====== Writing to a file byte-by-byte ======
BASIC code:
<code>
10 FS=8192:FE=16384
20 OPEN 2,8,2,"JUST A FILENAME,P,W"
30 IF ST<>0 THEN GOTO 70
40 A=PEEK(FS):FS=FS+1
50 PRINT#2,CHR$(A);
60 IF FE>FS THEN GOTO 30
70 CLOSE 2
</code>
Assembler code:
<code>
file_start = $2000    ; example addresses
file_end   = $4000

        LDA #fname_end-fname
        LDX #<fname
        LDY #>fname
        JSR $FFBD     ; call SETNAM

        LDA #$02      ; file number 2
        LDX $BA       ; last used device number
        BNE .skip
        LDX #$08      ; default to device 8
.skip   LDY #$02      ; secondary address 2
        JSR $FFBA     ; call SETLFS

        JSR $FFC0     ; call OPEN
        BCS .error    ; if carry set, the file could not be opened

        ; check drive error channel here to test for
        ; FILE EXISTS error etc.

        LDX #$02      ; filenumber 2
        JSR $FFC9     ; call CHKOUT (file 2 now used as output)

        LDA #<file_start
        STA $AE
        LDA #>file_start
        STA $AF

        LDY #$00
.loop   JSR $FFB7     ; call READST (read status byte)
        BNE .werror   ; write error
        LDA ($AE),Y   ; get byte from memory
        JSR $FFD2     ; call CHROUT (write byte to file)
        INC $AE
        BNE .skip
        INC $AF
.skip
        LDA $AE
        CMP #<file_end
        LDA $AF
        SBC #>file_end
        BCC .loop     ; next byte
.close
        LDA #$02      ; filenumber 2
        JSR $FFC3     ; call CLOSE

        JSR $FFCC     ; call CLRCHN
        RTS
.error
        ; Akkumulator contains BASIC error code

        ; most likely errors:
        ; A = $05 (DEVICE NOT PRESENT)

        ... error handling for open errors ...
        JMP .close    ; even if OPEN failed, the file has to be closed
.werror
        ; for further information, the drive error channel has to be read

        ... error handling for write errors ...
        JMP .close

fname:  .TEXT "JUST A FILENAME,P,W"  ; ,P,W is required to make this an output file!
fname_end:
</code>

You may open more than one file if you use different file numbers and secondary addresses for them. File numbers and secondary addresses should be in the range of 2 to 14. It's usually a good idea to use the same number for both to keep confusion low.