commodore.gen.tr

!to "colorbars.prg", cbm

*=$0801

!word nextline			;start basic listing
!word 2009			;2009
!byte $9e			;sys
!text "2061"			;2061
!byte 0
nextline: 
!word 0				;end basic listing

start:
lda #$00			;choose black color
sta $d020			;set border color
;sta $d021			;set background color
;lda #$01			;chose white color
;jsr $e536			;set character color & clear screen

jsr prepare_irq			;prepare irq

end:
jmp end				;endless loop

prepare_irq:
sei				;disable interrupts
lda #$7f
sta $dc0d			;disable cia interrupts
lda $d01a
ora #$01
sta $d01a			;activate raster irq
lda $d011
and #$7f
sta $d011			;irq raster line < 256
lda #$00
sta $d012			;irq raster line = 0
lda #<irq_routine
sta $0314
lda #>irq_routine
sta $0315			;set irq routine address
lda #$01
sta $d019
cli				;enable interrupts
rts

irq_routine:
inc $d019			;irq ok, prepare for the next one
lda #$00			
jsr wait_raster			;wait for raster line
jsr draw_colorbar		;draw color bar
lda $dc01			;read keypress
cmp #$ef			;if not space
bne continue			;continue, else
sei				;disable interrupt
jsr $ff81
jsr $ff84
lda #$31
sta $0314			;restore lo byte
lda #$ea
sta $0315			;restore hi byte
cli				;enable interrupts
jmp $a474			;exit program
continue:
jmp $ea31			;give the control to the cpu

wait_raster:
cmp $d012			;compare $d012 with register a
bne wait_raster			;wait for raster line
rts

wait_timer:
dey				;decrement register y
bne wait_timer			;until it reaches zero
rts

draw_colorbar:
ldx #$00			;reset counter
loop:
ldy #$21			;delay timer = 33 
jsr wait_timer			;wait timer to finish
stx $d020			;change border color
;stx $d021			;change background color
inx				;increment counter
bne loop			;loop 
rts

;sta $d021			;set background color
;lda #$01			;chose white color
;jsr $e536			;set character color & clear screen

;stx $d021			;change background color