"DIAMOND JEFF" <[EMAIL PROTECTED]> wrote in message
news:41819@palm-dev-forum...
>
> The PalmOS is set up to handle up to 256 megabyte cards and a
> theoretical 4 gigabytes of total address space.
>
> But the Dragonball cores only have 24 address lines, and so can only
> address 16 megabytes.
>
> Currently, system ROM requires separate addresses, forcing an 8 meg card
> limit. Now I suppose it would be possible to intercept the low 2
> megabytes into ROM and give the user a "14 megabyte" card (like the
> Atari did.)
>
> But short of switching to a different 68K core (like ColdFire) or ARM
> core, does the PalmOS have any tricks available that would allow
> addressing multiple 16 meg address segments?
The VZ chip has slightly better memory expansion possiblities. Already you
can have a VZ device with 16MB of RAM, 16MB of Flash in the Spri, and the
system ROM; witness some of the very tricked-out Visor Prisms out there.
You have to look at the whole pin-out. The Dragonball VZ may have 24
address lines, but there are also chip select lines that get used by the
memory manager hardware to pick a bank of memory. The VZ has four chip
select lines, plus another four which can be used in a static RAM system,
but are used as DRAM CAS/RAS lines otherwise.
Some of this chip select logic may be used for the Palm's memory protection
scheme; this is what lets the Palm detect writes to database memory. This
UPSIZ register in the Dragonball's logic is why the Palm's dynamic heap is
at most 256K -- if it grew larger, Palm couldn't do hardware write
protection as they would have to leave the entire chip select region
unprotected. I'm not sure what OS changes were made for the VZ -- the EZ
also
It looks like the original Dragonball, if I've decoded the Motorola data
book right, didn't have this protection region concept. They must have
aliased multiple chip selects to the same memory device, with the smaller
R/W memory region intercepted by the SIM first, and writes to other regions
seen by the wider ranged chip select and intercepted. The original DB also
has pinouts defined for A31:0, but only A23:0 are connected, as many of the
high address lines alias other system ports.
--
Ben Combee
Veriprise Wireless <http://www.veriprise.com>
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