Article 1285 of alt.sys.perq:
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Subject: Microcode Part 2
Message-ID: <21NOV199416591459@siva.bris.ac.uk>
Date: 21 Nov 94 15:59:00 GMT
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Here, as promised, is the second part of the introduction to microcode. This
bit covers reading an input port.
----

Reading from an Input Port
--------------------------
The last section showed the standard method for outputing data to a port, an
operation that is performed in one instruction cycle. The converse operation,
reading data from a port, is less simple - it takes 2 instructions and thus 2
cycles. The first instruction addresses the input port and copies the data into
an internal IOD (I/O Data) register in the CPU. The second instruction transfers
the data into the ALU for processing.

In order to try out the example programs, it is necessary to have a PERQ fitted
with an OIO card having the PERQlink circuitry. The examples use the 16 data
lines of the PERQlink input as a 16-bit User input port, and display the decimal
value read from this port on the CRT.

Here is the microcode for reading the port :

    !userin.micro
    !read value from user input port (=perqlink in)
    define(r330,330);
    place(7000,7777);
    loc(7000),
    iob(42),next;   ! Copy User input to IO latch
    r330:=iod,next;  ! Read IO latch into register
    tos:=r330,push,next;  ! Push onto stack
    nextinst(0);
    end;

The 'iob(42)' phrase addresses the perqlink input port and copies the data into
the IOD register. The 'r330:=iod' phrase in the next instruction then transfers
it into a CPU register. The other instructions should be familiar by now.

Since this program requires no arguements from the pascal driver, it may be
called using the 'jumpcontrolstore' procedure, as illustrated by the following
pascal program.

program userin(input,output);
imports controlstore from controlstore;
var microcode:microfile;
    x:integer;
begin;
  reset(microcode,'userin.bin');
  loadcontrolstore(microcode);
  close(microcode);
  repeat;
    jumpcontrolstore(#7000);
    storexpr(x);
    write('x=',x:0,' ');
  until false;
end.

This program repeatedly calls the microcode and displays the returned value.
There are no new concepts here, and so no explanation will be given.

Transfer the 2 files to the PERQ using kermit (or type them in using the
editor), and build them in the usual way. When the program is executed it will
print x= -1 repeatedly. Since the input lines are pulled up by resistors in the
PERQ, an unconnected input is read as a 1, and thus the octal value 177777
(decimal -1) is read.

If an input data bit is grounded, the displayed value will change to reflect the
new value read from the port. The input lines can be found on the top connector
on the OIO board with pin 39 as bit 0, pin 37 as bit 1, etc. All the
even-numbered pins (component side) are ground.

Since an unconnected pin is read as a logic 1, it is sensible to treat the port
as having active-low data lines and to invert the data before displaying it.
This can be done by changing the microcode to the following :

    !userin.micro
    !read value from user input port (=perqlink in)
    define(r330,330);
    place(7000,7777);
    loc(7000),
    iob(42),next;   ! Copy User input to IO latch
    r330:=iod,next;  ! Read IO latch into register
    tos:=not r330,push,next;  ! Push onto stack
    nextinst(0);
    end;

The only change is that the complement of r330 is pushed onto the stack. As in
the case of active-low data output, this change needs no more instructions and
takes no more machine cycles.

When the userin.pas program is executed with this microcode, it prints 0
repeatedly, and grounding a particular data line causes 2^(that line) to be
displayed.


-tony

Bristol University takes no responsibility for the views expressed in this
posting. They are the personal views of the user concerned.