Efficient sign extension

The EightThirtyTwo ISA – Part 14 – 2020-01-17

Here’s an optmisation I should have spotted much sooner: sign extension.

In the C programming language char and short types are converted to integers before being subjected to mathematical operations – so called “integer promotion”. There are circumstances where this can be avoided – and vbcc has a hook for precisely this purpose, allowing CPUs which have byte- or word-oriented versions of their arithmetic operations to avoid promotion. EightThirtyTwo doesn’t provide this luxury.

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That’s more like it…

The EightThirtyTwo ISA – Part 13 – 2020-01-12

My project time over the last few weeks has been spent improving the vbcc backend for the EightThirtyTwo CPU, trying to find ways of making the generated code more efficient. Many of these changes have been simple tweaks, like avoiding sign-extensions when comparisons are only concerned with equality. I’ve also put some effort into recognising when a previous operation has left a value in the tmp register and thus when reloading it can be avoided.

The biggest improvement of all, however, has come from changing how the code generator handles loading and storing values. Previously I had reserved two GPRs – r0 and r1 – for the code generator, but on an architecture with only eight general purpose registers, one of which is the PC and one of which is busy being the stack, depriving the compiler of two registers hurts performance – so I’ve re-worked the code generator so that it reserves just one register for its own use.

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Good news… and bad news!

The EightThirtyTwo ISA – Part 12 – 2019-12-07

Since my last post I’ve put some further work into the EightThirtyTwo vbcc backend, and I now have it working well enough to compile a working Dhrystone benchmark. That’s the good news. Here’s the bad news:

User time: 1541
Microseconds for one run through Dhrystone: 61
Dhrystones per Second: 16223
VAX MIPS rating * 1000 = 9231

OK, that’s not terrible, but it’s not as high as I was hoping.

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Two threads are better than one…

The EightThirtyTwo ISA – Part 10 – 2019-11-16

One of the main goals with a pipelined CPU is keeping the pipleline as full as possible at all times. The biggest obstacle to this goal is the existence of ‘hazards’ – situations where what’s about to happen at the beginning of the pipeline depends on an outcome that’s yet to be determined further down the line. This is a particular weakness of the EightThirtyTwo CPU: because we have only eight registers, and because many operations involve the tmp register, there’s a good chance that any given instruction will depend on the result of the previous one. One way around this is to use results forwarding where, for example, a result calculated in the ALU can be sent directly to the ALU’s inputs for the next instruction rather than being first written to and then read again from the register file.

I haven’t yet attempted to implement this for the EightThirtyTwo. Instead, I’ve attempted something far less sane for a lightweight CPU – dual-threading!

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The EightThirtyTwo ISA – Part 9 – 2019-10-28

Having managed to get the CPU working well enough to run a Hello World program, I’ve since persuaded it to run the LZ4 decompression program I posted a week or two back, too – and was pleasantly surprised to find it running happily at 133MHz on my DE2 board.

I’ve been pondering how best to handle interrupts on this CPU, and realised that I’ve painted myself into a bit of corner by using the tmp register as a link register when branching; this means that without adding some kind of temporary storage register or stack logic especially for the task, I can’t change the program flow externally – such as in response to an interrupt – without losing the contents of the tmp register.

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Some real-life code…

The EightThirtyTwo ISA – Part 7 – 2019-09-26

Last time I said I would show some real-life code for this new ISA. I’ve already shown some very simple “Hello World” code, and I’m now in the slow, careful process of implementing a CPU to run this – and exploring the wonderful world of HDL simulation in the process! But in the meantime, by far the best way to get a feel for how well an instruction set works is to write some actual code for it, and see which issues and limitations you bump into.

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Evolving the ISA…

The EightThirtyTwo ISA – Part 6 – 2019-09-15

In my baby-steps towards a working VBCC backend I now have something complete enough at least to send the text “Hello world!” to a UART. There’s a long way to go yet before it (a) works and (b) produces code that’s even remotely efficient, but it’s a start. In the process I’ve found and addressed some shortcomings in the ISA, and made a few tweaks to the instruction set and encoding.

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Assembly language is all very well…

The EightThirtyTwo ISA – Part 5 – 2019-08-24

Before proceeding very much further with the EightThirtyTwo design I wanted to have some way of generating code for it from a higher-level language, just to get a better feel for which instructions are useful for compilers, which will be pretty much unused and where I might find any gaps that make the generated code unneccessarily clumsy. The most obvious route to this goal is to find a C compiler that can be easily retargetted to a new architecture. (Anyone who’s attempted this before is probably laughing now at my use of the word ‘easily’!)

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