INT 33h AX=0003h: read position and buttons — 8086 assembly guide

Return where the pointer is (CX = x, DX = y, in pixels; in text mode a cell is 8 × 8) and which buttons are held down now (BX).

INT 33h AX=0003h is the polling interface to the mouse: it returns the pointer's position and the buttons held down at this moment. It does not wait and does not consume anything, so a program calls it once per frame and acts on the answer.

Register	Returns
CX	x, 0..639 for most modes (0..638 in steps of 2 in mode 13h)
DX	y, 0..199 in 80×25 text and the 200-line modes, 0..479 in mode 12h
BX bit 0	left button down
BX bit 1	right button down
BX bit 2	middle button down (three-button mice)

In text mode divide both coordinates by 8 to get the column and row of the character under the pointer; the driver keeps them at multiples of 8 there anyway. The example places the pointer on column 10, row 5, reads it back once and converts:

prints: 80 40 0 -> 10,5 (x, y, buttons, then column and row; move the mouse over the DOS screen while it runs and you may see other numbers)

XOR     AX, AX        ; reset the driver
INT     33h
MOV     AX, 0004h     ; put the pointer on column 10, row 5
MOV     CX, 10*8      ; x = column x 8
MOV     DX, 5*8       ; y = row x 8
INT     33h
MOV     AX, 0003h     ; read position and buttons, once
INT     33h           ; CX = x, DX = y, BX = buttons
MOV     SI, CX        ; keep them: printing uses DX
MOV     DI, DX
MOV     AX, SI
CALL    PDEC          ; x: 80
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, DI
CALL    PDEC          ; y: 40
MOV     DL, " "
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, BX
AND     AX, 0007h     ; the three button bits
CALL    PDEC          ; 0: nothing pressed
MOV     DX, OFFSET ARROW1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
MOV     CL, 3
MOV     AX, SI
SHR     AX, CL        ; column = x / 8
CALL    PDEC          ; 10
MOV     DL, ","
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AX, DI
SHR     AX, CL        ; row = y / 8
CALL    PDEC          ; 5
 
ARROW1  DB      " -> $"
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

Clicks

BX is the state now, not a history. A click shorter than the time between two polls is missed entirely, and a button held down is seen as "pressed" on every poll. To react once per click, remember the previous BX and act when a bit goes from 0 to 1. To catch clicks that happen between polls, use AX=0005h (presses since the last call, with the position of the last one) or an event handler, INT 33h AX=000Ch.

MOV     AX, 0003h
INT     33h
MOV     AX, BX
XOR     AX, OLDB1  ; bits that changed since the last poll
AND     AX, BX     ; ...and are down now: fresh presses
MOV     OLDB1, BX
TEST    AL, 1      ; the left button went down
JNZ     CLICK1

Upper bits of BX are not always zero: drivers with wheel support (CuteMouse, DOSBox-X) put the wheel movement in BH. Mask with AND BX, 7 before comparing.