FAQ: MILO gets memory size wrong

[Jay.Estabrook@digital.com]

>>> Wojtek Puchar said:
>
> MILO detects only 24MB of RAM (I have 2x32MB RAM)
> 
> I boot MILO from SRM console.
> 
> I will try using aboot...

There are two ways in which the LINUX kernel will have accurate information
about how much memory is installed in the box:

1. boot the kernel *DIRECTLY* from SRM console (via aboot).

2. boot the kernel via MILO from ARC or AlphaBIOS console.

The LINUX kernel expects an array of "memory descriptors" to be pointed to
by the HWRPB (see the Alpha Architecture Reference Manual for details on
what the HWRPB is, and what the memory descriptors look like).

These "descriptors" describe the use of main memory pages (basically "free"
or "occupied").

#1 works because the SRM console sets up the HWRPB and "memory descriptors",
while reserving some memory for itself, the HWRPB, and PALcode.

#2 works because "linload.exe" passes along to MILO memory size information,
that it itself gets from ARC or AlphaBIOS. MILO then builds the appropriate
"memory descriptors" for the amount of memory it has been told, also keeping
some pages for itself, PALcode, HWRPB.

So, if you boot MILO from SRM console, MILO gets *NO* information about the
size of memory; in this case, which it fortunately detects, it chooses a
default size (24 for NONAME/UDB, 32 for the others, IIRC), and builds the
"memory descriptors" for that size.

Using "set MEMORY_SIZE 64", or some such, will work, as MILO will re-do the
memory descriptors appropriately. But it's not the correct solution, IMHO.

Remember, on many ALpha platforms, kernels built to boot from SRM will *not*
boot from MILO, and vice versa. Fortunately, I believe NONAME/UDB is one of
those platforms whose kernels will boot from either... ;-}

--Jay++

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Jay A Estabrook                         Alpha Motherboards - LINUX Project
Digital Equipment Corp.                 (508) 841-3241 or (DTN) 237-3241
334 South Street, Shrewsbury, MA 01545  Jay.Estabrook@digital.com
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