2600 101: The Joy of Sticks

So, moving forward...up until now, we've been making non-interactive demos. A well-coded, graphically impressive demo can be a joy to behold, but our demos haven't been well-coded or graphically impressive. Interaction, obviously, is the hallmark of programming the atari. The most common form of input is the beloved atari joystick. (The same 9-pin format was used for many other systems, such as the Commodore 64 and the Colecovision. (With some enhancements.) In fact, you can use a Sega Genesis controller in an Atari, using the center button for firing.

It turns out that the Atari's controller sockets can be used for input or output. (This might bring to mind the idea of making tons of funky little homebrew electronic interfaces controlled by your Atari. Needless is to say this page of tutorial is not going to get in that deep.) In fact, you can set pins individually for input or output.

The Atari joystick has 4 switches; pressing N,S,E, or W activates one switch, pressing diagonally activates two switches. Before trying to read the direction of the controller, you need to have set the correct I/O Port for "input". (Port A is used for the controllers, Port B for the console switches). You can do this explicitly by writing 0's to all the bits of the memory location SWACNT...or, if you do the "generic start up stuff" routing I've been using, it's one of the memory locations that will be zero'd out in that loop.

Reading the direction, then, is pretty easy. The memory location SWCHA is set as follows:
Data BitDirectionPlayer
A zero in a data bit means the joystick has been pressed that direction, and all ones means no joystick is being moved. So if (in binary), you got 11101001, that would mean Player 0 (the left player) was pressing up, and Player 1 (the right player) was pressing down and left. If you read 11001100 you would be confused...someone must be mucking with the hardware.

You may also be interested in whether the firebutton is pressed. Finding that out is easy, but with a few small potential gotchas. When the left joystick button is pressed, D7 (the leftmost bit, used for the sign of the number) of the memory location "INPT4" is set. (INPT5 for the other joystick.) The other bits of the locations are set to random values (well not really, but they probably won't be all zeros so you can't use BEQ/BNE--I was burned by that once) So typical code might look like
	BMI ButtonNotPressed

	;do something here because the button *is* pressed

The other gotcha is that the behavior of this is modified by what you set D6 of our old friend VBLANK to...the safest behavior is to make sure D6 of VBLANK is 0...this is easily done if you just set VBLANK to #2 (%#00000010) when you start the vertical blank, since after that is when you'll likely be doing the button checking. (Thanks to Eckhard Stolberg and some other Stella-ites for some advice on the buttons.)

So, for today's program, we'll move yesterday's dot around. Just for kicks, we'll change the screen color when the button is pressed. (The color value will be the vertical position of the, so keep the button held while moving around.)

Again, new stuff is in red, and you might notice the only changes take place during the vertical blank...the kernal is the same as the last lesson's.
; move a dot with the joystick by Kirk Israel

	processor 6502
	include vcs.h
	include macro.h
	org $F000

YPosFromBot = $80;
VisibleMissileLine = $81;

;generic start up stuff...

	lda #$00
	sta COLUBK	;start with black background
	lda #66
	sta COLUP0
;Setting some variables...
	lda #80
	sta YPosFromBot	;Initial Y Position

	lda #$20
	sta NUSIZ0	;Quad Width

;VSYNC time
	lda #2
	sta VSYNC
	sta WSYNC
	sta WSYNC
	sta WSYNC
	lda #43
	sta TIM64T
	lda #0
	sta VSYNC

;Main Computations; check down, up, left, right
;general idea is to do a BIT compare to see if
;a certain direction is pressed, and skip the value
;change if we're not moving that way

;Not the most efficient code, but gets the job done,
;including diagonal movement

; for up and down, we INC or DEC
; the Y Position

	lda #%00010000	;Down?
	bit SWCHA
	bne SkipMoveDown
	inc YPosFromBot

	lda #%00100000	;Up?
	bit SWCHA
	bne SkipMoveUp
	dec YPosFromBot

; for left and right, we're gonna
; set the horizontal speed, and then do
; a single HMOVE.  We'll use X to hold the
; horizontal speed, then store it in the
; appropriate register

;assum horiz speed will be zero
	ldx #0

	lda #%01000000	;Left?
	bit SWCHA
	bne SkipMoveLeft
	ldx #$10	;a 1 in the left nibble means go left

	lda #%10000000	;Right?
	bit SWCHA
	bne SkipMoveRight
	ldx #$F0	;a -1 in the left nibble means go right...
			;(in 4 bits, using "two's complement
			; notation", binary 1111 = decimal -1
			; (which we write there as hex "F" --
			; remember?))

	stx HMM0	;set the move for missile 0

; while we're at it, change the color of the background
; if the button is pressed (making sure D6 of VBLANK has
; appropriately set above) We'll set the background color
; to the vertical position, since that will be changing
; a lot but we can still control it.

	lda INPT4		;read button input
	bmi ButtonNotPressed	;skip if button not pressed
	lda YPosFromBot		;must be pressed, get YPos
	sta COLUBK		;load into bgcolor

	lda INTIM
	bne WaitForVblankEnd
	ldy #191
	sta WSYNC

	sta WSYNC
	sta HMOVE

;main scanline loop...
;(this probably ends the "new code" section of today's
; lesson...)

	sta WSYNC

; here the idea is that VisibleMissileLine
; is zero if the line isn't being drawn now,
; otherwise it's however many lines we have to go

	cpy YPosFromBot
	bne SkipActivateMissile
	lda #8
	sta VisibleMissileLine

;turn missile off then see if it's turned on
	lda #0
	sta ENAM0
;if the VisibleMissileLine is non zero,
;we're drawing it
	lda VisibleMissileLine
	beq FinishMissile
	lda #2
	sta ENAM0
	dec VisibleMissileLine

	bne ScanLoop

	lda #2
	sta WSYNC
	ldx #30
	sta WSYNC
	bne OverScanWait
	jmp  MainLoop

	org $FFFC
	.word Start
	.word Start

Next: Happy Face
Introduction - The Development Environment - Into The Breach - My First Program -
Kernal Clink - The Joy of Sticks - Happy Face - PlayerBufferStuffer