From http://www.jwz.org/xscreensaver/xscreensaver-5.36.tar.gz

[xscreensaver] / hacks / images / m6502 / greynetic.asm

diff --git a/hacks/images/m6502/greynetic.asm b/hacks/images/m6502/greynetic.asm
new file mode 100644 (file)
index 0000000..1218d87
--- /dev/null
+++ b/hacks/images/m6502/greynetic.asm
@@ -0,0 +1,96 @@
+;Port of Greynetic
+;Jeremy English 2013
+
+lda #$0
+sta $0
+lda #$2
+sta $1
+
+start:
+lda $1
+cmp #$6
+bne randOffset
+lda #$2
+sta $1
+
+randOffset:
+;move position by some random offset
+clc
+lda $fe
+adc $0
+sta $0
+lda $1
+adc #$0
+sta $1
+cmp #$06 ;Did we go out of range
+bne setRect ;Nope
+lda #$02 ;Start back at the top
+sta $1
+
+setRect:
+lda $fe
+and #$f
+tax
+inx ;at least 1
+stx $2 ;width
+stx $3 ;working copy
+lda $fe
+and #$f
+tax
+inx ;at least 1
+stx $4 ;height
+
+lda $fe
+sta $5 ;color
+
+ldy #0
+draw:
+lda $5
+sta ($0), y
+dec $3
+beq down
+
+lda $0
+clc
+adc #$1
+sta $0
+lda $1
+adc #$0
+sta $1
+cmp #$06 ;Did we go out of range
+beq done ;yes
+jmp draw
+
+down:
+
+;;Move back to the start of this row
+ldx $2 ;The width of the rectangle
+dex
+lda $0
+stx $0
+sec
+sbc $0
+sta $0
+lda $1
+sbc #$0
+sta $1
+
+;;Move down one row
+dec $4
+beq done ;;Are we done drawing?
+
+lda $2
+sta $3 ;reset the width counter
+
+lda $0
+clc
+adc #$20
+sta $0
+lda $1
+adc #$0
+sta $1
+cmp #$06 ;Did we go out of range
+beq done ;yes
+jmp draw
+done:
+jmp start