INT 16h AH=01h: check for a key without waiting — 8086 assembly guide

Ask whether a key is waiting, without waiting and without taking it: ZF = 1 means none; otherwise AX shows the key, which stays in the buffer.

INT 16h AH=01h looks at the head of the keyboard buffer and returns at once. The answer is in the zero flag: ZF = 1 means the buffer is empty, ZF = 0 means a key is there, and then AX holds it exactly as INT 16h AH=00h would return it. The key is not removed: call 01h ten times and you see the same key ten times. To consume it, follow with AH=00h, which then returns immediately.

The example empties the buffer, stores an "X", and peeks at it several times before reading it:

prints: - 2D58 2D58 -

FLUSH1:
MOV     AH, 01h       ; BIOS: is a key waiting?
INT     16h
JZ      EMPTY1        ; ZF = 1: the buffer is empty
MOV     AH, 00h       ; take the key out
INT     16h
JMP     FLUSH1
EMPTY1:
CALL    PEEK1         ; empty: prints -
MOV     AH, 05h       ; store "X": scan 2Dh, ASCII 58h
MOV     CX, 2D58h
INT     16h
CALL    PEEK1         ; 2D58
CALL    PEEK1         ; 2D58 again: 01h leaves the key where it is
MOV     AH, 00h       ; now take it out
INT     16h
CALL    PEEK1         ; empty again: -
 
PEEK1   PROC    NEAR  ; INT 16h AH=01h, then print the key or "-"
MOV     AH, 01h
INT     16h           ; ZF = 1: nothing waiting
JZ      NONE1
CALL    PHEX4         ; the key, still in the buffer
JMP     GAP1
NONE1:
MOV     DL, "-"
MOV     AH, 02h
INT     21h
GAP1:
MOV     DL, " "
MOV     AH, 02h
INT     21h
RET
PEEK1   ENDP
PHEX4   PROC    NEAR  ; print AX as four hex digits; keeps every register
PUSH    AX
MOV     AL, AH
CALL    PHEX2         ; the high byte first
POP     AX
CALL    PHEX2         ; then the low byte
RET
PHEX4   ENDP
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

The polling loop

This is how a program does its own work and still reacts to keys: check, do one step, check again. The loop below never blocks; when a key turns up it is read and handled, and the work goes on.

MAIN1:
; ... one frame of work: move, draw ...
MOV     AH, 01h  ; a key?
INT     16h
JZ      MAIN1    ; no: carry on
MOV     AH, 00h  ; yes: take it
INT     16h
CMP     AL, 1Bh  ; Esc ends the loop
JNE     MAIN1

Pitfalls

Test ZF straight after the INT; the BIOS returns it by changing the flags saved on the stack, and any CMP or arithmetic in between destroys the answer. The value in AX when ZF = 1 is undefined: some BIOSes leave AH = 01h, some zero it. Like 00h, this function hides the extended-keyboard keys: in DOSBox-X a waiting F11 or F12 is quietly removed from the buffer and the call reports the key after it, or none. INT 16h AH=11h is the enhanced version. The DOS equivalent is INT 21h AH=0Bh, which also checks for Ctrl+C.