Hi Paul,
You wrote:
>... looking at the various sites for memory brokers ... there seems
>to be a few different flavors of the PC100 SDRAM that are required
>for the G4 [with Gigabit Ethernet] (one site touted something called
>2-2-2 RAM). Looking at Mac-specific retailers, the prices were
>much, much higher for what would, at first glance, seem to be the
>same PC100 chips.
>
>Can anyone provide some insight on the difference between all these
>chips? Will anything described as PC100 work in these boxes without
>compromising performance?
Intel's PC100 SDRAM DIMMs conform to an Intel memory standard --
specifying trace lengths and widths, the number of printed circuit
board layers, and other values -- that helps ensure that packaged
PC100 DIMMs will work well with system buses running at 100 MHz.
One attribute of PC100 DIMMs is their Column Access Strobe (or
"CAS") latency: the speed at which other components can access a
single column of RAM. Memory with a CAS latency speed of 2 can be
accessed 33% faster than memory with a CAS latency speed of 3.
Some of the typical latency speed codes associated with various
kinds of PC100 DIMMs, ordered from slowest to fastest (and generally
also, from least to most expensive), are:
3-3-3 (found in earlier PC100 DIMMs or the slower PC66 DIMMs)
3-2-3
3-2-2
2-2-2
Under some circumstances -- and presumably only on newer Macintosh
models capable of using PC100 DIMMs -- the Apple System Profiler
application is apparently capable of reporting on the CAS latency
values of installed DIMMS. This is described in the AppleCare Tech
Info Library article:
http://til.info.apple.com/techinfo.nsf/artnum/n88005
Apple's developer notes and technical specifications for the Power
Macintosh G4 (Gigabit Ethernet) only appears to mention that standard
PC100 DRAM should be used with this Macintosh model. These
specifications do not appear to require the use of 2-2-2 DIMMs:
Developer notes:
<URL:http://developer.apple.com/techpubs/hardware/Developer_Notes/Macintosh_CPUs-G4/PowerMac_G4/PMG4.4e.html>
Technical specifications:
http://til.info.apple.com/techinfo.nsf/artnum/n58666
http://www.info.apple.com/info.apple.com/applespec/applespec.taf?RID=394
At least in the Blue & White G3 models, the slowest DIMM will
dictate the speed of memory access. Thus, to achieve performance
benefits (if any) from using 2-2-2 latency DIMMs, all of the
installed DIMMs must be identical. (Apple's memory controller in
this model is apparently configured to detect CAS latencies ranging
from 1 to 3.)
I've come across conflicting reports as to whether using 2-2-2
DIMMs in these older Macintosh models offer any measurable
performance advantages that might justify the higher price for these
faster DIMMs. There does not appear to be any widely-recognized,
compelling reason to use faster PC100 DIMMs in these models, however.
Regarding G4-based models (such as your Power Macintosh G4 Gigabit
Ethernet model), I wasn't readily able to find any comparable
performance claims. Perhaps some other, more hardware-oriented
MAGNet members might be able to comment on this.
As an aside, one PC-oriented RAM vendor, Savin Hill Computer,
claims that 2-2-2 PC100 DIMMs, apart from any performance advantages
which they may (or may not) offer:
- May be more stable as internal temperature and power requirements change.
- Can work with bus speeds as high as 125 MHz.
Aron Roberts
Workstation Software Support Group
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