Trying to understand how much memory Oracle uses

Hello Experts,
I'm trying to understand how Oracle uses memory.
I have an AIX 5.3 server with 3GB of memory. The sga_max_size for the 10.2.0.4 database is set to 316MB.

The server is only running this one instance, so I'm wondering why is it using 90%+ of its memory?

I assumed that Oracle wouldn't use more than 316MB, but that must not be the case.

The top 30 memory processes are all Oracle.

< > ps auxw | sort -r +3 | head -30
USER         PID %CPU %MEM   SZ  RSS    TTY STAT    STIME  TIME COMMAND
oracle    114700  0.0  4.0 97428 110060      - A      Apr 21  2:53 ora_cjq0_ISDQA
oracle    397546  0.0  4.0 97236 105248      - A      Apr 21  1:58 ora_pmon_ISDQA
oracle    319566  0.0  4.0 95876 108168      - A      Apr 21  1:15 ora_mmon_ISDQA
oracle    495860  0.1  4.0 95644 111584      - A    10:07:43  0:13 oracleISDQA (LOCAL=NO)
oracle    651296  0.0  4.0 95448 105752      - A    13:58:18  0:04 oracleISDQA (LOCAL=NO)
oracle   1126596  0.0  4.0 95248 111188      - A    10:09:49  0:01 oracleISDQA (LOCAL=NO)
oracle    569452  0.0  4.0 95216 105504      - A      Apr 29  0:01 oracleISDQA (LOCAL=NO)
oracle    303286  0.0  4.0 95004 105340      - A      Apr 21  0:30 ora_dbw0_ISDQA
oracle   1093704  0.0  4.0 95000 110940      - A    09:56:06  0:01 oracleISDQA (LOCAL=NO)
oracle   1114356  0.0  4.0 94868 110808      - A    10:21:01  0:00 oracleISDQA (LOCAL=NO)
oracle    659570  0.0  4.0 94764 107556      - A    13:58:18  0:02 oracleISDQA (LOCAL=NO)
oracle    782476  0.0  4.0 94760 105204      - A      Apr 29  0:00 oracleISDQA (LOCAL=NO)
oracle    594010  0.0  4.0 94752 105496      - A      Apr 29  0:00 oracleISDQA (LOCAL=NO)
oracle    618678  0.0  4.0 94500 110440      - A    09:14:17  0:00 oracleISDQA (LOCAL=NO)
oracle    409818  0.0  4.0 94296 105452      - A      Apr 21  0:00 ora_q000_ISDQA
oracle    249966  0.0  4.0 94140 107380      - A      Apr 21  0:36 ora_smon_ISDQA
oracle    274452  0.0  4.0 94096 105324      - A      Apr 21  1:30 ora_ckpt_ISDQA
oracle    561164  0.0  4.0 93320 105092      - A      Apr 21  0:00 ora_d000_ISDQA
oracle    876700  0.0  4.0 92624 105364      - A    13:58:18  0:00 oracleISDQA (LOCAL=NO)
oracle    667710  0.0  4.0 92624 105364      - A    13:58:18  0:00 oracleISDQA (LOCAL=NO)
oracle    778296  0.0  4.0 92624 105300      - A    13:58:17  0:00 oracleISDQA (LOCAL=NO)
oracle    196812  0.0  4.0 92508 105220      - A      Apr 21  0:00 ora_reco_ISDQA
oracle    516204  0.0  4.0 92432 105416      - A      Apr 21  2:48 ora_mmnl_ISDQA
oracle    520362  0.0  4.0 92308 104984      - A      Apr 21  0:01 ora_q001_ISDQA
oracle    573456  0.0  4.0 92272 105092      - A      Apr 21  0:00 ora_s000_ISDQA
oracle    442516  0.0  4.0 92268 104980      - A      Apr 21  0:03 ora_qmnc_ISDQA
oracle    327810  0.0  4.0 92256 105004      - A      Apr 21  0:15 ora_mman_ISDQA
oracle    372940  0.0  4.0 92244 105080      - A      Apr 21  0:14 ora_psp0_ISDQA
oracle    299016  0.0  4.0 107464 105116      - A      Apr 21  0:30 ora_lgwr_ISDQA


Total System Global Area  331350016 bytes
Fixed Size                  2083824 bytes
Variable Size             205521936 bytes
Database Buffers          117440512 bytes
Redo Buffers                6303744 bytes

+-topas_nmon--#=PURR Stats-------Host=basalt---------Refresh=2 secs---11:39.18---
| Memory ------------------------------------------------------------------------
|          Physical  PageSpace |        pages/sec  In     Out | FileSystemCache  
|% Used       91.3%      6.9%  | to Paging Space   0.0    0.0 | (numperm) 41.2%  
|% Free        8.7%     93.1%  | to File System    0.0    6.0 | Process   25.7%  
|MB Used    2805.9MB   567.7MB | Page Scans        0.0        | System    24.4%  
|MB Free     266.1MB  7624.3MB | Page Cycles       0.0        | Free       8.7%  
|Total(MB)  3072.0MB  8192.0MB | Page Steals       0.0        |           ------ 
|                              | Page Faults       0.5        | Total    100.0%  
|------------------------------------------------------------ | numclient 41.2%  
|Min/Maxperm     578MB( 19%)  2312MB( 75%) <--% of RAM        | maxclient 75.3%  
|Min/Maxfree     960   1088       Total Virtual   11.0GB      | User      62.0%  
|Min/Maxpgahead    2      8    Accessed Virtual    1.6GB 14.6%| Pinned    25.5%

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