The EightThirtyTwo ISA – Part 2 – 2019-08-10

In part 1 I talked about why I wanted to design my own microprocessor, and how I’d settled upon an architecture using eight-bit instruction words and eight addressable thirty-two bit registers.

Because our instruction words won’t have room to encode two registers in them, each instruction will only take one operand – so we need a way of supplying a second operand to instructions such as add, xor, load immediate, etc. For this I’ve defined a ninth register, which I’ll call temp. This register isn’t addressable via the instruction word – it’s simply used implictly wherever it’s needed. One of the key decisions to make will be whether the result of arithmetic instructions will go the nominated register or the temp register.

It’s not entirely clear without giving the matter some thought (and ideally writing some test programs) exactly which instructions we need to implement, so initially I’ll list all the possible instructions that come to mind, and then figure out what’s mandatory, what’s optional, and what’s most beneficial in terms of keeping code size down, in the hope that we can hit our target of 25 instructions.

The EightThirtyTwo ISA – Part 1 – 2019-08-09

In 2019 there are any number of off-the-shelf CPU cores which can be used in FPGA projects, some imitating long-established CPUs such as x86, MC68000, Z80, MIPS, ARM, etc – and some more specifically targetted at the FPGA space, such as NIOS, Microblaze, ZPU, Moxie and suchlike. So why on earth would I consider creating a brand new one from scratch?

As always, because I wanted to learn something, and because even though there are so many existing options, there are still applications for which none of them is ideal.

My design goal is to create a CPU that’s reasonably small – not much bigger than ZPUFlex – so takes up somewhere in the region of 1500 logic elements, while being somewhat faster and offering better code density. Ideally I want something that can supplant the ZPU for control module applications and allow me to reduce the amount of block RAM I have to devote to the code.