Monitor Program Update

A project log for CPU4

This is my fourth home brew CPU

agp.cooperagp.cooper 08/02/2017 at 05:377 Comments

Monitor Program

I have rewritten the monitor program for a 16 bit address and a 4 MHz clock.

Still needs to be simulated before burning to PROM:

You get a feel for just how primitive these simple CPUs are from the code above.

The CPU OpCode interpreter would commence on line 116.



agp.cooper wrote 08/04/2017 at 07:56 point

Yes, it is in the pipe line (I have a Subleq version already).
The problem is space is a super premium for these simple CPUs.
The thing about this style of code (and this version is better than most) is it does your "head in". Especially self modifying code.

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Ed S wrote 08/04/2017 at 12:12 point

Ah, you have both a space constraint and low code density!

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agp.cooper wrote 08/04/2017 at 13:28 point

That seems to be the problem with these simple CPU.
I want to do it in 8 bits but there is not enough space to do it in.
Thus the demise of the 8 bit Subleq CPU. Absolutely no chance of a simple monitor program in 8 bits! I was actually lucky to get it to fit in my 8bit TTA and to leave space for a program.

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Ed S wrote 08/04/2017 at 13:34 point

A possible way forward is nested interpreters: for example the Tiny Basic interpreter is written in an interpreted bytecode.  Also we have Sweet 16. And I think the TI/99's Basic is also written in an interpreted bytecode.

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agp.cooper wrote 08/05/2017 at 03:12 point

I think another name for "nested interpreters" is Forth?
Forth is an option (but I think it would need a couple of 'k' of ROM) but at the moment the key issue is to get the CPU to talk to a Front Panel. It all get very complicated very quickly without a front panel. Another reason Subleq died was because it was getting very complicated to program it (direct memory access and a Arduino!).
Once you add an Arduino then the project loses the authentic retro feel (but I understand!). There is a guy building a transistor 8080 but he uses ATMEL microcontroler for "house keeping"! Go figure!
Intrepreted ByteCode (perhaps P-Code) is where I am headed!

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Ed S wrote 08/05/2017 at 05:25 point

P-code would be the same kind of idea too - there may be  a way to do it. Or you might be able to add a few instructions to help make programs shorter.

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Ed S wrote 08/04/2017 at 07:19 point

Is this is the sort of thing where a macro assembler would help?

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