INT 21h AH=07h: read a key: no echo, no Ctrl+C — 8086 assembly guide

Wait for a key and return it in AL without printing it. Ctrl+C comes back as 03h instead of stopping the program.

INT 21h AH=07h waits for a key and returns it in AL, and that is all: nothing is echoed and no key is special. Ctrl+C comes back as 03h. It is the read for programs that handle every key themselves: editors, games, menus, and anything that has hooked a vector and must not be stopped halfway.

prints: 03 00 3B

MOV     SI, OFFSET KEYS1
CALL    STUFF1                   ; Ctrl+C, then F1
MOV     AH, 07h                  ; DOS: read a key, no echo, no Ctrl+C check
INT     21h                      ; AL = 03h: Ctrl+C, an ordinary key here
CALL    HEX2
MOV     DL, " "
MOV     AH, 02h                  ; DOS: print the character in DL
INT     21h
MOV     AH, 07h
INT     21h                      ; AL = 00h: a special key, the scan code follows
CALL    HEX2
MOV     DL, " "
MOV     AH, 02h                  ; DOS: print the character in DL
INT     21h
MOV     AH, 07h
INT     21h                      ; AL = 3Bh: the scan code of F1
CALL    HEX2
 
KEYS1   DW      2E03h, 3B00h, 0  ; Ctrl+C, F1: scan code and ASCII
STUFF1  PROC    NEAR             ; "type" the keys at DS:SI: words, 0 ends the list
CLD
ST1:
LODSW                            ; AH = scan code, AL = ASCII
TEST    AX, AX
JZ      ST2
MOV     CX, AX                   ; CH = scan code, CL = ASCII
MOV     AH, 05h                  ; BIOS: store a keystroke in the keyboard buffer
INT     16h
JMP     ST1
ST2:
RET
STUFF1  ENDP
HEX2    PROC    NEAR             ; print AL as two hex digits
PUSH    AX
MOV     CL, 4
SHR     AL, CL                   ; the high digit first
CALL    NIB1
POP     AX
AND     AL, 0Fh                  ; then the low one
CALL    NIB1
RET
HEX2    ENDP
NIB1    PROC    NEAR             ; print AL (0-15) as one hex digit
ADD     AL, "0"
CMP     AL, "9"
JBE     NIB2
ADD     AL, 7                    ; 10-15 become "A"-"F"
NIB2:
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET
NIB1    ENDP

The example puts Ctrl+C (scan code 2Eh, ASCII 03h) and F1 (scan code 3Bh, ASCII 0) into the BIOS buffer through INT 16h AH=05h, then reads three bytes. Ctrl+C is just a byte. F1 takes two calls: the first returns 0, which means "a special key", the second its scan code. A key loop handles that like this:

MOV     AH, 07h  ; DOS: read a key, no echo
INT     21h
TEST    AL, AL   ; 0: a special key
JNZ     PLAIN1
MOV     AH, 07h  ; read again: the scan code
INT     21h
; AL = 48h up, 50h down, 4Bh left, 4Dh right, 3Bh F1 ...
PLAIN1:

Service	Waits for a key	Echo	Ctrl+C
`INT 21h AH=01h`	yes	yes	^C, INT 23h: normally the end of the program
`INT 21h AH=06h` (DL = FFh)	no: ZF = 1 when none is waiting	no	returned as 03h
`INT 21h AH=07h`	yes	no	returned as 03h
`INT 21h AH=08h`	yes	no	^C, INT 23h
`INT 21h AH=0Ah`	for a whole line, up to Enter	yes	^C, INT 23h

Pitfalls

A program that reads only with 07h has switched off the user's emergency exit, so it should offer one of its own, usually Esc (1Bh). The call still reads handle 0: with input redirected from a file it returns the file's bytes, and a program waiting for a particular key may wait past the end of the file. For the keyboard itself, whatever the redirection, use INT 16h AH=00h, which also returns the scan code of every key in AH in one call.