Excellent & detailed write-up. The later ZX81 is best regarded as a ZX80 with some minor (but important!) tweaks.
Hard to understand, is effect of the semi-separated data buses (D0..D7 vs D0'..D7'). This is an intricate play between cpu, ROM code, signal timing & external hardware (external to cpu, that is).
Aeons ago I designed a ZX81 ULA replacement in programmable logic, and couldn't shake the feeling that with adjusted signal timing, it might be possible to get things working on a single 'unified' data bus. If I ever touch this again, I'd have a serious look at that.
But I suspect the way it was done, is because the signal timings allow for very slow ROMs and/or RAMs (up to ~450 ns iirc). This was important, since (to keep costs down) Sir Clive was buying memory ICs wherever he could find 'em cheap. Remember £99 for a real, user-programmable computer was the ZX80's #1 selling point.
As gxd noted, this was when people did not have computers in their homes. No PC, laptop, tablet or smartphone. NONE. Perhaps a pong console if lucky. So people were very curious & eager to find out what could be done with such a machine. Regardless its limitations, or how crappy its keyboard ;-)
Despite quickly becoming obsolete, the early ZX computers (ZX80 and ZX81) were a marvel of cost-benefit at the time. Most people had no access to computing of any kind... The difference between no computing and a machine that can perform thousands of operations per second is massive.
Over 40 years ago I worked with a guy who had bought a ZX80, and had designed and built his own expansion systems for it. Eventually it reached the point where it consumed most of the space in one of the rooms of his house.
He kept the thing running for many years beyond the point where the technology was obsolete.
(2019, who cares :-)
Excellent & detailed write-up. The later ZX81 is best regarded as a ZX80 with some minor (but important!) tweaks.
Hard to understand, is effect of the semi-separated data buses (D0..D7 vs D0'..D7'). This is an intricate play between cpu, ROM code, signal timing & external hardware (external to cpu, that is).
Aeons ago I designed a ZX81 ULA replacement in programmable logic, and couldn't shake the feeling that with adjusted signal timing, it might be possible to get things working on a single 'unified' data bus. If I ever touch this again, I'd have a serious look at that.
But I suspect the way it was done, is because the signal timings allow for very slow ROMs and/or RAMs (up to ~450 ns iirc). This was important, since (to keep costs down) Sir Clive was buying memory ICs wherever he could find 'em cheap. Remember £99 for a real, user-programmable computer was the ZX80's #1 selling point.
As gxd noted, this was when people did not have computers in their homes. No PC, laptop, tablet or smartphone. NONE. Perhaps a pong console if lucky. So people were very curious & eager to find out what could be done with such a machine. Regardless its limitations, or how crappy its keyboard ;-)
Despite quickly becoming obsolete, the early ZX computers (ZX80 and ZX81) were a marvel of cost-benefit at the time. Most people had no access to computing of any kind... The difference between no computing and a machine that can perform thousands of operations per second is massive.
This book
https://archive.org/details/Cheap_Video_Cookbook_Don_Lancast...
talks about how to build homebrew video systems similar to the ZX80.
Over 40 years ago I worked with a guy who had bought a ZX80, and had designed and built his own expansion systems for it. Eventually it reached the point where it consumed most of the space in one of the rooms of his house.
He kept the thing running for many years beyond the point where the technology was obsolete.
Back in the early 1980's this could also be bought at electronic shops and be mounted at home, DYI style.
I remember seeing them on the electronics store my father used to go to, for work related components.
Yeah, let's go back to the ZX80 (or 81) because we can all afford 1kb RAM (some of use might even get the 16kb extension).