Hi again!
This time im stretchin my newbie brain muscles creating (or ate least
trying to)
a perfect blank template. Trying to be really careful in every detail, as im
plannng to use this template for all demos, tutorial and exercices from now on
(or at least until the source gets so big i need to break it in modules)
So, after digging hints here and there (like the variables section, 100%
extracted from
Andres tutorial), I present you with my 1st attempt of the perfectly clean,
standard,
modern template for simple 2600 programs. 73 bytes that do absolutely
nothing :) Please feel absolutely free to make any corrections,
suggestions, recomendations, whatever, about style, conventions,
optimizations, anything you think is
worth pointing out.
And I already some questions (the main reason for posting a file that
doesnt work :)
- Altought things look nice, this code generates 260 scanlines. Why are
there missing 2? Replacing the timers for scanline counting loops, i
figured there is one scanline missing in vblank section and the other in
overscan. The very places im using timers instead of a counter. What am i
doing wrong?
- Is there any way to accuratly find out how many time cycles are left in
each section? (marked with XXXX till i find out). Every attempt i made
using cycle-controled loops gave different or weird results. For example...
after finding the maxmum number of cycles for 262 scanlines, fine-tuned by
SLEEP n, if i change it to SLEEP n+1, scanlines go up to 280!! Wow, 18
scanlines for a single cycle!
Im completely clueless... and i swear i tried hard before bugging you...
Cya,
Rodrigo ; 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 = Freeware that should remain Free"
processor 6502 ; Atari 2600 CPU
include "vcs.h" ; TIA 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
sta VSYNC ; Starts VSYNC
sta WSYNC ; End of VSYNC Line 1
sta WSYNC ; End of VSYNC Line 2
sta WSYNC ; End of VSYNC Line 3
lda #%00000000 ; Get ready to turn off VSYNC
sta VSYNC ; Turn off VSYNC
lda #44 ;
sta TIM64T ; Start the timer for VBlank
;----------------------------------------------------------------------
; 37 scanlines of vertical blank... have fun with XXXX cycles
; End of fun
;----------------------------------------------------------------------
WaitEndOfVBlank lda INTIM ; check timer...
bne WaitEndOfVBlank ;
lda #%00000000 ; Get ready to turn off VBLANK
sta WSYNC
;----------------------------------------------------------------------
; Do 192 scanlines
; 1st line has 5 cycles less then the others ,
; so blew it off if kernel is too tight
sta VBLANK ; Turns VBLANK off
ldx #192 ; scanline counter
;dex ; uncomment here and below if needed
;sta WSYNC ; (prematuraly ends 1st scanline, but
; guarantees that all the other lines
; have exactly the same cycle counting)
Kernel
;--Go kernel, go!
;----------------
sta WSYNC
dex ;
bne Kernel ;
;----------------------------------------------------------------------
; 30 scanlines of overscan...
lda #%01000010 ; Get ready for Blanking
sta VBLANK ; End of screen - enter blanking
lda #36 ;
sta TIM64T ; Start Timer for Overscan
;----------------------------------------------------------------------
; Have more fun with XXXX cycles left for overscan
; Sorry pal, time's up - get ready for next frame
;----------------------------------------------------------------------
WaitEndOfOverscan lda INTIM ; check timer...
bne WaitEndOfOverscan
sta WSYNC
jmp StartOfFrame
;///////////////////////////////////////////////////////////////////////////////
ECHO [*-$F000+6]d,"out of 4096 ROM bytes used",[$FFF9-*]d,"bytes left"
ORG $FFFA
InterruptVectors
.word Reset ; NMI
.word Reset ; RESET
.word Reset ; IRQ
END
; EOF
