yet another explosion type

2 files changed, 140 insertions, 69 deletions

diff --git a/explosion.s b/explosion.s
index a4e461f..dbc000d 100644
--- a/explosion.s
+++ b/explosion.s
@@ -1,102 +1,70 @@
 ; explosion seen when we're hit by enemy fire.
-; this draws something that looks a bit like TV static, only
-; it's a lot more regular and less random. it somewhat approximates
-; the apple version's explosion, but not all that closely.
+
+; flash the part of the screen where the lorchas are displayed.
+; previously this flashed the whole screen, but it gave me a
+; headache.
 
  .export _explosion
  .include "atari.inc"
  .importzp tmp1, tmp2
  .import _jsleep
 
-static_loop_count = tmp1
+counter = tmp1
+color2save = tmp2
 
 ; extern void explosion(void);
-; {
 _explosion:
+; {
+ ldy #3 ; loop counter, counts 3 2 1
 
- ldy #3 ; explosion_loop counter, counts 3 2 1
-
+ lda COLOR1
+ and #$0f
+ sta tmp2
 ;   {
-@explosion_loop:
- lda #$03 ; static_loop runs 3 times
- sta static_loop_count
+@loop:
+ lda #$0a
+ sta counter
 
 ;     {
-@static_loop:
- ldx #0         ; jsleep(2)
- lda #$02
- jsr _jsleep
-
- ldx #2
-
-;         {
-;           {
-; let the top part of the screen display normally.
-; garbage begins to display on gr.0 line 8 (the one above the tops
-; of the top row of lorchas).
 @wait4scanline:
+;       {
  lda VCOUNT
  cmp #(4+8)*4
  bne @wait4scanline
-;           }
+;       }
 
-;           {
-; load a different random font address on every scanline.
-; CHBASE is the hardware address, not the OS shadow!
-; This isn't as random as I'd like, because ANTIC ignores the
-; bottom 9 bits for GR.0: fonts are 1K and start on a 1K boundary,
-; so the code below picks one of 32 possible "fonts".
-; The LSR below limits us to the first 32K of memory, because:
-; - setting the font address to $d4 - $d7 causes ANTIC to read from
-;   the CCNTL (cart control) area at $d5xx, which makes SpartaDOS X
-;   spontaneously bankswitch (crashing the Atari).
-; - addresses $e0 - $e3 would be the ROM font, which doesn't look
-;   like static. On an XL/XE, $cc - $cf are the international
-;   font too.
-; - reading from the I/O chips in the $d000 area doesn't seem like
-;   a safe thing to do (though it might be OK).
-; If the 'sta WSYNC' were removed, we'd be switching fonts a lot
-; more than once per scanline... but ANTIC only seems to pay attention
-; to CHBASE at the start of a GR.0 modeline, so it's wasted effort.
+ sta WSYNC
 
-@randfont:
+;       {
+@scanlines:
+ lda COLOR2
+ and #$f0
+ ora tmp2
  sta WSYNC
- lda RANDOM
- lsr ; avoid reading upper 32K of address space
- sta CHBASE
+ sta COLPF2
+ lda #0
+ sta COLPF1
+
  lda VCOUNT
  cmp #(4+23)*4
- bne @randfont ; stop garbage 2 lines after the bottom row of lorchas
-;           }
-
-; temporarily restore ROM font pointer, so the rest of the frame will
-; display normal spaces.
- lda #$e0
- sta CHBASE
+ bne @scanlines ; stop changing color 2 lines after the bottom row of lorchas
+;       }
 
- lda RTCLOK+2
-;           {
-; wait for start of next TV frame.
-; OS will restore hw CHBASE reg from CHBAS shadow.
-@wait4frame:
-cmp RTCLOK+2
- beq @wait4frame
-;           }
+ lda COLOR2 ; put colors back like they were for the rest of the frame
+ sta COLPF2
+ lda COLOR1
+ sta COLPF1
 
- dex
+ dec counter
  bne @wait4scanline
-;       }
-
- dec static_loop_count
- bne @static_loop
 ;     }
 
- ldx #0         ; jsleep(10)
- lda #$0a
- jsr _jsleep
+ ldx #0      ;\
+ lda #$0a    ; | jsleep(10);
+ jsr _jsleep ;/
 
  dey
- bne @explosion_loop ; we're done if Y==0
+ bne @loop ; we're done if Y==0
 ;   }
 
  rts
diff --git a/explosion.s.font b/explosion.s.font
new file mode 100644
index 0000000..a4e461f
--- /dev/null
+++ b/explosion.s.font
@@ -0,0 +1,103 @@
+; explosion seen when we're hit by enemy fire.
+; this draws something that looks a bit like TV static, only
+; it's a lot more regular and less random. it somewhat approximates
+; the apple version's explosion, but not all that closely.
+
+ .export _explosion
+ .include "atari.inc"
+ .importzp tmp1, tmp2
+ .import _jsleep
+
+static_loop_count = tmp1
+
+; extern void explosion(void);
+; {
+_explosion:
+
+ ldy #3 ; explosion_loop counter, counts 3 2 1
+
+;   {
+@explosion_loop:
+ lda #$03 ; static_loop runs 3 times
+ sta static_loop_count
+
+;     {
+@static_loop:
+ ldx #0         ; jsleep(2)
+ lda #$02
+ jsr _jsleep
+
+ ldx #2
+
+;         {
+;           {
+; let the top part of the screen display normally.
+; garbage begins to display on gr.0 line 8 (the one above the tops
+; of the top row of lorchas).
+@wait4scanline:
+ lda VCOUNT
+ cmp #(4+8)*4
+ bne @wait4scanline
+;           }
+
+;           {
+; load a different random font address on every scanline.
+; CHBASE is the hardware address, not the OS shadow!
+; This isn't as random as I'd like, because ANTIC ignores the
+; bottom 9 bits for GR.0: fonts are 1K and start on a 1K boundary,
+; so the code below picks one of 32 possible "fonts".
+; The LSR below limits us to the first 32K of memory, because:
+; - setting the font address to $d4 - $d7 causes ANTIC to read from
+;   the CCNTL (cart control) area at $d5xx, which makes SpartaDOS X
+;   spontaneously bankswitch (crashing the Atari).
+; - addresses $e0 - $e3 would be the ROM font, which doesn't look
+;   like static. On an XL/XE, $cc - $cf are the international
+;   font too.
+; - reading from the I/O chips in the $d000 area doesn't seem like
+;   a safe thing to do (though it might be OK).
+; If the 'sta WSYNC' were removed, we'd be switching fonts a lot
+; more than once per scanline... but ANTIC only seems to pay attention
+; to CHBASE at the start of a GR.0 modeline, so it's wasted effort.
+
+@randfont:
+ sta WSYNC
+ lda RANDOM
+ lsr ; avoid reading upper 32K of address space
+ sta CHBASE
+ lda VCOUNT
+ cmp #(4+23)*4
+ bne @randfont ; stop garbage 2 lines after the bottom row of lorchas
+;           }
+
+; temporarily restore ROM font pointer, so the rest of the frame will
+; display normal spaces.
+ lda #$e0
+ sta CHBASE
+
+ lda RTCLOK+2
+;           {
+; wait for start of next TV frame.
+; OS will restore hw CHBASE reg from CHBAS shadow.
+@wait4frame:
+cmp RTCLOK+2
+ beq @wait4frame
+;           }
+
+ dex
+ bne @wait4scanline
+;       }
+
+ dec static_loop_count
+ bne @static_loop
+;     }
+
+ ldx #0         ; jsleep(10)
+ lda #$0a
+ jsr _jsleep
+
+ dey
+ bne @explosion_loop ; we're done if Y==0
+;   }
+
+ rts
+; }