IIfx SIMMs; Was: Re: Tristate Buffer Output if Input is High-Z?

[Jeff Walther]

>Date: Thu, 29 Sep 2005 11:55:18 -0700
>From: Eric J Korpela <korpela at ssl.berkeley.edu>
>Subject: Re: Tristate Buffer Output if Input is High-Z?

>Anyone have a datasheet for Hitachi HM511000AJP8 DRAMs? It's used in at
>least some IIfx SIMMS.
>
>What's the deal with parity on the IIfx? Is the parity bit stored off the
>SIMM? The technote
>http://developer.apple.com/technotes/hw/pdf/hw_25.pdfmentions parity,
>are there 9-chip SIMMS on the IIfx? Or is the parity stored
>on a separate RAM on the mainboard? Since your max memory is 8x(4Mx8bit bit
>SIMMS) accessed 32 bits at a time (I assume) you'd only need 1 Mbit of
>parity RAM if you do parity across 32 bits.

You are unlikely to ever see a IIfx that actually implements parity. 
It was an option for the IIfx but was very rare in practice.   In 
virtually all cases the IIfx uses an 8 bit 64 pin SIMM with no 
parity.  The presence of parity on the SIMM will not affect 
operation, because on a non-parity IIfx, the SIMM pins for parity 
connections are NC.

A IIfx SIMM built for parity will have 1/8 more capacity.  The extra 
storage for parity is on each individual SIMM.  The pinout for the 
SIMMs is in "The Guide to the Macintosh Family Hardware".  It's one 
of those Addison Wesley books.

I don't have that particular datasheet on hand.  But if you get one, 
I'd like a copy as well.  I imagine it's a pretty standard 1M X 1 
DRAM chip.

>Date: Thu, 29 Sep 2005 14:01:37 -0700
>From: Eric J Korpela <korpela at ssl.berkeley.edu>

>I also notice that the table on page 2 of the technote contradicts the IIfx
>section. The table says you can put up to 128MB in a IIfx, but the IIfx
>section says the biggest SIMM allowed is 256kx8.

I think you misread that somehow.  The tech note states that the IIfx 
does not support 256KB SIMMs.  It gives examples of memory 
configurations up to 32 MB, using 4 MB SIMMs, although there's a 
consistent typo in the example where megabit is written instead of 
megabyte.

But the maximum memory capacity is 128 MB, using eight 16 MB SIMMs. 
That was/is the maximum capacity for most of the Mac II family.  The 
exceptions are the original Mac II (68020 based), the Mac IIsi 
(supports 16 MB SIMMs, but only has 4 slots) and the abomination with 
variations known as the IIvi and IIvx.

As far as I know, there can't be 30 pin SIMMs with capacities larger 
than 16 MB, because there are only 12 address pins available and 12 X 
2 = 24 and 24 address bits yields 16M addresses.  There aren't any NC 
pins left to convert to additional address lines, unless one steals 
the parity pins.

Jeff Walther