1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
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
; }
|
