Tube: tube_write

       Name: tube_write                                              [Show more]
       Type: Subroutine
   Category: Tube
    Summary: As the parasite, send a byte across the Tube to the I/O processor
  Deep dive: Tube communication in Elite-A
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * boot_in calls tube_write
             * CATS calls tube_write
             * CHPR calls tube_write
             * CLYNS calls tube_write
             * CPIX2 calls tube_write
             * DET1 calls tube_write
             * DIL2 calls tube_write
             * DILX calls tube_write
             * DKS1 calls tube_write
             * DKS4 calls tube_write
             * ECBLB calls tube_write
             * HANGER calls tube_write
             * HLOIN calls tube_write
             * LL30 calls tube_write
             * MSBAR calls tube_write
             * MT26 calls tube_write
             * PIXEL calls tube_write
             * RDKEY calls tube_write
             * read_0346 calls tube_write
             * SCAN calls tube_write
             * scan_fire calls tube_write
             * SPBLB calls tube_write
             * TT217 calls tube_write
             * TTX66 calls tube_write
             * UNWISE calls tube_write
             * update_pod calls tube_write
             * write_0346 calls tube_write
             * WSCAN calls tube_write


 Tube communication in Elite-A uses the following protocol:

 Parasite -> I/O processor

   * Uses the FIFO 1 status and data registers to transmit the data
   * The parasite calls tube_write to send a byte to the I/O processor
   * The I/O processor calls tube_get to receive that byte from the parasite

 I/O processor -> Parasite

   * Uses the FIFO 2 status and data registers to transmit the data
   * The I/O processor calls tube_put to send a byte to the parasite
   * The parasite calls tube_read to receive that byte from the I/O processor

 This routine is called by the parasite to send a byte to the I/O processor.

 The code is identical to Acorn's MOS routine that runs on the parasite to
 implement OSWRCH across the Tube.


.tube_write

 BIT tube_r1s           \ Check whether FIFO 1 is available for use, so we can
                        \ use it to transmit a byte to the I/O processor. We do
                        \ this by checking bit 6 of the FIFO 1 status register

 NOP                    \ Pause while the register is checked

 BVC tube_write         \ If FIFO 1 is available for use then bit 6 of the
                        \ status register will be set, so this loops back to
                        \ tube_write until FIFO 1 is available for us to use

 STA tube_r1d           \ FIFO 1 is available for use, so store the value we
                        \ want to transmit in the FIFO 1 data register, so it
                        \ gets sent to the I/O processor

 RTS                    \ Return from the subroutine