Mr. Backup Z64 FAQ

You must remember that the bios is just software,
that is if you upgrade the bios on a PCI only motherboard you won't
get AGP. That's the same with the support for 64MB SIMMS; the Z64 does
not have the correct dram refresh hardware, in fact if you want to
make a very expensive test you can (try) to get a 64M, 60ns EDO 5V
powed and data rails (3.3V, 5V tolerant part won't work -- even for
32M). Once you put the SIMM in and turn on the Z64 you will be greeted
by a memory error message and you won't be able to load even a 8M
game. To make the 64M simm work you'd need to rework (code and burn a
new one and replace the old) the asic that andles the DRAM to
PI interface; evenmore the revision 3 of the hardware has the ASIC
embedded in the CPU, so there's totally no way. Besides, there's more
or less no need for such thing now, as only 2 games are 512Mbits and
given a 64M SIMM costs around 150-200 euro, so even if it worked is
not worth the expense...

The Z64 is a pc compatible hardware running caldera OpenDOS, but the
device has 512KBytes of RAM (eventually expandable to 1M via soldering
one chip to the Z64 main board). The N64 "virtual cart" is handled by
an external memory area that is accessed by the Z64 CPU via an I/O
port. That is, it is NOT part of the main memory.

The virtual memory works by detecting accesses of the
CPU to a memory area that is marked via an MMU table to generate an
exception on access. When this happens, the OS/Driver/Whateve, saves
the least recently used block of memory to disk, and replaces it with
a block loaded from the disk, then the block is "remapped" via the MMU
at the address which caused the exception, and the execution is
resumed...

This means that to allow virtual memory to work, you'd need to detect
accesses from the N64, and load/unload parts of the memory. The
programs you mentioned can do that *ONLY* for the 386 cpu, so it's
useless for expanding the Z64. Even having a fast enough mechanism to
detect these accesses (but this mechanism does NOT exist), you
could not perform the "page-in", because the PI bus does not offer any
handshake line: the N64 assumes that the memory is always available,
and there's no way to stop it to allow loading.

The Z64 actually passes thru the "high half" of a 512M cart: in fact
you can play a 512M game by putting the original cart in the slot and
loading the Z64 DRAM with the lower half (256M) of the rom.

The trick i to build a device that has 256M of DRAM and behaves like
the "high half" of a cart. All you need is a PI<->dram bridge
hardwired to appear at 0x12000000 instead of 0x10000000 (as a normal
cart would do).

After this you need a bios for the Z64 able to load data into the
device...

In the end it is technically feasible, but the extremely small amount
of games and the relative complexity of the work to be done, coupled
with the cost of the parts and constructions do suggest that you'd
better buy the 512M carts you want...

You need an altera flex chip, the flex chip burner, the
altera compiler, some memory, pcb design tools, pcb etching
equipment, blank pcb, a 32 megs simm (which, atleast in Italy, it's
more than 50 euros alone), sockets and soldering tools. Assuming you
have the basics (soldering and etching tools) it will cost you around
100-150 euros (given you're going to build just one), assuming,
ofcourse that you get the pcb and pld right the first time...