CrowTRobo writes:

I don't know why I didn't pick this up before, but while doing some
coding last night I noticed why the CD64 doesn't support 512mbits.

The CD64 has three memory maps. One is 'Cart' which maps the cart
plugged into the CD64 to the N64 cart address 0xb0000000. The second is
'Dram' which maps the CD64's DRAM to the cart address. The third one is
'Bios' which lets the N64 control the comm port, cd-drive, etc. Here is
some of the mapping:

0xb2000000 to 0xb4000000 = Cart
0xb4000000 to 0xb6000000 = DRAM

As you can see, in BIOS mode the CD64 can only address 256mbits of a
cart and dram. The memory mapping is controlled by the hardware, so in
order to support 512mbit, the CD64 would technically need new hardware
and a new bios.

But I've come up with an idea. In DRAM mode the CD64's ram is mapped to
the N64 cart space, so basically you can address any size that the N64
supports. What I am thinking is this:

In BIOS mode, receive say 1mbyte into a buffer. Switch to DRAM mode,
write that 1mbyte, then switch back to BIOS mode and repeat for
64mbytes. This gets around the 256mbit limit of the BIOS mode.

Of course this is all theory and I have no idea if it will work. I would
like to test it though. What I need is someone with a CD64 and a 64mbyte
simm that works with it. By works I mean that the CD64 will recognize
and use the first 32mbytes.

Any thoughts on this long message?

CrowTRobo

and then:

This idea is dead.  

You cannot write to cart space while in DRAM mode, so that kills the
whole theory.

On a positive note though, the PPA is no longer a problem.

CrowTRobo