Got a chance to spend most of today sitting down and hacking out some
code.. I started from Nick's "How to Draw a Playfield", and modified some
stuff. The scrolling is the most difficult part to understand about
Nick's code (there's a lot of math in there to process the scrolling,
that you wouldn't need in a real game), and the reversed storage of the
playfield patterns is also confusing, so here's a simplified version for
anyone who had trouble with the first one.
This works on PC Atari for me.. I don't have a Supercharger yet (ordered
one, though) so I'd appreciate it if someone with a 'charger could test
it for me. thanks!
BEGIN
;How to Draw a Playfield II, by Erik Mooney (emooney@xxxxxxxxxxxxxxxx)
;Heavily indebted to Nick S. Bensema's "How to Draw a Playfield"
(thanks!)
;I removed most of Nick's comments.. refer to the original for more
;complete documentation.
;
;This program draws a bunch of colorful vertical stripes on the screen
;using the playfield. It also does some minimal joystick reading,
;to demonstrate how to do that.
;Use DASM to assemble.
;
; A diagram of the entire frame output by the TIA:
; ----------------------------------------------------------
; | VSYNC period (3 scanlines, 3*76 = 228 machine cycles | 3 ^
; | Set VSYNC=1 at beginning, VSYNC=0 at end | |
; +--------------------------------------------------------+ |
; | Vertical Blank time (37 scanlines, 2812 cycles) | |
; | Set VBLANK=0 at end | 37 2
; | | 6
; | | 2
; +------------+-------------------------------------------+
; |Horizontal | | ^ s
; |Blank | Actual TV Picture | | c
; | | <----160 pixels (53 cycles)----> | | a
; |68 pixels | 192 scanlines (pixels) tall | 192 n
; |(23 cycles) | | | l
; |wide, 192 | | | i
; |scanlines | | V n
; |tall | | e
; +------------+-------------------------------------------+ s
; | Overscan (Set VBLANK=1 at the beginning) | |
; | 30 scanlines (2280 cycles) | 30 |
; | | |
; +--------------------------------------------------------+ V
;
; <---------------228 pixels, 76 machine cycles---------->
include vcs2600.h ;same as Nick's posted vcs.h except the directive
org $F000 ;"processor 6502" was moved into vcs2600.h
Score0 = $80 ;RAM locations
Score1 = $81
Joystick0 = $90
Start
SEI ; Disable interrupts, if there are any.
CLD ; Clear BCD math bit.
LDX #$FF
TXS ; Set stack to top of RAM.
LDA #0 ;Zero everything except VSYNC.
B1 STA 0,X
DEX
BNE B1
; At this point in the code we would set up things like the data
; direction registers for the joysticks and such.
JSR GameInit
MainLoop
JSR VerticalBlank ;Execute the vertical blank.
JSR CheckSwitches ;Check console switches.
JSR GameCalc ;Do calculations during Vblank
JSR DrawScreen ;Draw the screen
JSR OverScan ;Do more calculations during overscan
JMP MainLoop ;Continue forever.
VerticalBlank ;Beginning of the frame - at the end of overscan.
LDA #2 ;VBLANK was set at the beginning of overscan.
STA WSYNC
STA WSYNC
STA WSYNC
STA VSYNC ;Begin vertical sync.
STA WSYNC ; First line of VSYNC
STA WSYNC ; Second line of VSYNC.
; Atari says we have to have 37 scanlines of VBLANK time. Since
; each scanline uses 76 cycles, that makes 37*76=2888 cycles.
; We must also subtract the five cycles it will take to set the
; timer, and the three cycles it will take to STA WSYNC to the next
; line. Plus the checking loop is only accurate to six cycles, making
; a total of fourteen cycles we have to waste. 2888-14=2876.
;
; We almost always use TIM64T for this, since the math just won't
; work out with the other intervals. 2880/64=44.something. It
; doesn't matter what that something is, we have to round DOWN.
LDA #44 ;Set timer to activate during the last line of VBLANK.
STA TIM64T
;
; Now we can end the VSYNC period.
;
LDA #0
STA WSYNC ; Third line of VSYNC.
STA VSYNC ; Writing zero to VSYNC ends vertical sync period.
RTS
CheckSwitches
LDA #0 ;Clear collision latches
STA CXCLR ;In a real game, we'd probably check the collision
;registers before clearing them.
LDA SWCHA ;Read joystick 0
STA Joystick0 ;Store for later use
RTS
GameCalc
LDA #0
STA COLUBK ;Background will be black.
LDA Joystick0 ;Load the joystick switches
AND #$F0 ;Only care about top four bits, which is joystick 0
CMP #$F0 ;If top four=1111, stick centered, don't change
BEQ NoStick
LDA #$88 ;otherwise, make the background medium blue
STA COLUBK
NoStick
LDA #$55 ;Alternate pixels: 01010101 = $55
STA PF0
STA PF2 ;Store alternating bit pattern to the playfield registers
ASL ;Because PF1 displays in the opposite bit order from PF0
STA PF1 ;and PF2, we need 10101010 instead of 01010101.
LDA #1
STA CTRLPF ;Let's reflect the playfield just cause we feel like it
=)
RTS
;To refresh your memory:
; PF0 | PF1 | PF2
; 4 5 6 7|7 6 5 4 3 2 1 0|0 1 2 3 4 5 6 7
;This pattern is the left half of the screen; it's then either repeated
;or reflected depending on bit 0 of CTRLPF. For a playfield that doesn't
;repeat or reflect, you'd have to alter the playfield registers in the
;middle of each scanline.
DrawScreen
LDA INTIM
BNE DrawScreen ;Loops until the timer is done - that means we're
;somewhere in the last line of vertical blank.
STA WSYNC ;End the current scanline - the last line of VBLANK.
STA VBLANK ;End the VBLANK period. The TIA will display stuff
;starting with the next scanline. We do have 23
cycles
;of horizontal blank before it displays anything.
LDY #192 ;We're going to use Y to count scanlines.
;Everything is already set, so let's just count scanlines.
;We're at the beginning of WBLANK of the first TV line right here.
ScanLoop
STY COLUPF ;Keep changing the playfield color every line for some
;neat-looking stripes.
STA WSYNC ;Wait for end of scanline
DEY
BNE ScanLoop ;Count scanlines.
RTS
OverScan ;We've got 30 scanlines to kill.
LDX #30 ;In a real game, we'd probably be doing calculations
here,
KillLines ;possibly using the timer again to tell us when overscan
STA WSYNC ;is done instead of counting the scanlines.
DEX
BNE KillLines
RTS
GameInit ;Usually called to start a new game.. we're not using it yet.
LDA #0 ;Just example code to show what could be done here.
STA Score0
STA Score1
RTS
;Starting positions for PC
org $FFFC
.word Start
.word Start
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