Date: Sat, 23 Jul 2005 11:41:29 +1000
From: David Elmo <[EMAIL PROTECTED]>
I will get a new battery soon, but I simply do not understand this stuff and
never have. I don't even know why it does not lose the date and time when
you yank the battery out and replace it a minute later? Is there some
capacitor maybe holding a bit of charge to keep it going temporarily. The
pram circuit is a big secret, a huge mystery, even theologians would argue
the toss on this for years...
I missed the early discussion, so this may be a bit irrelevant...
Both the PRAM and the NVRAM save some system settings (different for
each of them) which need to be available either before the OS loads
(before hard drive access) or for which it is convenient to be the
same no matter which volume you boot from.
When there is no power to the machine, they are both maintained by
the PRAM battery. If there is power to the machine, but the machine
is off, then they are maintained by the 5V trickle.
The PRAM physically resides in the CUDA chip which also acts as the
ADB controller. It is (apparently) a 6510 processor or derivative
and some integrated memory and a real time clock. The chip itself is
a 28 pin SOIC (small outline integrated circuit) surface mount chip
labeled 341S0788. PRAM has been around on the Mac since the
beginning, with some mutations. Of course, the PRAM and ADB
controller did not use to be integrated together. That happened
somewhere in Quadra time. CUDA chips from old Quadras can be used on
PCI Macs which need one.
The NVRAM physically resides in a memory chip. The memory chip is a
type of memory called Static RAM. Static RAM needs very little power
to maintain its contents. The most noteworthy resident here is the
NVRAMrc file, which is a file of Open Firmware code which is executed
at boot time before the computer accesses any disk drives. Some
extensions (and other programs) insert patches into the NVRAMrc to
fix firmware bugs, or add capabilities. I believe that "Virtual
Firmware" feature of one of the CPU upgrade sellers is actually an
Open Firmware patch implemented via a NVRAMrc insertion. NVRAM was
introduced with the PCI PowerMacintoshes because those are the first
to use Open Firmware and NVRAM is all about supporting Open Firmware
features.
The NVRAM chip is also a 28 pin SOIC surface mount chip. Static RAM
in this capacity (8 KB) was a comodity part, so you may see any of
several manufacturers' chips filling the NVRAM position on the board.
I've seen Sony, Winbond (always(?) on Umax machines), Samsung and
Hyundai (, and Hynix?). The part number will probably have a '64'
near the end, because these parts are sold by their capacity in bits,
and 8 KB is 64 Kbits.
If your PRAM battery is low and the machine is left without power,
the contents of these chips don't necessarily go to zeros, resetting
everything to default values. Some cells may lose their contents,
while other cells manage to keep theirs. This can result in some
bizarre and unintended contents in the PRAM or NVRAM.
I don't think that corrupted PRAM contents could actually cause data
corruption, because I can't think of any parameter stored there that
would be that catastrophic. I think the worst that the PRAM could do
is reset your boot volume to something unintended, which could be
(but probably wouldn't be) messy if you have a version of the OS on
that volume which is older than your machine supports.
Corrupt NVRAMrc contents can do all kinds of bizarre things and
*might* cause data corruption depending on the luck of the draw.
One particularly virulent NVRAM corruption occurs on the Umax
S900/J700. If you load PowerLogix's E100 Enabler (lets the
SCSI/Fast Enet card work with later OSs) in your extensions and also
enable or disable speculative processing in the wrong order, then the
machine won't boot any more. It will chime on power up and just sit
at the black screen. I can't remember the exact details of how it
happens, but it's quite disconcerting when it appears that you've
killed your machine and often it isn't even apparent that the E100
Enabler caused the problem. Similar symptoms sometimes occur with a
bad OSX install apparently.
If you've got a NVRAM corruption problem you can usually get past it
with a preemptive PRAM zap. This consists of getting the cmd-opt-p-r
keys down before the machine stops chiming. If you press the keys
this early, it behaves differently than a later PRAM zap and appears
to bypass the NVRAM--or something. I don't know exactly what it
does, but it will get you past a black screen machine, if the cause
is corrupted NVRAM.
Heh. But because I haven't been following this thread, I don't if
any of this is relevant to the problem at hand. I just saw the words
"pram" and "big secret" and data dumped....
Jeff Walther
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