    CX
PUSH    DX
MOV     DH, AL        ; keep the byte
MOV     CL, 4
SHR     AL, CL        ; the high nibble
CALL    HEXN1
MOV     AL, DH
AND     AL, 0Fh       ; the low nibble
CALL    HEXN1
POP     DX
POP     CX
POP     AX
RET
HEXN1:
ADD     AL, "0"
CMP     AL, "9"
JBE     HEXN2
ADD     AL, 7         ; past "9": "A".."F"
HEXN2:
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET
PHEX2   ENDP

Pitfalls

Do not reset after every successful operation: a recalibration moves the heads to track 0 and costs time. On a floppy, error 06h (disk changed) is not a failure but a warning that the disk in the drive is a new one; retry, and drop anything cached from the old disk. In DOSBox-X the reset always succeeds when no disk images are attached, since there is no controller state to lose.