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PORTA = $6001
DDRA = $6003
SER = %00000001
RCLK = %00000010
SRCLK = %00000100
.org #$f800
reset:
ldx #$FF ; initialize call stack to $1FF
txs
ldx #%00000111 ; set output on 7seg control pins of PORTA
stx DDRA
lda #$a5 ; 10100101, data to show
ldy #$8 ; 8 bits to shift
loop:
rol a
bcc zerobit ; C=0, bang a zero to SER
onebit:
ldx #(SER) ; C=1, bang a one to SER
stx PORTA
ldx #(SER | SRCLK)
stx PORTA
ldx #(SER)
stx PORTA
jmp next
zerobit:
ldx #$0 ; C=0, bang a zero to SER
stx PORTA
ldx #SRCLK
stx PORTA
ldx #$0
stx PORTA
jmp next
next:
dey
bne loop
output:
ldx #0 ; bang RCLK to store output in latch
stx PORTA
ldx #RCLK
stx PORTA
ldx #0
stx PORTA
term:
jmp term
nmi:
rti
irq:
rti
.org #$fffa
.word nmi
.word reset
.word irq
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