2600 101: Kernal Clink

OK! We made it through our first program. Our second is going to be darn similar. Baby steps and all that.

The last program was a line, because that was about the easiest thing I could think to make, since we didn't have to do anything on every scanline. (Heck, we didn't even really have to do everything for every frame, except for the HMOVE.) So now we're going to make something just slightly more complex, a moving dot.

Why is a dot more complex than a line? Because we have to figure out when to turn the dot on, and when to turn it off. The code that you do during the scanlines (as opposed to during the Overscan or Vertical Blank) is called the "kernal" code, and it has to be tight, because you have very little time to do program logic. Sometimes, you might need 2 scanlines to do all your thinking, thus making a "two line kernal". (Though in that case, you have to be careful...since a lot of the kernal is setting up the graphical "things", if you change those things while the line is being drawn, weird things might result. Also wonderful things, if you're very careful with the timing... more careful than I've learned to be.)

Anyay, lets dive right into the code. I'll try to focus on commenting mostly on the new stuff, which I'll put in red for your reading convenience. (This means you should copy and paste from the web browser directly, not using "View Source"...and of course you still need the vcs.h file.)

Actually, we'll use one other file: "macro.h" is a file full of convenient "macros", or little bitty programs DASM runs as it puts your program together. It's a lot like vcs.h. The current standard can be downloaded from DASM's official homepage, in the "Atari 2600 Support Files" Zip file. Here's a local copy of macro.h. 
; a moving dot by Kirk Israel

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



;we start by setting up two "variables"
;this means we tell DASM that when we say
;variablename, we mean this specific memory
;location (we have $80 to $FF to play with)

;we'll use this one to store the vertical position
YPosFromBot = $80;
;more on the use of this variable below
VisibleMissileLine = $81;

Start

;generic start up stuff from macro.h...
	CLEAN_START



	lda #$00
	sta COLUBK
	lda #66		;Lets go for purpley!
	sta COLUP0

	lda #80
	sta YPosFromBot	;set Initial Y Position

	;NUSIZ0 sets the size and duplication
	;of the sprite and missiles --see the Stella
	;guide for details
	lda #$20
	sta NUSIZ0 ;Quad Width for now


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

	;#% is a way of indicating a binary actual number
	;(just like #$ starts a hex number and # a decimal number)

	lda #%00010000  ;put value of 1 in the left nibble (slow move right)
	sta HMM0	;set the move for missile 0

WaitForVblankEnd
	lda INTIM
	bne WaitForVblankEnd
	ldy #191
	sta WSYNC
	sta VBLANK

	sta WSYNC
	sta HMOVE

;main scanline loop...
ScanLoop
	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

; there are more efficient ways of doing this


; we see if this is the line (line # stored in Y) is the
; one that we start the missile on
CheckActivateMissile
	cpy YPosFromBot		;compare Y to the YPosFromBot...
	bne SkipActivateMissile ;if not equal, skip this...
	lda #8			;otherwise say that this should go
	sta VisibleMissileLine	;on for 8 lines
SkipActivateMissile

;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	;load the value of what missile line we're showing
	beq FinishMissile	;if zero we aren't showing, skip it
IsMissileOn
	lda #2			;otherwise
	sta ENAM0		;showit
	dec VisibleMissileLine 	;and decrement the missile line thing
FinishMissile

	dey		;decrement scanline counter
	bne ScanLoop	;lather rinse repeat


;overscan same as last time
	lda #2
	sta WSYNC
	sta VBLANK
	ldx #30
OverScanWait
	sta WSYNC
	dex
	bne OverScanWait
	jmp  MainLoop

	org $FFFC
	.word Start
	.word Start
Next: The Joy of Sticks
Introduction - The Development Environment - Into The Breach - My First Program -
Kernal Clink - The Joy of Sticks - Happy Face - PlayerBufferStuffer