gistfile1.asm

DrawNewColumn:
	lda scroll		;calc new address using scroll register
	lsr a
	lsr a
	lsr a			;shift right 3 times
	sta columnLo	;$00 to $1F, screen is 32 tiles wide
	
	lda nametable	;calc new col addr using current nametable
	eor #$01		;invert lo bit
	asl a			;shift up, A = $00 or $02
	asl a			;$00 or $04
	clc 
	adc #$20		;add hi byte of nametable addr ($2000)
	sta columnHi	;new addr = $20 or $24 
	
	lda columnNum	;col number * 32 = col data offset
	asl a
	asl a
	asl a
	asl a
	asl a
	sta sourceLo
	lda columnNum
	lsr a
	lsr a
	lsr a
	sta sourceHi
	
	lda sourceLo	;col data start + offs = addr to load col data from
	clc
	adc #<columnData
	sta sourceLo
	lda sourceHi
	adc #>columnData
	sta sourceHi
	
DrawColumn:
	lda #%00000100	;increment +32 mode
	sta $2000
	
	lda $2002		;reset latch
	lda columnHi
	sta $2006
	lda columnLo
	sta $2006
	ldx #$1E		;copy 30 bytes
	ldy #$00
DrawColumnLoop:
	lda (sourceLo), y
	sta $2007
	iny
	dex
	bne DrawColumnLoop
	
	rts