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Day 9: Status Register and Subtraction

A project log for Detritus, the 8-bit ... computer?

I have a lot of random spare parts doing nothing and a need to make them do things...

Jorj BauerJorj Bauer 05/24/2017 at 10:210 Comments

Remember all that hoo-ha over the subtractor yesterday? The insane architecture where this seems to be the way I'm going to multiplex the input in to the adder, so that I can perform twos-compliment math for subtraction?

Well, funny story.

I placed orders in three different places for the parts for this build. Looking at them last night, I realized that I'd accidentally ordered two XOR gates ... from two different vendors.

That's right, one of the packages that's still enroute has something like $1 in parts that will turn that multiplexer design in to a single box in this architecture:

Harrumph. I feel like my hard work was wasted, and now I'm not gonna get to use the kooky logic I built. Like running a copy of VMWare on Windows, which is running on a qemu emulator on a Linux machine. Totally impractical but there's some sense of accomplishment in there. For whatever reason.

So this is the result at the end of today's lunch:

... there was to be a time lapse of the work itself, but apparently I shut off the camera right after it started. Well, maybe tomorrow.

The changes today: the wiring to the adder was cleaned up; then I added the beginnings of the status register, and wired in the XOR gate for the input from the B register to the adder.

I realized two things in the building stage. One: I wanted an INVERT operation in the ALU, so I might as well use the 8-bit XOR for that. Slap on a bus driver and it's a done deal. And Two: I need to put in a status register for the Carry flag.

I'm not entirely sure that I need to put the status register on the bus yet. I'd like to limit the status register to 4 bits, because I have only one '173 remaining. And it has a built-in TRIS, so maybe I don't need a bus driver for it... I suppose that all depends on whether the control logic needs the carry flag at the moment it has been asserted from the ALU, or during a subsequent operation. We'll see.

Here's the ALU from the photo above, with some annotations:

(The wiring from the XOR to the adder obviously needs some cleanup tomorrow.)

The status register, with its lonesome one bit, is showing that A (%1110 1011) + B (%1010 1101) has carry set (the LED above the status register); but that last clock cycle, it was not set (the LED below the status register). When the clock next pulses, the status register will latch in the carry.

Maybe that means that the ADD microcode will need an extra clock cycle to be sure that the carry flag is appropriately set. Thinking about it, that seems fine - I've allotted two microcode clock cycles for each machine instruction - and for the ALU operations, I don't have anything in the second cycle yet. I'll note this for later, I think, and we'll see how the control logic fits when we get that far.

Goals for Day 10:

... but I'm quickly running out of board space. I need to order some more solderless breadboards so I can extend the busses. And I'm almost out of blue wire, which I'm using for all of the bus interconnects. More of that should be arriving today; I might have to put off some of the bus connections another day.

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