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9.1 Accessing the registers
The mode of communication desired is established and monitored through the bit values
of the internal read and write registers. The register set of the SCC includes 16 write registers
and 9 read registers. These registers only occupy four address locations, which start at the
MPAP-100's physical base address that is configured via the on board switches. This and all
other addresses are referenced from this base address in the form Base+Offset. An example of
this is Base+1 for the SCC Control Port, Channel A.
There are two register locations per SCC channel, a data port and a control port.
Accessing the internal SCC registers is a two step process that requires loading a register pointer
to perform the addressing to the correct data register. The first step is to write to the control port
the operation and address for the appropriate channel. The second step is to either read data from
or write data to the control port. The only exception to this rule is when accessing the transmit
and receive data buffers. These registers can be accessed with the two step process described or
with a single read or write to the data port. The following examples illustrate how to access the
internal registers of the SCC. Table 3 on page 26 describes the read registers and Table 4 on
page 27 describes the write registers for each channel.
The MPAP-100 has been designed to assure that all back to back access timing
requirements of the SCC are met without the need for any software timing control. The standard
of adding jmp $+2 between I/O port accesses is not required when accessing the MPAP-100.
Example 1: Enabling the transmitter on channel A.
mov dx, base ; load base address
add dx, ContA ; add control reg A offset (1)
mov al, 05H ; write the register number
out dx, al
mov al, 08H ; write the data to the register
out dx, al
Example 2: Monitoring the status of the transmit and receive buffers in RR0 of Channel A.
Register 0 is addressed by default if no register number is written to WR0 first.
mov dx, base ; load base address
add dx, ContA ; add control reg A offset (1)
in ax, dx ; read the status