39 lines
744 B
ArmAsm
39 lines
744 B
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# X16 * #IGNORE
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// #LAYOUT# * KERNAL_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Well-known routine, described in:
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//
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// - [CM64] Computes Mapping the Commodore 64 - page 219
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// - http://sta.c64.org/cbm64scrfunc.html
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//
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// input: .X - screen line to clear
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//
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// Checked that original routine preserves X (convenient for loops),
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// and leaves space screen code in .A, so duplicate this behaviour
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//
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screen_clear_line:
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// First, calculate PNT and USER
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phx_trash_a
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jsr screen_calculate_PNT_USER_from_X
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plx_trash_a
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// Now clear the line
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ldy #39
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!:
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lda COLOR
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sta (USER), y
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lda #$20 // space character screen code
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sta (PNT), y
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dey
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bpl !-
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// Done
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rts
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