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Experiments, experiments and errors

A project log for A 16-bit CPU in Logisim

A custom 16-bit CPU created in Logisim

MartianMartian 04/12/2018 at 20:585 Comments

First I'd like to thank Yann Guidon's advice from his Processor Design Principles (PDP).

So this is the Phoenix, the Ph-16.  A wholly 16-bit system with 16 registers and 64K of RAM.  This is at the highly experimental/research stage, I've come up with a basic layout in Logisim.

Ignore the lower left, it's not connected. It will eventually be the instruction decoder and the 4K ROM will hold the 12-bit microcode, I will look at hardwiring it or doing it with another project, but microcode is just easier for me to fix.   But I did follow Yann Guidon's advice of having 16 registers for a 16 bit machine and they can point to the whole 64K.

The top left is the keyboard and top right is the TTY output, the big multiplexor allows crude channels for other devices, currently. INP and OUT instructions will input/output to/from a given register.

At present I'm playing with the control lines, figuring out what I can get it to do, working out the microcode, instruction set etc.

Discussions

Yann Guidon / YGDES wrote 04/12/2018 at 22:50 point

Would you like to provide me with a zoomed image of your ALU  for illustration of my new entry to PDP ?

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Martian wrote 04/13/2018 at 02:01 point

No problem.  The reason for the wiring as is was I wanted to save space and I -was- going for a  16-operation ALU with 4bits selecting the op via the multiplexor.  In the end I just settled for 8 operations and 3bits to select.  I've sent the image via a personal message.  ;)

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Yann Guidon / YGDES wrote 04/13/2018 at 02:18 point

Thanks a lot !

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Yann Guidon / YGDES wrote 04/12/2018 at 22:49 point

Awesome !
I'm glad my rants have helped somebody :-)
Keep us informed and please log all your design decisions !

BTW your post leads me to write a new entry to #PDP - Processor Design Principles  :-)

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Martian wrote 04/13/2018 at 02:01 point

Cool!

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