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⌂ ∞ Elite on the BBC Micro and NES

Tube: tube_read

[Elite-A, Parasite]

       Name: tube_read                                               [Show more]
       Type: Subroutine
   Category: Tube
    Summary: As the parasite, fetch a byte that's been sent over the Tube from
             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:
             * DKS1 calls tube_read
             * DKS4 calls tube_read
             * MT26 calls tube_read
             * RDKEY calls tube_read
             * read_0346 calls tube_read
             * scan_fire calls tube_read
             * TT217 calls tube_read
             * WSCAN calls tube_read




 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 receive a byte from the I/O
 processor.

 The code is identical to Acorn's MOS routine that runs on the parasite to
 implement OSWRCH across the Tube (except this uses FIFO 2 instead of FIFO 1).



.tube_read

 BIT tube_r2s           \ Check whether FIFO 2 has received a byte from the I/O
                        \ processor (which it will have sent by calling its own
                        \ tube_put routine). We do this by checking bit 7 of the
                        \ FIFO 2 status register

 NOP                    \ Pause while the register is checked

 BPL tube_read          \ If FIFO 2 has received a byte then bit 7 of the status
                        \ register will be set, so this loops back to tube_read
                        \ until FIFO 2 contains the byte transmitted from the
                        \ I/O processor

 LDA tube_r2d           \ Fetch the transmitted byte by reading the FIFO 2 data
                        \ register into A

 RTS                    \ Return from the subroutine