INT 16h AH=00h

INT 16h AH=00h takes the oldest key out of the BIOS keyboard buffer and returns it: AL = the ASCII character, AH = the scan code of the key. If the buffer is empty it waits, with interrupts enabled, until the keyboard handler puts something there. There is no timeout and no echo, and Ctrl+C is just character 03h: nothing aborts the program.

Because both bytes come back, one call tells apart keys that DOS would need two calls for. A key with a character has AL ≠ 0; a key without one (function keys, arrows, Alt+letter) has AL = 0 and is identified by AH alone. The example stores "A" and F1 in the buffer, then reads them back:

MOV     AH, 05h       ; put "A" in the buffer: scan 1Eh, ASCII 41h
MOV     CX, 1E41h
INT     16h
MOV     AH, 05h       ; and F1: scan 3Bh, no ASCII
MOV     CX, 3B00h
INT     16h
MOV     AH, 00h       ; BIOS: wait for a key (one is there)
INT     16h           ; AH = scan code, AL = ASCII
CALL    PHEX4         ; 1E41
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AH, 00h
INT     16h           ; F1: AL = 0 marks a key without a character
CALL    PHEX4         ; 3B00
 
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

Telling keys apart

The usual dispatch tests AL first. Comparing AH instead makes the program independent of Shift and Caps Lock, which is what games want for movement keys; comparing AL is what a text editor wants for letters.

MOV     AH, 00h
INT     16h
TEST    AL, AL    ; a character?
JZ      SPECIAL1  ; no: look at the scan code
CMP     AL, 1Bh   ; Esc
JE      QUIT1
; ... AL is a character ...
SPECIAL1:
CMP     AH, 48h   ; Up
JE      UP1
CMP     AH, 3Bh   ; F1
JE      HELP1

Pitfalls

The call blocks. A program that must keep running (a game loop, an animation) checks first with INT 16h AH=01h and only reads when a key is there. On a 101-key keyboard the grey arrows, F11 and F12 are filtered for compatibility: a grey arrow comes back as the keypad one (48E0h becomes 4800h) and F11/F12 are silently dropped while the call keeps waiting. Use INT 16h AH=10h to see them. DOS functions such as INT 21h AH=08h read the same buffer, but return an extended key in two steps: first 0, then the scan code on the next call.

In DOSBox-X the wait does not spin the emulated CPU in a tight loop; the BIOS idles until a key arrives, so a program sitting in AH=00h costs little host CPU.