Routines
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E9C8: shift screen line up/down
Used by the routines at E8EA and E965.
E9C8 29 03 AND #%00000011 mask 0000 00xx, line memory page
E9CA 0D 88 02 ORA $0288 OR with screen memory page
E9CD 85 AD STA $AD save next/previous line pointer high byte
E9CF 20 E0 E9 JSR $E9E0 calculate pointers to screen lines colour RAM
E9D2 A0 27 LDY #$27 set the column count
E9D4 B1 AC LDA ($AC),Y get character from next/previous screen line
E9D6 91 D1 STA ($D1),Y save character to current screen line
E9D8 B1 AE LDA ($AE),Y get colour from next/previous screen line colour RAM
E9DA 91 F3 STA ($F3),Y save colour to current screen line colour RAM
E9DC 88 DEY decrement column index/count
E9DD 10 F5 BPL $E9D4 loop if more to do
E9DF 60 RTS
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