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