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ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 3:01 pm
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 This also had some small error and was superceded later that year.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’?
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 5:30 pm
by Moggy
Deleted due to erroneous rubbish spouted on my part without checking things out!
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 5:50 pm
by Lardo Boffin
I did wonder about the vagueness of the comments about the ‘some small error’.
https://www.ebay.co.uk/itm/Complete-set ... 890.l49292
And of course about never having heard of it before.
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 6:03 pm
by Moggy
Deleted for reasons mentioned above.
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 6:11 pm
by Lardo Boffin
I seem to have a couple of 622s as well. Not that rare I guess!
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 6:14 pm
by Moggy
Do I spy a Motorola chip there Lardo?
That's one I never got my mitts on.
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 6:16 pm
by Moggy
A 649 too.
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 7:58 pm
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!!
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 10:06 pm
by 1024MAK
Now need a binary dump of the last two to see where the differences are...
Mark
Re: ZX81 ROMs 3 types?
Posted: Sun Jan 17, 2021 10:54 pm
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.