.data zp
.space _na 1	; a的临时存放处
.text
printbyte:
    pha
    sta _na
    txa
    pha
    ldx #8	    ; 打印8bit
*   lda #$30	; a = '0'
    asl _na		; 左移一位，溢出到c
    adc #0		; a = a + c + 0
    jsr chrout	; putchar(a)
    dex			; x--
    bne -		; if(x != 0) goto 上个星号
    lda #$20
    jsr chrout
    pla
    tax
    pla
    rts