2026-09-21 The first code to run on the A5

This is the code with comments removed. It is explained in detail here https://a5computer.com/code/firstever.html
but briefly016 04000020 :ADDRESS 16#100010 017 04000020 018 04000020 .START 019 04000020 ;Experiment to move an immediate value in and out of A3 020 04000020 000000 000 0 11 100 000 010 011 MOV (PC++) DAR 021 04000021 001 100 110 011 001 100 110 011 :WORD 16#333333 022 04000022 000000 000 0 00 011 111 000 000 MOV DAR A3 023 04000023 000000 000 0 00 111 011 000 000 MOV A3 ADR 024 04000024 000000 000 0 00 000 000 000 000 NOP 025 04000025 026 04000025 ;Jump back to START 027 04000025 000000 000 0 11 100 000 010 011 MOV (PC++) DAR 028 04000026 000 100 000 000 000 000 010 000 :WORD .START 029 04000027 000000 000 0 00 011 100 000 000 MOV DAR PC

The three digit number on the extreme left is the line number in decimal.

The number following it is the memory address of the instruction in octal.

The binary numbers where they appear are the instruction stored at the address or the constant :WORD value.

They are broken into blocks of binary digits because each block is a part of the instruction and has a particular meaning;

23-18 ALU Function
17-15 3bit Test Condition if any use 000 for no test
14 Test for 0 or 1 use 0 for no test
13 Ax_CountOrHold
12 Ax_DownOrUp
11-9 Source Address
8-6 Destination Address
5-3 Source Data
2-0 Destination Data

Then follows the mnemonic and the parameters. (PC++) means the contents of the memory location pointed to by PC and then increment PC again after this instruction.

The PC is the Program Counter

A3 is an address register connected to the address bus

DAR the Data bus to Address bus Register

ADR the Address bus to Data bus Register

Discussions

tomdehavas wrote 3 hours ago

This was the first ever code to be assembled initially by hand and then by the cross assembler I wrote.

The code proves the operation of the instruction decoder, the address register operations including post increment, the effectiveness of immediate addressing mode, jump instructions and the fetch/execute instruction cycle.

Now I am ready to build the ALU and test that.

Are you sure? yes | no

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