The easiest way of sending a command to the disk drive is by simply using the command string as filename when calling OPEN.

BASIC code:

OPEN 15,8,15,"I":CLOSE 15

Assembler code:

        LDA #cmd_end-cmd
        LDX #<cmd
        LDY #>cmd
        JSR $FFBD     ; call SETNAM

        LDA #$0F      ; file number 15
        LDX $BA       ; last used device number
        BNE .skip
        LDX #$08      ; default to device 8
.skip   LDY #$0F      ; secondary address 15
        JSR $FFBA     ; call SETLFS

        JSR $FFC0     ; call OPEN
        BCS .error    ; if carry set, the file could not be opened
.close
        LDA #$0F      ; filenumber 15
        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

cmd:    .TEXT "I"     ; command string
cmd_end:

Another way of sending a command to the disk drive is by leaving the filename string empty and send the command string afterwards. The advantage of this is that you can send several commands but only open the command channel one time.

BASIC code:

OPEN 15,8,15:PRINT#15,"I":CLOSE 15

Assembler code:

        LDA #$00
        LDX #$00
        LDY #$00
        JSR $FFBD     ; call SETNAM (no filename)

        LDA #$0F      ; file number 15
        LDX $BA       ; last used device number
        BNE .skip
        LDX #$08      ; default to device 8
.skip   LDY #$0F      ; secondary address 15
        JSR $FFBA     ; call SETLFS

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

        LDX #$0F      ; filenumber 15
        JSR $FFC9     ; call CHKOUT (file 15 now used as output)

        LDY #$00
.loop   LDA cmd,y     ; get byte from command string
        JSR $FFD2     ; call CHROUT (send byte through command channel)
        INY
        CPY #cmd_end-cmd
        BNE .loop
.close
        LDA #$0F      ; filenumber 15
        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

cmd:    .TEXT "I"     ; command string
        .BYTE $0D     ; carriage return, needed if more than one command is sent
cmd_end: