The 64KB of RAM for the 65C102 co-processor is provided by eight MB8264 64k x 1-bit chips. Each chip provides one bit of memory for every address, meaning that all eight chips are used for every memory location. In many instances, the failure of a RAM chip will affect all memory locations (including the 6502's page 0 registers), meaning that the co-processor won't be able to boot far enough to run any memory tests! But let's assume that the co-processor boots, and you want to test the rest of the memory.
Before you start testing specific memory locations, you'll need to know what use is made of different locations. The User Guide for the 65C102 Co-Processor includes a memory map for the host (I/O) processor and the co-processor. When using Hi-BASIC, it is as follows:
Used by MOS
HIMEM -> -------------- &C000
PAGE --> -------------- &0800
Used by MOS
Note that the I/O processor will also have a memory space with addresses from &0000 to &FFFF. Because programs can access memory from either processor, references to the co-processor's memory are prefixed with FFFF. So the command ?&2000=&55 would write the hexadecimal value &55 at &2000 in the I/O processor's memory, but ?&FFFF2000=&55 would write it at &2000 in the co-processor's memory.
Testing Memory Locations (without crashing the machine)
In my experience, most BBC Micro and Master RAM faults show up at many (if not all) possible locations, so you shouldn't worry about testing the highest and lowest 2K of RAM. Between PAGE and HIMEM, BASIC will arrange it's variables as follows:
| BASIC |
| stack |
+-----------+ STACK ^
| free | |
| space | |
+-----------+ VARTOP increasing
| heap | memory
| program |
So any addresses between the values returned by STACK and VARTOP are safe for you to PEEK and POKE at as much as you like. Writing and reading a selection of values (e.g. &00, &5A, &A5, %FF) from each address, and comparing the output with what you wrote, is a good strategy. You can use the PEEK and POKE commands or the indirect operators to do so. There are a few example BASIC programs on this forum thread to help you get started. Remember to prefix your memory locations with FFFF to use the co-processor's memory!
If you want to test the area of co-processor memory used for the language ROM (e.g. BASIC), this memory space is written to by the Master when copying BASIC there in the first place. You can read it back and compare it against the original. I suggest using BASIC rather than Hi-BASIC for this, as you can then test directly against the copy in the I/O processor's memory: testing for ?&FFFF8000 <> ?$8000 (and looping through to $BFFF) will pick out any memory locations that don't match.
Additionally, when using regular BASIC (instead of Hi-BASIC) on the co-processor, the 14k of memory from &C000 to &F7FF is free RAM, so you can test this memory be PEEKing and POKEing with the same BASIC programs as earlier.
Identifying the Faulty Chip
If you find any faults when writing to and reading from a memory location, you should be able to identify which bits of that byte are incorrect. The bits D0 to D7 (from most to least significant) correspond to the RAM chips IC18 to IC25 on the co-processor board, as seen in this circuit diagram.
D0 is provided by IC18
D1 is provided by IC19
D2 is provided by IC20
D3 is provided by IC21
D4 is provided by IC22
D5 is provided by IC23
D6 is provided by IC24
D7 is provided by IC25