After correcting both code and comment bugs (thanks Andrew and all who
answered my questions), here goes the final version of my template.
I believe its a good framework for beginners like me... its heavily
commented, indented, and every detail was carefully thought to be as easy
to learn as possible. For example I used binary notation when writing to
addresses like VBLANK, to make clear that in theory im not writing 2, but
rather setting D1 bit.
I also precisely calculated (and tested) the # of cycles left for sections
like vblank and overscan. I know this may sound useless (pretty much like
the whole template), but when there is no need add any unpredicted "sta
WSYNC" it gave me a warm felling of a robust, steady beggining...
So, I thought it would be worth sharing... and, as always, any comments,
suggestions, compliments, flames, advices, etc would be greatly welcome
Cya,
Rodrigo
; Framework for simple Atari demos - Full version (heavily commented)
;
; Copyleft 2003 By Rodrigo E.S. Silva - atari@xxxxxxxxxxxxx
; Under GPL - GNU Public Licence - http://www.fsf.org/copyleft/gpl.html
; You may copy, distribuite, use, edit, sell, whatever, as long as such freedom
; is propagated in evey version and my credit for original work is kept
; "GPL = Free software that should remain Free"
processor 6502 ; Atari 2600 CPU
include "vcs.h" ; Stella (TIA & PIA & RIOT) Definitions
include "macro.h" ; Useful Functions
;///////////////////////////////////////////////////////////////////////////////
SEG.U Variables
ORG $80
;/////////////////// Global Variables
GlobalDummy ds 0
;------------------------------------------------------------------------------
; OVERLAYS!
; These variables are overlays, and should be managed with care
; That is, variables are ALREADY DEFINED, and we're reusing RAM for other
; purposes
; EACH OF THESE ARE VARIABLES (TEMPORARY) USED BY ONE ROUTINE (AND IT'S
; SUBROUTINES) THAT IS, LOCAL VARIABLES. USE 'EM FREELY, THEY COST NOTHING
; TOTAL SPACE USED BY ANY OVERLAY GROUP SHOULD BE <= SIZE OF 'Overlay'
;------------------------------------------------------------------------------
Overlay ds 0 ; overlay (share) variables (make sure this is as big
; as the biggest overlay subsection)
;/////////////////// 1st set of Overlay Variables - repeat block as necessary
ORG Overlay
ov1_Dummy ds 0
ECHO [*-$80]d,"out of 128 RAM bytes used",[$100-*]d,"bytes left"
;///////////////////////////////////////////////////////////////////////////////
SEG
ORG $F000
Reset
CLEAN_START ; Clear RAM and all TIA registers
; Now A = X = Y = 0
;----------------------------------------------------------------------
; Once-only initialization... take your time
; End of once-only initialization
;----------------------------------------------------------------------
StartOfFrame lda #%00000010 ; Get ready to turn VSYNC on
sta VSYNC ; Starts VSYNC
sta WSYNC ; End of VSYNC Line 1
sta WSYNC ; End of VSYNC Line 2
lda #%00000000 ; Get ready to turn VSYNC off
sta WSYNC ; End of VSYNC Line 3
sta VSYNC ; Turn off VSYNC
lda #43 ; Get timer ready:43*64/76=36.2 scanlin
sta TIM64T ; Start the timer for VBlank
;----------------------------------------------------------------------
; 37 scanlines of vertical blank... have fun with 2792 cycles
; (37 scanlines * 76 cycles - 20 cycles already
; used from one sta WSYNC to another)
; End of fun
;----------------------------------------------------------------------
WaitEndOfVBlank lda INTIM ; check timer...
bpl WaitEndOfVBlank ;
lda #%00000000 ; Get ready to turn off VBLANK
sta WSYNC ;
;----------------------------------------------------------------------
; Do 192 scanlines
sta VBLANK ; Turns VBLANK off
ldx #192 ; scanline counter
Kernel
;--Go kernel, go! 68 cycles max
;----------------
sta WSYNC ;
dex ;
bne Kernel ;
;----------------------------------------------------------------------
; 30 scanlines of overscan...
lda #%01000010 ; Get ready for Blanking
sta VBLANK ; End of screen - enter blanking
lda #35 ; 35*64/76=29.4 scanlines
sta TIM64T ; Start Timer for Overscan
;----------------------------------------------------------------------
; Have more fun with 2256 cycles left for overscan
; Sorry pal, time's up - get ready for next frame
;----------------------------------------------------------------------
WaitEndOfOverscan lda INTIM ; check timer...
bpl WaitEndOfOverscan ;
sta WSYNC ;
jmp StartOfFrame ;
;///////////////////////////////////////////////////////////////////////////////
ECHO [*-$F000+6]d,"out of 4096 ROM bytes used",[$FFFA-*]d,"bytes left"
ORG $FFFA
InterruptVectors
.word Reset ; NMI
.word Reset ; RESET
.word Reset ; IRQ
END
;EOF; Framework for simple Atari demos - Short version (less comments, no overlay)
;
; Copyleft 2003 By Rodrigo E.S. Silva - atari@xxxxxxxxxxxxx
; Under GPL - GNU Public Licence - http://www.fsf.org/copyleft/gpl.html
; You may copy, distribuite, use, edit, sell, whatever, as long as such freedom
; is propagated in evey version and my credit for original work is kept
; "GPL = Free software that should remain Free"
processor 6502 ; Atari 2600 CPU
include "vcs.h" ; Stella (TIA & PIA & RIOT) Definitions
include "macro.h" ; Useful Functions
;///////////////////////////////////////////////////////////////////////////////
SEG.U Variables
ORG $80
GlobalDummy ds 0
ECHO [*-$80]d,"out of 128 RAM bytes used",[$100-*]d,"bytes left"
;///////////////////////////////////////////////////////////////////////////////
SEG
ORG $F000
Reset
CLEAN_START
;----------------------------------------------------------------------
; Once-only initialization
;----------------------------------------------------------------------
StartOfFrame lda #%00000010
sta VSYNC
sta WSYNC
sta WSYNC
lda #%00000000
sta WSYNC
sta VSYNC
lda #43
sta TIM64T
;----------------------------------------------------------------------
; Vertical blank - 2792 cycles avaliable
;----------------------------------------------------------------------
WaitEndOfVBlank lda INTIM
bpl WaitEndOfVBlank
lda #%00000000
sta WSYNC
;----------------------------------------------------------------------
; Do 192 scanlines
sta VBLANK
ldx #192
Kernel
;--68 cycles max
;----------------
sta WSYNC
dex
bne Kernel
;----------------------------------------------------------------------
; 30 scanlines of overscan
lda #%01000010
sta VBLANK
lda #35
sta TIM64T
;----------------------------------------------------------------------
; Overscan - 2256 cycles avaliable
; End of Overscan
;----------------------------------------------------------------------
WaitEndOfOverscan lda INTIM
bpl WaitEndOfOverscan
sta WSYNC
jmp StartOfFrame
;///////////////////////////////////////////////////////////////////////////////
ECHO [*-$F000+6]d,"out of 4096 ROM bytes used",[$FFFA-*]d,"bytes left"
ORG $FFFA
InterruptVectors
.word Reset ; NMI
.word Reset ; RESET
.word Reset ; IRQ
END
;EOF
