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Elite on the BBC Micro

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 * TT217 calls tube_read * WSCAN calls tube_read * read_0346 calls tube_read * scan_fire 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