INT 16h AH=02h: read the shift flags — 8086 assembly guide

Return the shift flags byte: which of Shift, Ctrl and Alt are held down and which of Scroll, Num, Caps Lock and Insert are on.

INT 16h AH=02h returns one byte, the shift flags the keyboard handler keeps at 0040:0017h. The low four bits say which modifier keys are held down right now; the high four say which toggles are on. It is the only BIOS way to see Shift, Ctrl or Alt on their own, since pressing them puts nothing in the buffer.

Bit	Meaning when 1
0	right Shift held
1	left Shift held
2	Ctrl held (either)
3	Alt held (either)
4	Scroll Lock on
5	Num Lock on
6	Caps Lock on
7	Insert on (toggled by the Ins key)

The function only copies that byte, so the byte itself and the function always agree. The example tests the modifier bits, then flips the Insert bit in the BIOS data area by hand, reads the flags again, puts the bit back, and prints which bits changed. Insert has no LED, so the experiment is invisible to the keyboard.

prints: no Shift, Ctrl or Alt held / changed bits: 80 (two lines; the first says a key is down if you hold Shift, Ctrl or Alt while it runs)

MOV     AH, 02h                 ; BIOS: get the shift flags
INT     16h                     ; AL = the flag byte
MOV     BL, AL                  ; keep it: printing changes AL
MOV     DX, OFFSET FREE1
TEST    BL, 0Fh                 ; bits 0-3: Shift, Ctrl, Alt held?
JZ      SHOW1
MOV     DX, OFFSET HELD1
SHOW1:
MOV     AH, 09h
INT     21h
MOV     AX, 40h
MOV     ES, AX                  ; ES = the BIOS data area
XOR     BYTE PTR ES:[17h], 80h  ; flip the Insert flag by hand
MOV     AH, 02h
INT     16h                     ; AL = the flags as they are now
XOR     BYTE PTR ES:[17h], 80h  ; and flip it back
XOR     AL, BL                  ; 1 bits: what changed
MOV     BL, AL
MOV     DX, OFFSET DIFF1
MOV     AH, 09h                 ; DOS: print the $-string at DS:DX
INT     21h
MOV     AL, BL
CALL    PHEX2                   ; 80: only bit 7
 
FREE1   DB      "no Shift, Ctrl or Alt held", 13, 10, "$"
HELD1   DB      "a Shift, Ctrl or Alt key is down", 13, 10, "$"
DIFF1   DB      "changed bits: $"
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

More detail

The byte does not say which Ctrl or Alt is held. Enhanced keyboards answer that through INT 16h AH=12h: AL is the same byte, AH adds left Ctrl (bit 0), left Alt (bit 1), right Ctrl (bit 2), right Alt (bit 3) and SysRq (bit 7), taken from 0040:0018h and 0040:0096h. Remember the toggles are states, not keys: bit 6 says Caps Lock is on, not that the key is being pressed.

Pitfalls

Writing to 0040:0017h changes what the BIOS believes. Setting the Caps Lock bit really switches Caps Lock on: the next letters come out in capitals, and many BIOSes (DOSBox-X's among them) notice the change on the next timer tick and update the keyboard LEDs. Programs used that to force Num Lock on at startup. If you change the byte, change it back.