The serial port

A 6551 ACIA at $9000, running 19200 baud, eight data bits, no parity, one stop bit. The Kernal sets it up at power-on and wires its receive interrupt into the same ring buffer the keyboards use, so incoming serial needs nothing from you: it arrives through Chrin like any other typing.

Going out is where the choices are.

Sending

Chrout	Wherever the console is pointed
SerialChrout	Down the cable, whatever the console is doing
SetIOMode	Point the console at the cable, or back at the screen

SerialChrout is the useful one on a machine with a monitor: your program keeps printing to the screen, and sends a running commentary to a terminal on your laptop at the same time.

  lda #<Trace
  ldy #>Trace
  jsr GetIOMode
  pha
  lda #1
  jsr SetIOMode                 ; console → serial
  jsr PrintStr                  ; ... so PrintStr goes down the cable
  pla
  jsr SetIOMode                 ; and back

There is no SerialPrintStr, which is why the mode gets flipped around PrintStr like that. For a single character SerialChrout is simpler.

The registers, if you want them

SC_DATA $9000	Read a received byte, write one to send
SC_STATUS $9001	Read: bit 7 set means this chip caused the interrupt
SC_RESET $9001	Write anything to reset the chip
SC_CMD $9002	Receive interrupts, and the RTS line
SC_CTRL $9003	Baud rate and framing — $1F is 19200 8-N-1

The Kernal's interrupt handler does the flow control for you: when the ring buffer gets close to full it raises RTS to ask the other end to stop, and drops it again once Chrin has drained things. Talk to SC_CMD yourself and you are taking that over.

Changing the baud rate

Write a different framing byte to SC_CTRL. Both ends have to agree, and the machine at the other end of the cable is usually the harder one to change — which is why 19200 is the default and why nothing in these guides moves it.

Moving files: XModem

XModem is the protocol behind LOAD and SAVE when there is no card in the machine, and it is available directly:

XModemLoad	Receive. XFER_PTR = where to put it. On return XFER_REMAIN = how many bytes arrived
XModemSave	Send. XFER_PTR = the bytes, XFER_REMAIN = how many

Both answer with the carry flag: clear worked, set failed.

  lda #<Incoming
  sta XFER_PTR
  lda #>Incoming
  sta XFER_PTR + 1
  jsr XModemLoad
  bcs TransferFailed
  ; XFER_REMAIN holds the byte count

Underneath it is the 1977 protocol, unchanged: 128-byte blocks, each with a block number, its complement, and a one-byte checksum; the receiver starts the conversation by sending NAK until a block arrives; every block is answered ACK or NAK; the sender finishes with EOT. Because the receiver leads, the order of operations at the two ends matters — start the receiving side first, then the sending side, and the transfer begins on its own.

A transfer is not a background job

XModemLoad and XModemSave do not return until the transfer is over, one way or the other. Nothing else in your program runs while a file is moving, and a cable pulled halfway through is a retry loop followed by a set carry flag.

Since blocks are 128 bytes and files rarely divide by 128, the last block is padded. XFER_REMAIN counts the padding too — it is the number of bytes received, not the length of the file that was sent. Anything that needs an exact length carries its own header, which is what a .prg does with its first two bytes.

When there is no serial card

The Kernal checks. HW_SC in HW_PRESENT says whether the card is fitted, and the console falls back to the screen when it is not. Guard your own transfers the same way:

  lda HW_PRESENT
  and #HW_SC
  beq NoCable

Next: the clock and the bytes that last.