After a reset the mouse pointer is hidden: the driver tracks the mouse, but draws nothing. INT 33h AX=0001h asks it to draw. Internally the driver keeps a counter, −1 after a reset; show adds 1 and never goes past 0, hide (INT 33h AX=0002h) subtracts 1, and the pointer is on screen exactly when the counter is 0.

Once shown, the pointer is the driver's business: it saves what is under it, draws it, and restores the screen as the mouse moves, from inside its interrupt handler. In text mode it inverts the colours of one character cell; in graphics modes it draws an arrow from a mask.

Driver version 7.02 and later can report the counter (INT 33h AX=002Ah, AL = counter as a signed byte), which makes the rule visible. The example prints it after each step:

prints: FF 00 00 FF (the pointer flashes on the DOS screen for a moment)
XOR     AX, AX        ; reset: the pointer starts hidden
INT     33h
CALL    COUNTER1      ; FF: -1
MOV     AX, 0001h     ; show: -1 -> 0, the pointer appears
INT     33h
CALL    COUNTER1      ; 00
MOV     AX, 0001h     ; show again: the counter stays at 0
INT     33h
CALL    COUNTER1      ; 00
MOV     AX, 0002h     ; hide: back to -1
INT     33h
CALL    COUNTER1      ; FF
 
COUNTER1 PROC   NEAR  ; print the cursor counter: 00 shown, FF, FE... hidden
MOV     AX, 002Ah     ; mouse (driver 7.02+): AL = the counter
INT     33h
CALL    PHEX2
MOV     DL, " "
MOV     AH, 02h
INT     21h
RET
COUNTER1 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

Pitfalls

Because the counter stops at 0, an extra show is harmless, but an extra hide is not: one too many and the next show leaves the pointer invisible. Show the pointer after setting the video mode and resetting the driver, not before, since a mode change hides it. While the pointer is visible, anything a program writes to the screen may land under it and be overwritten when the driver restores the background; see INT 33h AX=0002h for the hide/draw/show pattern.