Part 9) Figuring out the exact address space (RAM, more)
We know where the main ROM is visible, now what about the RAM and VideoBob?
Usually you would just write a few patterns here and there, read back and check if parts of what you wrote are copies of what you wrote previously, or if its there at all.
OK, but what do you write? AA$,55$,0,1,2,3,4...FF?
That is boring! What about ... Literature?!
I went to the Gutenberg project, took the first text I found, and WROTE it in the device memory. I used "LE GRAND MEAULNES" by Alain-Fournier.
You can't be fooled by literature much. It doesn't contain blatant word/page synced data!
TOP: Original, Bottom what I read back.
0000:0FFF UNIQUE RAM (same text and offset as source)
8000:F7FF UNIQUE RAM (same text and offset as source)
F800:F87F FF's (unmapped ...probably)
F880:F88F Weird combination of Read Only and R/W ... VideoBOB!
This means there is a minimum of $8800 (34816 Bytes) of RAM in there.
(possibly more paged)... but this HY6264 only contains $2000 (8KiB)??
I double checked and the HY6264's VCC is not tied to normal VCC but to a lithium battery. Maybe this is the so called "Option Table" from the Patent?
... In any case its probably only seen from VideoBob as its !WR line doesn't budge through writing the entire address space.
So this means either there is more than 32KiB of RAM on VideoBob?.
Or the CPU ram is shared with the Video Ram? A mix of the two?