The Totally Unscientific Code Density Competition!

The EightThirtyTwo ISA – Part 22 – 2020-05-04

What better way to celebrate Star Wars Day than with a multi-CPU code density shoot-out?! (Well, OK, most people can probably name a dozen better ways without even trying – but this is how I’m choosing to spend it!)

I was curious to know just how well the code density of EightThirtyTwo code generated by the VBCC backend stacks up against other architectures, so I compiled a fixed codebase, namely the OSD control module from one of the many Minimig variants, targetting 832, m68k, MIPS, OpenRISC, RISC-V ARM, and even i386 and x86-64. The results make for interesting reading…

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Endian matters, linker libraries and constructors.

The EightThirty Two ISA – Part 20 – 2020-04-03

I’m continuing to make gradual improvements to the EightThirtyTwo toolchain, and over the last few days I’ve given my attention to linker libraries, and endian issues.

While I initially intended EightThirtyTwo to be a big endian CPU, it occurred to me early on that I could make endian-ness configurable with a generic parameter. This I did, and having set that “littleendian” parameter to “true” for testing, I never actually changed it back, so all my testing over the last few months has actually been in little endian mode!

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Mitigating Hazards

The EightThirtyTwo ISA – Part 17 – 2020-02-16

I was looking today at ways of improving the throughput of the EightThirtyTwo CPU. The design as it stands is very simple, and didn’t make any attempt to perform result forwarding or instruction fusing. These are both strategies for improving the performance of certain constructs, and I wasn’t sure which of these two techniques I should use.

In brief, without either mechanism implemented, when the CPU encounters code such as:

    li 0
mr r0

it has to wait until the first instruction has finished writing to the tmp register before moving its new contents into the pipeline, and only then finally writing it to r0.

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Assembling is the easy bit!

The EightThirtyTwo ISA – Part 16 – 2020-02-08

In my last post I touched briefly on the 832a assembler which I wrote as the first part of my solution to improving the code density of compiled C code.

An assembler that takes a single source file and spits out a ready-to-run binary file is not particularly difficult to write, but it’s not particularly useful either – in order to be useful we need to be able to link together multiple code modules.

Thus, 832a now has a cousin: 832l, the linker.

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Time to stop being lazy!

The EightThirtyTwo ISA – Part 15 – 2020-02-06

I’ve joked a few times in this series about being too lazy to write an assember – but it would be more true to say that the stop-gap solution I was using was adequate, so my time was better spent on the more enjoyable aspects of the project. I am now feeling the limitations of using the GNU assembler to produce a bytestream for a target it knows nothing about, and to improve either the performance or code density of the vbcc backend’s output any further, I need to address the problem I’ve had so far with cross-module references…

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Choosing the Instruction Set

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.

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