* = $1000

SEI
LDA #<IRQ
LDX #>IRQ
STA $0314
STX $0315
LDA #$30
STA $D012
LDA #$7F
STA $DC0D
LDA #$18
STA $D011
LDA #$01
STA $D01A

lda #$00
tax
tay
sta $d021

CLI

JMP *

IRQ
LDA $D019
AND #$01
STA $D019

cpy #$01
beq down

up
inc $d011
inx
cpx #$02
bne end
ldy #$01
jmp $ea7e

down
dec $d011
dex
bne end
ldy #$00

end
JMP $EA7E