Question about PAE

Hey. I've been doing some research into 32/64 bit operating systems and how to use 4+GB of RAM. I think I have some of it down, but I still could use some clarification. Is this correct:

You have a machine with 8GB of memory, and PAE is enabled. 4GB of the memory is used by the OS, and 4GB is used by applications. Since its 4GB, even 32 bit software not set up to use PAE can use it.

I have a feeling that's not correct, but which part exactly is incorrect (or am I right?)

 

I probably sound like an idiot, but can't 32 bit systems only utilize 3 GB of ram?

 

I believe the technical limit is 4, though due to some other things (like space used for address mapping, not really clear on what that is), the amount of recognized memory might do down.

 

Your application must be optimized to make use of PAE. No consumer apps are optimized. PAE retards performance so seems silly to use more RAM I would presume for a performance increase when the method of making use of it in fact reduces performance? I do not know the upper limit of PAE.

PAE is useful for certain specialized situations, I have read using very large databases and some server functions.

PAE is not the answer for consumers. As I have said before in many threads. If you have to ask not a good idea to do.

 

/PAE was a way of old INTEL CPUs to recognize more that 3gb memory. It does not apply to AMD chips and new Intels. CoolGamer is correct. 32bit does recognize 4gb but after all the mapping it'll show up as 3.6 or less.

 

I know the consumer apps won't take advantage of PAE, but can the OS use it? In other words, if the OS sees 8GB of RAM, won't it give 4 to the applications, and 32-bit applications can use 4GB of RAM, can't they?


Also, perhaps a more important question, if I'm only getting 4GB of RAM, will a 64-bit OS help me at all? Is that fact that Vista recognizes only around 3.xGB related to the fact that it's 32-bit, or will that memory be used up even in 64-bit Vista?

 

No the OS will not make use of it in a way that is beneficial to the applications. You really have re asked the same question and the answer is still the same.

Your second question? I am confused. 32bit 2^32 is 4GB of memory addresses. RAM uses these so do other things. The total is 4GB if other components are using RAM cannot. 64bit 2^64 can make use of all 4GB of RAM as it has plenty of memory addresses. I believe some of the versions of Vista go as high as 128GB (artificial limitation). But remember chipsets are more likely to be the limiting factor. Some that support 8GB actually can only see 6.8GB (same issue as 32bit I think).

No reason to ever use PAE unless your software manufacturer says to.

 

Err... you lost be a bit there. What's the difference between memory and RAM? Forgetting about PAE:


machine A has a 32-bit OS and 4GB of memory installed.

machine B has a 64-bit OS and 4GB of memory installed.


Will machine B be better than machine A in some way, or will they be the same?

(sorry if this has gone off-topic)

 

Machine B will be better.
64-bit OS provides slightly better performance, as compared to a 32-bit OS. (esp. with 64-bit applications)

Machine B can support 4GB or more RAM. (Not considering any hardware limitations)

 

The 4GB limitation (32bit) is not on RAM. It is on memory addresses. Since more than RAM uses memory addresses RAM can never use the full 4GB.

32bit will only be able to make use of around 3GB of the 4GB of RAM installed. The OS will gain zero benefit from the extra. It might as well not even be there, it is unused.

The 64bit can make full use of all 4GB.

So machine B will have more RAM (usable).

 

http://www.codinghorror.com/blog/archives/000811.html

 

^^Nice find Andy, but I thought I said it.

 

I just wanted to show the nos. for 32bit and 64bit

(I don't know how to use the CODE tags )

 

Err... ok. My previous understanding of the situation was wrong.

The OS isn't benefiting, but is anything else? It might as well not be there as far as the OS in concerned? Or as far as the entire system is concerned?


If you had a 32-bit machine with 3GB of memory, would all 3GB of the memory be available, unlike the way that when you have 4GB, only around 3 is available? If the answer is yes, I'm very confused...

 

Cool we are no longer talking PAE.

All 3GB will be available to 32bit when 3GB are installed. What is the confusion? We will try and explain just ask.

 

Alright, on a 32-bit system, if I have 4GB of physical memory, the OS will only see around 3GB. But if I have 3GB of physical memory, the OS will see all of it. What is it about going over 3GB that will cause the OS to stop seeing some amount of the memory?

 

Read the link ^.

 

As Andy said and pointed. But sometimes saying a different way helps it click. For the OS to see and make use of RAM the RAM must have memory addresses. If I told you the OS only had 2GB of memory addresses? You would not be surprised it could only use 2GB of RAM correct? Well Vista 32 has 4GB of memory addresses. But almost 1GB are used for I/O device mapping. Well if the total addresses are 4GB and 1GB is used for devices 4GB-1GB=3GB, only 3GB left for RAM. Now do you see? The OS has no space left to connect with that last 1GB so it can't connect/can't see/can't use.

 

Ohh, okay - now I think get it. A 32-bit OS has 4GB of memory addresses (which is just like saying it has 4,294,967,296 unique addresses. I just like saying that - bytes makes me think of space, not an amount of addresses). One quarter of those addresses (of which there are always 4 billion w/e), not one quarter of the memory (which may or may not be 4GB) have to be used by the OS for device mapping (don't really know what that is, but w/e). So there are only around 3 billion addresses left, and this is the case regardless of how much physical memory there is.

Now, is it always a consistent 1 billion addresses used for mapping? Or does it vary based on what devices you have? Is there any way to know around how many of the addresses are going to be used for mapping before actually purchasing a computer?

 

1GB was a made up number. The only way I know of finding out how many are used is ask other owners.

You wrote out not using prefix which I don't do as easier for me to talk GB than the written out numbers. For fun tonight you could write out the memory addresses for 2^64 as a number no prefix?

Looks like you got it.

 

It's just that when I hear "bytes" I think of space to store data in, not a number of addresses. I think that may have been why I couldn't get this clearly from some articles.