INT 33h

INT 33h is not part of the BIOS or of DOS. It belongs to the mouse driver, a program loaded at boot (MOUSE.COM, or MOUSE.SYS in CONFIG.SYS) that talks to the mouse hardware on a serial port or the PS/2 port, keeps track of where the pointer is, draws it on the screen and answers questions through this vector. Microsoft defined the interface in 1983 and every later driver copied it. DOSBox-X has a driver built in, active from the start.

Unlike most services the function number is the whole of AX, not AH: MOV AX, 0003h, not MOV AH, 03h. Results come back in AX, BX, CX and DX.

Is there a driver?

Calling INT 33h when nothing is installed is a gamble. On DOS 2 the vector may be 0000:0000 and the call jumps into the interrupt table; later DOS versions point it at an IRET, so the call returns with AX unchanged. A careful program reads the vector first with INT 21h AH=35h, checks that it is not zero and does not point at an IRET (CFh), then calls INT 33h AX=0000h and checks for AX = FFFFh.

MOV     AX, 3533h               ; DOS: get the INT 33h vector
INT     21h                     ; ES:BX = the handler
MOV     AX, ES
OR      AX, BX                  ; 0000:0000: nothing installed
JZ      NONE1
CMP     BYTE PTR ES:[BX], 0CFh  ; only an IRET there?
JE      NONE1
XOR     AX, AX                  ; mouse: reset and detect
INT     33h                     ; AX = FFFFh if a mouse is there, BX = buttons
CMP     AX, 0FFFFh
JNE     NONE1
MOV     DX, OFFSET FOUND1
MOV     AH, 09h                 ; DOS: print the $-string at DS:DX
INT     21h
MOV     AX, BX
CALL    PDEC                    ; the button count
MOV     DX, OFFSET BUTTONS1
MOV     AH, 09h                 ; DOS: print the $-string at DS:DX
INT     21h
JMP     DONE1
NONE1:
MOV     DX, OFFSET NOMOUSE1
MOV     AH, 09h                 ; DOS: print the $-string at DS:DX
INT     21h
DONE1:
 
FOUND1  DB      "mouse driver found, $"
BUTTONS1 DB     " buttons", 13, 10, "$"
NOMOUSE1 DB     "no mouse driver", 13, 10, "$"
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

Coordinates

The driver reports the position in pixels of a virtual screen, whatever the video mode. In graphics modes that is the real resolution (640×480 in mode 12h; in mode 13h the driver still counts 0..639 across, so divide x by 2). In text mode the virtual screen is 640×200 for 80×25: every character cell is 8 × 8, so column = x / 8 and row = y / 8, and the driver only ever reports multiples of 8. The pointer in text mode is a cell drawn with inverted colours, not an arrow.

Mickeys

The mouse itself sends movement counts, mickeys, about 1/200 inch each. The driver turns them into pixels with a ratio, by default 8 mickeys per 8 pixels across and 16 per 8 pixels down, changeable with AX=000Fh, and applies acceleration when the mouse moves fast. AX=000Bh returns the raw mickeys since the last call, which is what a game uses for mouse-look, unaffected by the screen edges.

The cursor counter

Visibility is a counter: 0 means shown, anything below hidden. Reset sets it to −1 (hidden), INT 33h AX=0001h adds 1 but never goes above 0, INT 33h AX=0002h subtracts 1. So nested hides need as many shows, which lets a drawing routine hide and show without knowing whether its caller had the pointer visible.

In DOSBox-X

The built-in driver supports the Microsoft functions up to version 7 or so, including AX=002Ah (read the cursor counter). It integrates with the host pointer, so in a window the DOS pointer follows yours. The configuration setting int33=false turns the driver off, for programs that bring their own; then the vector check above reports no mouse.