Questions on a ZX81

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andychak
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Questions on a ZX81

Post by andychak »

So I recently acquired a ZX81 and I was looking to do some upgrades (RAM upgrade + composite video mod).
However on taking it apart I've found a board piggybacked on the CPU.

I cant seem to find any other similar boards from some general googling.

Can anyone shed any light on this ?

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1024MAK
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Re: Questions on a ZX81

Post by 1024MAK »

Don’t read this if you have (a) been drinking down the pub or at home or (b) you have a headache...

First there was the ZX80

Then Sinclair launched the ZX81

For existing owners, Sinclair offered the ZX81 ROM to ZX80 users, as it had been designed to be backwards compatible (see note). So some ZX80 users upgraded from the ‘old’ ROM to the ‘new’ ROM.

However, a bit later in time, a maths bug was discovered in the ZX81 ROM.

Sinclair though had a stock of these ROM chips. And being mask ROM chips, it’s not possible to change the code...

So for a quick fix, Sinclair came up with a hardware ‘fix’. What you have in your machine is a extra board mounted on the Z80 CPU that detects when the ROM location that contains the bug is accessed, it then changes the value on the data bus. Hence overrides the data from the ROM [this is the simple version, it’s a bit more complex than this].

Of course, soon after, Sinclair sorted out the bug and got a supply of new bug fixed ZX81 ROM chips....

There is more on this subject over on Sinclair ZX World forums [Relevant links added in the post below, this is just to the home page].

Note: Simply putting a ZX81 ROM in a ZX80 can’t magically add the extra circuitry to enable compute and display, so you can only use FAST mode, not SLOW mode.

Mark
Last edited by 1024MAK on Thu May 02, 2019 6:39 pm, edited 1 time in total.
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1024MAK
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Re: Questions on a ZX81

Post by 1024MAK »

:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
andychak
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Posts: 4
Joined: Thu May 02, 2019 5:02 pm

Re: Questions on a ZX81

Post by andychak »

Thanks! That explains that then.
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