NMS8280 vwry strange basic math division or multiplication error (defect ??? ) --> OVERFLOW

Expert (83)

21-12-2017, 22:43

I'm posting this strange problem here because I still believe it's hardware related... (maybe rom defect? But the issue is well confined to math operations (!))
I just repaired an NMS8280 (power supply faulty, required one diode rectifier replacement).
I was testing the overall functioning and it seems to work fine with any rom or cartridge I have... but...
A very strange behaviour occurs on some demo or test programs such as testram or the monitor test by NYYRIKKI (which stops with an overflow wthin a 'circle' statement in test)

I traced down the problem beginning from the suspect instructions reading the H2B location that stores monitor frequency , but then I found that the problem occurs on any division made

but the strangest thing occurs using a variable to store a value :

I don't believe it's a common NMS8280 defect but if anyone wants to try in his own... (I tried same configuration on BlueMSX and it behaves just as expected...)

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By marcoexo

Expert (83)

21-12-2017, 22:50

...testram (once executed deleting the lines that give overflow (they'r used just to display frequency and ram size) reports no error on ram or video ram... it reports ok any other mapper testing tool I used.

By FiXato

Scribe (1743)

21-12-2017, 23:12

Your last image requires me to sign in to 1drive before viewing it.

By marcoexo

Expert (83)

21-12-2017, 23:19

..should be ok now

By Grauw

Ascended (10768)

22-12-2017, 00:42

Maybe the BIOS / Basic ROM has gone bad? Dump the ROM, compare to existing ones online.

By marcoexo

Expert (83)

22-12-2017, 02:00

Ok, .... Any tool to dump the rom ? I have a minipro reader/programmer but roms are quite difficult to get access to...

(all programs work fine (also video digitizing sw) so I also think that a problem in basic rom exists)

By gdx

Enlighted (6215)

22-12-2017, 09:17

http://bifi.msxnet.org/msxnet/utils/saverom.html

You can also dump the main rom with the program Basic below.

10 CLEAR300,&H9000
20 FOR I=0 TO &H3FFF: POKE&H9000+I,PEEK(I): NEXT
30 BSAVE"main-rom.1",&H9000,&HCFFF
40 FOR I=&H4000 TO &H7FFF: POKE&H5000+I,PEEK(I): NEXT
50 BSAVE"main-rom.2",&H9000,&HCFFF

By NYYRIKKI

Enlighted (6067)

22-12-2017, 10:14

I still think this is a RAM problem... and I don't think gdx solution is very good since if it is, it is going to affect also the dump and not tell witch was the real problem... What you get, if you type PRINT FRE(0) ?

By gdx

Enlighted (6215)

22-12-2017, 11:10

Marcoexo can use TESTRAM, MEMTEST or TESTMAP to test the RAM.

By marcoexo

Expert (83)

22-12-2017, 11:46

Thanks for info, saverom very useful... Dumped BASIC Rom and it does not match on a few bytes, going to take it out and verify with reader...