ZX81 ROMs 3 types?

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
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ZX81 ROMs 3 types?

Post by Lardo Boffin »

I just saw an auction for three ZX81 ROM types. From the auction:


Type 550: This is the first release of ZX80 upgrade and ZX81 ROM with the infamous floating point maths error that caused square root of 0.25 to be calculated as 1.3591409 instead of 0.5. Sinclair corrected this error by including a small logic board in early ZX81s, but ZX80 owners who had upgraded could get a free replacement ROM on-request

Type 622: This is the ROM that was sent out to replace the 550 part. This also had some small error and was superceded later that year. This is the rarest of the 3 ROMs

Type 649: The final version of the Sinclair 8K ROM

I didn’t know there were three types? I knew about the two types but what is the 622 with ‘some small error’?
Last edited by Lardo Boffin on Fri Jan 22, 2021 2:32 pm, edited 2 times in total.
ZX80
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Moggy
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Re: ZX81 ROMs 3 types?

Post by Moggy »

Deleted due to erroneous rubbish spouted on my part without checking things out! :oops:
Last edited by Moggy on Sun Jan 17, 2021 7:47 pm, edited 6 times in total.
Lardo Boffin
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Re: ZX81 ROMs 3 types?

Post by Lardo Boffin »

I did wonder about the vagueness of the comments about the ‘some small error’. :D

https://www.ebay.co.uk/itm/Complete-set ... 890.l49292

And of course about never having heard of it before.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 ROMs 3 types?

Post by Moggy »

Deleted for reasons mentioned above.
Last edited by Moggy on Sun Jan 17, 2021 7:48 pm, edited 4 times in total.
Lardo Boffin
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Re: ZX81 ROMs 3 types?

Post by Lardo Boffin »

I seem to have a couple of 622s as well. Not that rare I guess!

9866D5AB-2F21-462F-86A5-BACFD50D9AFC.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 ROMs 3 types?

Post by Moggy »

Do I spy a Motorola chip there Lardo?

That's one I never got my mitts on. :(
Moggy
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Re: ZX81 ROMs 3 types?

Post by Moggy »

A 649 too.
Moggy
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Re: ZX81 ROMs 3 types?

Post by Moggy »

After opening my big gob as usual and casting doubt upon there being three types of ROM for the ZX81 it would seem (after getting my arse in gear and checking) that it is the case that three examples do exist.

Using Xsnap' s checksum program I obtained the following results..

Bugged ROM (NON NEC but a bugged example anyway) checksum-854885 OLD OK

NEC ROM 622 checksum 855106 NEW OK

NEC ROM 649 checksum 878117 ROM-FAULT.

Now this ROM works ok shows no other anomalies so can only conclude that there is in fact three versions of the ROM but what the differences are It would take someone a lot cleverer than myself to work out.

In future I will test things out before spouting a load of old cobblers!!
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1024MAK
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Re: ZX81 ROMs 3 types?

Post by 1024MAK »

Now need a binary dump of the last two to see where the differences are...

Mark
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Moggy
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Re: ZX81 ROMs 3 types?

Post by Moggy »

Mark I have all the time in the world for something like this.

Just instruct me how to go about it please and I will get on with it. I did think of dumping the ROM to a block in the ZXspand memory then saving as a P file,swap ROMs then same again but wouldn't that mean switching out the spand to access the standard ROM thus nullifying the spand P file save if you follow me? Either way let me know what's required and I'll give it a shout.
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