[Arm-netbook] laptop main board, power board and ingenic jz4775 cpu card

[Luke Kenneth Casson Leighton]

On Tue, Nov 24, 2015 at 3:30 PM, Paul Boddie <paul at boddie.org.uk> wrote:
> On Tuesday 24. November 2015 14.42.44 Luke Kenneth Casson Leighton wrote:
>> On Tue, Nov 24, 2015 at 9:06 AM, Paul Boddie <paul at boddie.org.uk> wrote:
>> > This is great news! I also like the 2GB RAM support - with many ARM
>> > single- board computers still only supporting 1GB, it makes sense to go
>> > for the maximum here -
>>
>>  those kinds of decisions i believe are usually made based on the
>> faulty logical analysis which is summarised as "but... but.... the
>> processor costs less than the memory!!!"

> I suppose that you could cluster several of them if they each only provided,
> say, 1GB RAM, but then you need the convenient infrastructure to make that
> easy. Indeed, a multi-slot cluster unit would be nice with EOMA-68, and I
> think someone mentioned something similar earlier in the year.

 yes.  primary justification for this is to enable rack-mount
low-power modular servers.

> As I understand it, 32-bit MIPS divides the addressable memory space in two,
> meaning that 2GB of actual memory is the limit.

  rright.  as ingenic's MIPS is home-grown they inform me that the
jz4775 can actually address up to 3GB of RAM.  however that would
involve a... rather complex board arrangement, of a mixture of ICs or
to simply put in 4GB DDR total and ignore 1GB of it.

> I haven't paid enough
> attention to 32-bit ARM recently to say whether similar limits apply there,

 absolutely they are - because the peripherals are all
memory-addressed, and the boot ROM also has to be addressed somehow.

 to make life a bit simpler (save some silicon space), usually only a
few of the address bits are utilised to make the decision "address
this peripheral yes/no".  so hilariously you can sometimes address the
exact same peripheral at regularly-spaced (large) memory intervals.
this was i believe the case with at least the intel pxa 270: it was
over 10 years ago but i do seem to recall seeing the exact same
peripherals at different addresses when reverse-engineering an XDA
smartphone.

 in short... yeah :)

l.