have tracked down a weird CPU bug: if i do a soft reboot from within SETUP with CTRL-SETUP (a key combination from the the manual) the Z80 corrupts the 808x address space with stack data.

What i found out:

* BIOS begins an extensive memory test (BITTEST_AX) -


* problem gets visible at this location:


- BX is current address
- AL contains XOR result of correct and actual value

Z80 writes to $07f0/$07f1 (reflected at 808x offset $87f0/$87f1).

RESULT: ERROR 19 Main Board RAM (0-64K)

Why does the Z80 interfere with the memory test?

There is a reasonable suspicion the ZFLIP flag is set or handled wrongly. I just cant find where...

P.S.: floppy test on the DIAG.DISK (from 'indiviual tests') now aborts with a 'Z80 response error'. Coincidence?

Any ideas? :-)

Screenshot of Z80 caught in the act:


Last edited by Bavarese; 06/16/16 08:04 PM.