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I'm designing an ISA and processor.
If anyone here is involved with low-level dev or processor design: what do you think of using non-8-bit bytes?
I'm thinking of making my bytes somewhere between 9 and 12 bits wide, with two bytes long words (between 18 and 24).
It would have a lot of advantages, such as more characters fitting into a byte, a larger address space, and more total information that can be stored in memory, but what do you all think?
2 years ago · 👍 justyb, bowman
Also, although I'm not doing this for any profit, I still have some useful goals for it, namely a nice, open, CISC processor that is just designed the way I want it to be designed.
I did spend most of my time on the general arch, but byte size is something that I think should be established early on in the design process. · 2 years ago
@justyb KISS is exactly why I'm considering departing from the standard, two byte words make my life significantly easier than three or four byte words would.
Also, with more characters I meant to solve exactly the problem you presented -- a 12-bit byte could store 4096 different values, and so I'd be able to fit more different characters into a single byte (in my plans, the entire Latin and Cyrillic alphabet, possibly some others and a bunch of misc characters).
I think the hardest part by far will be finding the right memory bus size and friends. · 2 years ago
I'm not really sure what you're design is, but the computation is less an issue now with higher speed chips and the relative cheap cost of them. Especially a hobby computer. ---- So don't worry about pros and cons for a hobby computer. Just do it, have a 18-bit byte or whatever, will bring some "oooo and aaahh" factor to your build if you should ever show it in some of the breadboard computer circles. ---- So if you're serious about your hobby build, worry less about 6-bit, 8-bit, whatever and more about the overall arch. And if this is your first build. K.I.S.S. Keep It Simple. Don't over engineer it, biggest fatal mistake by everyone. · 2 years ago
Advantage wise. You can use 5-bit baudot codes if you really want to go old school, but you get only caps. Going to 6-bit gives you lowwer and upper case, but few puncuation. 7-bit is when you first get somewhere most people found acceptable. ---- Now as you noted, you can encode in two bytes. But that'll carry it's own con in computation. Especially in char to char comparison. ---- The problem back in the day was processing was slow, so two byte chars were a problem, and so you did it if you had to, but you really wanted to not to. · 2 years ago
Nothing stopping you. A lot of older systems used to have six-bit bytes and did the whole 18 and 24. 8 was just a convenient power of two and eventually became codified. ---- Now it pratical terms, you'll find most bus tri-state transceivers, d-latches, up/down counters, and so forth as either octal or quad. There's a few of those that are hex that would let you do what you are aiming for. 74LS365 springs to mind. ---- Also, a lot of character displays are for 8-bit ASCII, so you'll need conversion, maybe in an EEPROM, to convert between encodings. · 2 years ago
Dont know jack about any of that but I am curious Is this a hobby project or something to do with your professional work? What drove you to start on this? · 2 years ago