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