A Question about SIMMS for IRIS 4D/25

[Wiltse Carpenter]

In article <9001051539.AA13873 at aero4.larc.nasa.gov>, blbates at AERO4.LARC.NASA.GOV ("Brent L. Bates AAD/TAB MS294 x42854") writes:
> 
>     I have been told that the 4D/20 and 4D/25 use the same memory.
> However, if you want 32Mb of memory on the main board you have to use
> 4Mb SIMMS, ALL of them must be the same.  So, if you currently have
> 1Mb SIMMS, you will have to remove them and replace them with the
> 4Mb SIMMS.

Whoever told you that 4D/20 and 4D/25 machines use the same memory was
right.  However, the 32MB maximum memory is achieved by using double-
high 1 MB SIMMs, not 4MB ones.  They don't all have to be the same, but
there are restrictions on mixing them.  Basically, it doesn't work to
have ``holes'' in the physical memory map.  Complicating the matter is
the way memory addressing works with dual height SIMMS: one layer at
a time.  So, all of the SIMM sockets must at least have single high
SIMMs in them before the first set of sockets can support double-high
ones.  4MB SIMMs haven't yet been qualified for the 4D/2x line and
may require a PROM fix to some machines; we're working on it.

	-Wiltse Carpenter