How to avoid writing an assembler

2019-08-14 – Part 4 – The EightThirtyTwo ISA

In order to experiment properly with the instruction set I need some way of assembling and running programs. Luckily I wrote a ZPU simulator some time ago, and this is easily re-purposed for the EightThirtyTwo instruction set. The simulator is part of the EightThirtyTwo project on github.

In the longer term, to be useful this ISA will need a fully-functional toolchain, which means writing a backend for GCC – or perhaps LLVM, or maybe even lcc, vbcc, or some other C compiler. That’s going to be a rabbit hole in its own right, however, and for now we just want a quick and easy way of turning an assembly listing into a binary file of bytes ready for execution.

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The EightThirtyTwo ISA

Part 3 – 2019-08-12 – Nailing down the instruction set

My initial plan for the EightThirtyTwo ISA was to define eight general purpose registers, and use the ninth “temp” register as an accumulator. Immediate values would be assigned here, as would the result of any arithmetic operations. The best way to test the concept is, of course, to write some actual code. So let’s see what simple looping looks like with this concept:

  li 99 // (needs two li instructions since 99 doesn't fit within a signed 6-bit value)
mr r0
.bottles_of_beer:
// take one down, pass it around
li 1
sub r0
mr r0
cond NEQ // Disable execution if we've reached zero
li .bottles_of_beer
mr r7 // if not, do another iteration
cond EX // return to normal execution

That’s not too bad – weighing in at 10 bytes – way better than MIPS, but 68K beats us easily thanks to its dbf instruction. Let’s try something more involved…

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The EightThirtyTwo ISA

2019-08-10 – Part 2 – Choosing the Instruction Set

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.

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A whole new ISA?

2019-08-09 – Part 1 – The Pipe Dream

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.

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