Where's the Plus-4 3+1 ROM kept?

[WimWalther]

Howdy folks,

I've a Plus-4:here in generally good working order, save one thing: The 3-Plus-1 ROM software crashes when I try to run it.

On invocation, it displays the copyright / credit screen. But next, it either shows a black screen with a single "P" in the lower left, or very rarely, it crashes to the monitor.

First order of business is verifying the ROM contents, but I'm unable to find out where in the memory map these ROMs reside. I assume they're normally banked out, so how can I expose them for testing?

Thanks for any help.

 

[WimWalther]

I learned from another source that the 32K of 3+1 ROM shares the space from 0x8000 to 0xFFFF that's normally occupied by BASIC, KERNAL and memory mapped IO registers.

The latter can be banked out to access the 3+1 ROMs, but this renders BASIC unusable, so any software that reads the ROMs has to be ML sans any KERNAL calls.

Might really be easier to pull the chips and use an external EPROM burner / reader to verify contents.