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
 jsr print7seg

term:
 jmp term

print7seg:
 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
 rol a				; restore A register
output:
 ldx #0				; bang RCLK to store output in latch
 stx PORTA
 ldx #RCLK
 stx PORTA
 ldx #0
 stx PORTA
 rts
  
nmi:
 rti

irq:
 rti

 .org #$fffa
 .word nmi
 .word reset
 .word irq