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kfest: add vapor lock code

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This commit is contained in:
Vince Weaver 2018-07-18 21:01:58 -04:00
parent f732e4d6e8
commit afa8586bac
1 changed files with 70 additions and 5 deletions
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75
kfest2018/raster.s
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@ -26,11 +26,6 @@ HOME = $FC58 ;; Clear the text screen
lda #0
sta DRAW_PAGE
; GR part
bit LORES
bit SET_GR
bit FULLGR
; Clear Page0
lda #$00
sta DRAW_PAGE
@ -53,6 +48,76 @@ HOME = $FC58 ;; Clear the text screen
ldy #38
jsr hline
;=====================================================
; 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 #$00
zxloop:
LDX #$04
wiloop:
CMP $C051
BNE zxloop
DEX
BNE wiloop
LDA #$55 ; now look for four all grey
zloop:
LDX #$04
qloop:
CMP $C051
BNE zloop
DEX
BNE qloop
; found first line of low-res grey, need to kill time
; until we can enter at top of screen
; so we want roughly 5200+4550 - 65 (for the scanline we missed)
; GR part
bit LORES
bit SET_GR
bit FULLGR
; want 9685
; Try X=34 Y=55 cycles=9681
lda #0 ; 2
lda #0 ; 2
ldy #55 ; 2
loopA:
ldx #34 ; 2
loopB:
dex ; 2
bne loopB ; 2nt/3
dey ; 2
bne loopA ; 2nt/3
jmp display_loop
.align $100
display_loop:
loop_forever:
jmp loop_forever