diff options
| -rw-r--r-- | explosion.s | 131 |
1 files changed, 98 insertions, 33 deletions
diff --git a/explosion.s b/explosion.s index dbc000d..678e2c0 100644 --- a/explosion.s +++ b/explosion.s @@ -2,69 +2,134 @@ ; flash the part of the screen where the lorchas are displayed. ; previously this flashed the whole screen, but it gave me a -; headache. +; headache. plus, the apple version's "tv static" effect only appears +; on the lorcha area of the screen, so this is closer to the original +; than a fullscreen flash would be. + +; we're not using a display list interrupt here. basically each flashed +; frame is: +; - sync CPU to ANTIC using VSYNC and WSYNC. +; - change HW color registers for text and background. +; - wait until enough scanlines have been drawn. +; - change the colors back to normal. + +; pseudo-C for explosion(): +; for(Yreg = flashes; Yreg > 0; --Yreg) { +; for(framecounter = jiffies; framecounter > 0; --framecounter) { +; // this loop body takes 1 jiffy to execute +; while(VCOUNT < startvcount) +; ; +; set_flashed_colors(); +; while(VCOUNT < endvcount) +; ; +; set_normal_colors(); +; } +; jsleep(jiffies); // unflashed display for equal amount of time +; } .export _explosion .include "atari.inc" .importzp tmp1, tmp2 .import _jsleep -counter = tmp1 -color2save = tmp2 +; how many times the screen will appear to flash. +flashes = 3 + +; timing, in jiffies. each flash shows this many 'flashed' frames, +; followed by the same number of normal frames. the whole explosion takes +; ((jiffies * flashes * 2) / 60) seconds, or 1 second for jiffies = 10 +; on NTSC (for PAL, it's 1.2 seconds). +jiffies = 10 + +; VCOUNT value where we will start the flash effect. Remember, VCOUNT is +; (scanlines / 2), and a GR.0 text line is 8 scanlines, or 4 VCOUNTs. +; There are 3 'blank for 8 lines' instructions at the top of the GR.0 +; display list, so the first visible GR.0 line starts at (4+0)*4. +; The value below is the start of the GR.0 line before the top row +; of lorchas. +startvcount = (4+8)*4 + +; VCOUNT value where we will stop the flash effect and restore normal +; playfield colors. Value below is 2 GR0 lines after the bottom row +; of lorchas. +endvcount = (4+23)*4 + +; use zero page rather than X register for frame counter, since +; _jsleep will trash the X register. +framecounter = tmp1 + +; bottom 4 bits of COLOR1. Will be ORed with the top 4 bits (the hue) +; of COLOR2, to get the 'flashed' text background color. The flashed +; text color is always 0 (black, or actually darkest luma of whatever +; hue the text BG color is). +textluma = tmp2 ; extern void explosion(void); _explosion: ; { - ldy #3 ; loop counter, counts 3 2 1 + ldy #flashes ; outerloop counter - lda COLOR1 - and #$0f - sta tmp2 + lda COLOR1 ; can't hardcode this, since it can be changed + and #$0f ; at the title screen. + sta textluma ; { -@loop: - lda #$0a - sta counter +; each time thru outerloop, show 'jiffies' frames of flash and another +; 'jiffies' frames of normal display (so the flashing toggles on and off +; 'flashes' times). +@outerloop: + lda #jiffies + sta framecounter ; inner loop, counts 10 to 1 +@frameloop: ; { -@wait4scanline: +@wait4startscanline: ; { - lda VCOUNT - cmp #(4+8)*4 - bne @wait4scanline + lda VCOUNT ; delay until start of flashing area + cmp #startvcount + bne @wait4startscanline ; } - sta WSYNC + ; set_flashed_colors(); + sta WSYNC ; finish current scanline (avoid tearing) -; { -@scanlines: - lda COLOR2 - and #$f0 - ora tmp2 - sta WSYNC - sta COLPF2 - lda #0 - sta COLPF1 + ; we're in the horizontal blank now. De Re Atari, Chapter 5, says we get + ; "from 17 to 26 cycles". We're not using P/M and there's no LMS here, + ; but 1 or 2 cycles may be stolen by memory refresh. The "sta COLPF1" + ; only has to finish before any non-space characters are displayed on + ; the scanline, which would give us a couple extra cycles if needed, + ; since there's a border of spaces around the lorchas. + + lda COLOR2 ; +4 = 4 + and #$f0 ; +2 = 6 + ora textluma ; +3 = 9 + sta COLPF2 ; +4 = 13 + lda #0 ; +2 = 15 + sta COLPF1 ; +4 = 18 +; { +@wait4endscanline: ; delay until end of flashing area lda VCOUNT - cmp #(4+23)*4 - bne @scanlines ; stop changing color 2 lines after the bottom row of lorchas + cmp #endvcount + bne @wait4endscanline ; } - lda COLOR2 ; put colors back like they were for the rest of the frame + ; set_normal_colors(); + sta WSYNC + lda COLOR2 ; put colors back like they were for the rest of the frame. sta COLPF2 lda COLOR1 sta COLPF1 - dec counter - bne @wait4scanline + dec framecounter + bne @frameloop ; inner loop done when framecounter == 0 ; } - ldx #0 ;\ - lda #$0a ; | jsleep(10); - jsr _jsleep ;/ + ldx #0 ;\ + lda #jiffies ; | jsleep(jiffies); + jsr _jsleep ;/ dey - bne @loop ; we're done if Y==0 + bne @outerloop ; we're done if Y == 0 ; } rts |