INT 33h AX=0000h: reset the driver and detect a mouse — 8086 assembly guide

Reset the mouse driver to its defaults and find out whether a mouse is there: AX = FFFFh if so, BX = the number of buttons.

INT 33h AX=0000h is the first mouse call a program makes. It answers whether a mouse is installed (AX = FFFFh yes, AX = 0000h no) and how many buttons it has (BX), and it puts the driver into a known state:

After a reset	Value
pointer	hidden (counter −1), in the middle of the screen
range	the whole virtual screen of the current video mode
event handler	removed (mask 0)
mickey ratio	8 per 8 pixels across, 16 per 8 down
button counts	zero
pointer shape	the default arrow or text block

prints: FFFF 0003 320 96 (in text mode the middle of 640×200 is 320,100; the driver reports whole cells, so y shows as 96)

MOV     AX, 0000h     ; mouse: reset and detect
INT     33h           ; AX = FFFFh: yes, BX = buttons
CALL    PHEX4         ; FFFF
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, BX
CALL    PHEX4         ; 0003 here
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, 0003h     ; where did the reset put the pointer?
INT     33h           ; CX = x, DX = y
MOV     BX, DX        ; keep y: printing uses DL
MOV     AX, CX
CALL    PDEC          ; 320: half of 640
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, BX
CALL    PDEC          ; 96: 100 rounded down to a text row
 
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
PDEC    PROC    NEAR  ; print AX in decimal; keeps every register
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BX, 10
XOR     CX, CX        ; digits on the stack
DECD1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
TEST    AX, AX
JNZ     DECD1
DECD2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    DECD2
POP     DX
POP     CX
POP     BX
POP     AX
RET
PDEC    ENDP

The button count

BX = 2 is a two-button mouse, 3 a three-button one (Mouse Systems, Logitech), and some drivers return FFFFh for two buttons. DOSBox-X reports 3. Do not rely on the middle button existing just because BX = 3.

Pitfalls

The reset can be slow on real hardware, up to a second or two with a serial mouse, because the driver resets the mouse too. INT 33h AX=0021h is the software-only reset (the same defaults, no hardware) and returns AX = FFFFh if a driver is there; use it when a program resets often. Call the reset after setting the video mode: the default range follows the mode current at the time. And check the vector before the first call (see INT 33h), because with no driver at all this call may go nowhere good.