Controller Design: ALU, MOV8 & SETAB Instructions

In my last post I covered off the controller design for the GOTO instruction. Being as I’ve decided to implement the controller on a new set of PCB cards I need to transfer over the existing ALU, MOV8 & SETAB instructions to the new controller design. At the relay level nothing really changes so I’ll just quickly recap the timing and implementations here using the new schematic style and then in my next post I can move on to putting the full controller schematic and PCB design together.

Let’s cover the three instructions off in order of complexity:

SETAB: Load Immediate

The SETAB instruction loads an immediate value in to either the A or B register. This is a really useful and quick way to get a value loaded in ready for performing further ALU calculations. The value to be loaded is limited though due to the number of available bits in the instruction opcode:

Load Immediate

0 1 r d d d d d
    r = destination register (0-A, 1-B)
ddddd = value (-16..15)

Here’s the associated timing chart for the SETAB instruction:

SETAB opcode timing chart
SETAB opcode timing chart (larger)

This is all very straightforward to implement in relays:

SETAB schematic
SETAB schematic (larger)

The only thing that perhaps needs a brief explanation is the CHAT line on the right. This is activated early in the fetch/increment cycle to indicate that the sequencer can now have an abort line set if required. All of the instructions on this page are 8-cycle and each of the instruction schematics will set AT08 when the CHAT line is active.

ALU: Arithmetic/Logic

The ALU instruction can perform a number of arithmetic and logic operations on the B and C registers (or just the B register for operations that only work on one value). The result of the operation can be placed in either the A or D register.

ALU Operation

1 0 0 0 r f f f
  r = destination register (0-A, 1-D)
fff = function code (000-NOP, 001-ADD, 010-INC, 011-AND, 100-OR, 101-XOR, 110-NOT, 111-SHL)

The timing chart for the ALU instruction looks like this:

ALU opcode timing chart
ALU opcode timing chart (larger)

The relay schematic for ALU uses five relays over SETAB’s three but is still easy to understand:

ALU schematic
ALU schematic (larger)

MOV-8: 8-bit Move

The ’not so aptly’ named MOV8 instruction copies an 8-bit value from one register to another. If the source and destination register are the same then the value in that register is cleared:

8-Bit Move

MOV8 8
0 0 d d d s s s
ddd = destination register (000-A, 001-B, 010-C, 011-D, 100-M1, 101-M2, 110-X, 111-Y)
sss = source register (000-A, 001-B, 010-C, 011-D, 100-M1, 101-M2, 110-X, 111-Y)

Here’s the MOV-8 timing chart:

MOV8 opcode timing chart
MOV8 opcode timing chart (larger)

The schematic for MOV-8 has a lot more relays going on but mostly that’s around decoding the source and destination registers for the operation:

MOV8 schematic
MOV8 schematic (larger)

You can see there’s quite a few bits of this schematic blanked out. I’ve done that for brevity here but effectively that’s where the M1, M2, X and Y registers can be selected as a source or destination. I won’t be implementing those for the moment although as you can probably tell they’re just a couple of relays away.

That it for these instructions. In my next post I’ll bring these together with the GOTO instruction and get the full controller schematic laid out and design the PCB lay out too and then I’m ready for some more board construction.

Published Dec 07, 2019