Sinclair ZX80 / ZX81 / Z88 Forums

I learned that there are two ROMs, a first, buggy one and a newer (less older, ) debugged one. There's an easy way to know what ROM I have, a PEEK on an address or something?

If I have the buggy ROM, should I worry?

Thanks in advice!

Err, we now know that there are THREE different ZX81 ROMs that were used in production ZX81s. See this topic. There is also a test program later on in that topic.

In addition, the first ROM, the one with the maths bug, ZX81s can have an extra circuit fitted that “intercepts” the data when the Z80 reads the dodgy part of the bugged ROM, this kinda corrects the maths bug. There is a bit of information about this here (but I have not yet updated this with the recent discovery of the third ROM).

So in total, that’s FOUR variations.

Here is a BASIC test program (courtesy of Moggy):

10 FAST
20 LET A = 0
30 FOR B = 0 TO 8191
40 LET A = A + PEEK B
50 NEXT B
55 PRINT “ROM TEST”
60 PRINT A
70 PRINT SQR 0.25

Note that it takes a while and while running, you will get a black screen.

Note the numbers it reports and report what it says.

Mark

Ref: ROMchip version test

I know I'm old fat and silly but I honestly cannot remember supplying that ROM checker.


If it is an NEC ROM then the it should be numbered either 550-(early bugged ROM) 622-(cured maths bug but supposedly faulty pause routine) 649-(last of the line bugs ironed out albeit not quite accurate with how numbers are held but can be worked around from BASIC)

1024MAK wrote: ↑Wed Mar 17, 2021 10:35 am
10 FAST
20 LET A = 0
30 FOR B = 0 TO 8191
40 LET A = A + PEEK B
50 NEXT B
55 PRINT “ROM TEST”
60 PRINT A
70 PRINT SQR 0.25

Note that it takes a while and while running, you will get a black screen.

Note the numbers it reports and report what it says.

Mark

ROM TEST
855106
0.5

That checksum is for the 2nd ROM issue 622. Division bug cured but supposedly having a fault which causes the listing to program vanish when PAUSE used without a suitable following POKE.

I only observed this once and try as I might cannot get it to manifest itself again.

I read the other post, it seems that I have the NEC ROM 622 (checksum 855106)

Thanks a lot!

For reference the three ROM types checksums.

550-854885

622-855106

649-878117

Other relevant links:

Test ROM program

And presumably where this program originated

ZX81 Versions/Issues

Topic with photos of the extra board to correct the ROM bug

Mark

I am seriously doubting my sanity as I have no remembrance of this at all.

Moggy wrote: ↑Wed Mar 17, 2021 2:29 pm For reference the three ROM types checksums.

550-854885

622-855106

649-878117

Well I have this ROM Placed on a CZ1500 (Argentinian Timex Portugal rebranded TS1500) . With this result:
Then I thought it would be a 649 but I have a 622 checksum result..