What tcode can I used to find out how much memory is allocated to R/3 in total? I know for Oracle I can get the SGA memory.  I am using SAP 4.7 / Oracle 9i on Unix AIX. 
 

Go to ST02 --> Detailed analysis --> Sap memory. This will let you know memory allocated for that app server.  You can also find out memory allocated for user (mode list) and Quotas. 

You can also use report RSMEMORY to find out how sap alloctes memory to Dia and Non Dia work processes. 
 

I'm afraid that is incorrect. The question is also not a simple answer. 

Go to ST02 --> Detailed analysis --> Storage is the correct path. The Virtual memory allocated line shows SAP's allocated memory. 

This memory does not include what the DB uses or the OS or other things running on the server. More importantly it does not include heap (private memory) allocated by WP's. It also does not include the executable size of the GW, the ICM and the dispatcher but it does include most of their memory in the shared memory areas. 
It also does not allow for additional memory that a Windows system may dynamically allocate to extended memory if the users use it all up.
 

Question on Shared Memory and IMODE

Can someone tell me about Shared memory (Code Definition) and IMODE? 
I have never heard these terms in SAP R/3, Shared momey sounds like its related to Operating System.
 

All memory is OS memory I'm afraid there is little distinction in that regard. 

Shared memory is memory that can be addressed by different processes as opposed to private memory that can only be addressed by the process that creates it. A good examlpe would be the Program Buffer (PXA) this buffer hold the recently used programs in memory for fast access and needs to be read by all work processes. A more impotrant shared memory area is extended memory. In 32 bit processing you need to keep shared memory areas at a minimum because otherwise you limit the amount of space in the address space that your program can use for its own data. I won't get into a sad chat about all this bat thats hared memory. 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

An IMODE is an internal mode. An example may be best, log onto a SAP system, create a new session and leave one at the main menu and with the other go to ST02. In ST02 near the bottom of the screen double click on the extended memory line. In the next screen click on the Mode List button. You will see you have the ST02 session marked with an X for being attached and another session in IMODE 0. 

With the other session got to SM36 then swap back to ST02, press enter and you will see the session has an IMODE 0 and an IMODE 1 use. Now from the SM36 screen click on the button Job Selection and jump back to St02 and press enter, you will see the session has an imode 0 1 and 2 active. 

Imode 0 is not really an IMOde but the main mode that all your information relevant in any transaction is available. As you call transactions inside transactions you need new memory and the old stuff needs to be save for when you green arrow back. This is why you have imodes so that when you green arrow back from the job selection screen above (I mode2) it is no longer the active mode and imode 1 is reactivated. 

Fast Links:
Program Memroy Request
What the Max Memory a good Program Should Request

Get help regarding your Basis problems
Do you have a SAP Basis Question?

SAP Basis Books
SAP System Administration, Security, Authorization, ALE, Performance Tuning Reference Books

SAP Basis Tips
SAP BC Tips and Basis Components Discussion Forum

Administration In SAP - Sapgui, Unix, SAP ITS, Router, Client Copy and IDES 

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com