Welcome to the MacNN Forums.

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Mac Desktops > Why RAM must be installed in pairs?

Why RAM must be installed in pairs?
Thread Tools
Mac Elite
Join Date: May 2002
Location: Los Angeles, CA.
Status: Offline
Reply With Quote
Aug 28, 2004, 04:17 AM
 
Does anyone know of the technical reason, why RAM must be installed in pairs?


was curious why we're back to the old days of installing RAM in pairs
     
Mac Elite
Join Date: Jan 2001
Status: Offline
Reply With Quote
Aug 28, 2004, 05:07 AM
 
I think it's just for dual processors - and if so, then one for each.

I could be way off though.
     
Senior User
Join Date: Nov 2000
Status: Offline
Reply With Quote
Aug 28, 2004, 05:23 AM
 
You're way off.

It's basically because the RAM isn't fast enough to be able to get to the speed Apple wants for their RAM. By requiring pairs of RAM you can basically double the speed of the RAM.

2 x DDR400 = 800MHz
On a 800, 900, 1000, 1250 MHz bus, you don't want to be using 400MHz RAM speeds now do you?

- proton
     
badtz  (op)
Mac Elite
Join Date: May 2002
Location: Los Angeles, CA.
Status: Offline
Reply With Quote
Aug 28, 2004, 05:34 AM
 
... but why would you NEED to install in pairs?


how would installing in twos double the bandwidth? why not install three?


     
Mac Elite
Join Date: Oct 2001
Location: Colorado Springs
Status: Offline
Reply With Quote
Aug 28, 2004, 08:20 AM
 
Originally posted by badtz:
... but why would you NEED to install in pairs?


how would installing in twos double the bandwidth? why not install three?


Dual Channel memory bus. Basically, two 400Mhz Channels (well, 200Mhz DDR). Each channel must have equal amounts of RAM. If you install two equally sized DIMMs in your system, one belongs to channel A, the other belongs to channel B, and they can be accessed simultaneously.

High-end chipsets on the PC side use the same concept. It works because modern processors like the G5, K8, and P4 have 128-bit memory busses, and DIMMs are 64-bits wide (2x64 bit channels to fully utilize the 128 bit bus). Some of those chipsets also support using single channel mode (if you don't want to use pairs of DIMMs), but only a moron would run their system that way (so don't be upset that the PowerMac G5 doesn't support this).

SIMMs usually had to be installed in pairs. The original Pentium had a 64 bit memory bus. 72 pin SIMMs were 32 bits wide, so to fully use the bus, you had to install memory in pairs (the first pentium systems used 72 pin SIMMs). A 486SX has a 32 bit bus and can use ONE 72 pin SIMM. 30 pin SIMMs were only 8-bits wide, so in a system like a 386SX, that had a 16 bit bus (the 386DX had a 32 bit bus), you had to install in pairs. On some old mac systems with 32 bit busses and 30 pin memory slots (some 68030 and a lot of the 68040 based systems) , you had to install your memory in sets of four.
Caffeinated Rhino Software -- Education and Training management software
     
badtz  (op)
Mac Elite
Join Date: May 2002
Location: Los Angeles, CA.
Status: Offline
Reply With Quote
Aug 29, 2004, 05:02 AM
 
jcadam,

perfect answer! EXACTLY what I needed

Thanks mucho!
     
Fresh-Faced Recruit
Join Date: Aug 2004
Location: Southern Ontario
Status: Offline
Reply With Quote
Aug 29, 2004, 09:13 PM
 
So with this dual channel ram and the size of the FSB, would it be possible to install some higher clocked ram and get benifits from it??
     
Mac Elite
Join Date: Aug 2001
Status: Offline
Reply With Quote
Aug 29, 2004, 11:12 PM
 
Originally posted by sphinx:
So with this dual channel ram and the size of the FSB, would it be possible to install some higher clocked ram and get benifits from it??
No. It has two double data rate 200MHz hookups to memory, allowing for dual channel DDR400. Anything faster than that runs faster than its connection to the northbridge does and is essentially useless.

<edit> if Apple upgraded the northbridge, though, the G5's FSB is certainly capable of handling faster ram. </edit>
     
Grizzled Veteran
Join Date: Apr 2001
Status: Offline
Reply With Quote
Aug 30, 2004, 09:56 PM
 
Quick lesson on how memory (DRAM variety) works:

To write to memory, you send an electronic signal to a capacitor. You either charge it, or don't (1 or 0).

To read, you check the capacitorto see if it is full or empty. The problem is, checking it always drains it, so you have to then write the same data back to the capacitor.

So, because you have to rewrite, you loose a bit of speed here, as a write or read closes off the row and column that capacitor is in on the memory chip.

Thus, if you interleave your memory (aka, install two identical DIMMs), the system can alternate back and forth, writing the first bit to the first dimm, and the second bit to the second dimm. When you go to read, you can read the first bit, and then read the second bit right away as the first dimm refreshes the memory.
<This space under renovation>
     
Junior Member
Join Date: Oct 2001
Status: Offline
Reply With Quote
Aug 31, 2004, 01:08 AM
 
Originally posted by timmerk:
I think it's just for dual processors - and if so, then one for each.

I could be way off though.
I'm sorry man, but I totally just pissed my goddamn pants at how stupid that post was. But like, I mean that in a nice way.
     
Mac Elite
Join Date: Oct 2001
Location: Colorado Springs
Status: Offline
Reply With Quote
Sep 1, 2004, 02:29 PM
 
Originally posted by Drakino:
Quick lesson on how memory (DRAM variety) works:

To write to memory, you send an electronic signal to a capacitor. You either charge it, or don't (1 or 0).

To read, you check the capacitorto see if it is full or empty. The problem is, checking it always drains it, so you have to then write the same data back to the capacitor.

So, because you have to rewrite, you loose a bit of speed here, as a write or read closes off the row and column that capacitor is in on the memory chip.

Thus, if you interleave your memory (aka, install two identical DIMMs), the system can alternate back and forth, writing the first bit to the first dimm, and the second bit to the second dimm. When you go to read, you can read the first bit, and then read the second bit right away as the first dimm refreshes the memory.
This is why the next PowerMac needs to switch to an ALL SRAM memory system. Anyone mind paying $15,000 for a PM?
Caffeinated Rhino Software -- Education and Training management software
     
   
Thread Tools
Forum Links
Forum Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On
Top
Privacy Policy
All times are GMT -4. The time now is 03:31 PM.
All contents of these forums © 1995-2015 MacNN. All rights reserved.
Branding + Design: www.gesamtbild.com
vBulletin v.3.8.8 © 2000-2015, Jelsoft Enterprises Ltd., Content Relevant URLs by vBSEO 3.3.2