dos33fsprogs/fireworks/vapor_lock.s

59 lines
1.1 KiB
ArmAsm
Raw Normal View History

2018-09-11 17:58:29 +00:00
;==============================
; setup graphics for vapor lock
;==============================
vapor_lock:
; Clear Page0
lda #$0
sta DRAW_PAGE
lda #$44
jsr clear_gr
; Make screen half green
lda #$11
ldy #24
jsr clear_page_loop
;=====================================================
; attempt vapor lock
; by reading the "floating bus" we can see most recently
; written value of the display
; we look for $55 (which is the grey line)
;=====================================================
; See:
; Have an Apple Split by Bob Bishop
; Softalk, October 1982
; Challenges: each scan line scans 40 bytes.
; The blanking happens at the *beginning*
; So 65 bytes are scanned, starting at adress of the line - 25
; the scan takes 8 cycles, look for 4 repeats of the value
; to avoid false positive found if the horiz blanking is mirroring
; the line (max 3 repeats in that case)
vapor_lock_loop: ; first make sure we have all zeroes
LDA #$11
zxloop:
LDX #$04
wiloop:
CMP $C051
BNE zxloop
DEX
BNE wiloop
LDA #$44 ; now look for our border color (4 times)
zloop:
LDX #$04
qloop:
CMP $C051
BNE zloop
DEX
BNE qloop
rts