Commodore 1571 firmware
    Format Routine
        .page  
	.subttl 'lccfmt4a.src'
jfmterr dec  cnt        ;  test for retry
        beq  jfmte10      
        jmp  jend         
jfmte10 ldy  #$ff        
        sty  ftnum      ;  clear format
        iny      
        sty  gcrflg      
        jmp  jerrr        
; this subroutine will be called with x*255 for amount of chars written
jclear  lda  pcr2	;  enable write
	and  #$ff-$e0   ;  wr mode=0
	ora  #$c0
	sta  pcr2
        lda  #$ff       ;  make port an output
        sta  ddra2      ;  clear pending
        
        lda  #$55       ;  write a 1f pattern
        ldy  #00         
1$	bit  pota1
	bmi  1$
	bit  byt_clr
        sta  data2       
        dey      
        bne  1$
        dex     	;  dex amount * 255
        bne  1$
        rts