ARCHIVED: In Mac OS, why is the system software taking up so much memory?

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Note: This document does not apply to Mac OS X.

Mac OS uses anywhere from under 2MB of RAM to well over 20MB, depending on the Mac OS version, processor type, and memory settings. To find out how much memory Mac OS is using on your computer, from the Apple menu, select About this Computer... in OS 7.6 and higher, or About this Macintosh... in System 7.0 through 7.5.5. When configured with default memory settings, the System or Mac OS should use an amount of memory within these general ranges:

Mac OS version	68k	PPC
7.0-7.1	1-3MB	2-5MB
7.5-7.6	3-6MB	6-10MB
8.0-8.1	5-10MB	8-14MB
8.5-9.0	N/A	12-20MB
9.0.4+	N/A	20-50MB

If you have a large amount of physical RAM, the system can take up significantly more space than these ranges suggest. However, if you find the system is still taking up too much memory, consider one of the following possibilities:

Virtual memory: On PowerPC-equipped Macintoshes, Mac OS, as well as any running application, will use anywhere between 25% and 50% more memory when virtual memory has been disabled. To turn virtual memory on, from the Apple menu, choose Control Panels and then Memory. Under "Virtual Memory", select the On radio button. Restart your computer for the change to take effect.
For systems with less than 64MB of RAM built in, use the up and down arrows to set the total memory to double the amount built in. On systems with more built-in memory, set the total to 1MB over the amount built in.
RAM Disk: A RAM Disk provides space for file storage in the RAM and places an icon for this space (similar to a floppy disk icon) on your desktop. With the exception of using a Macintosh as a server, there is little use for this feature and you should disable it. To do this, from the Apple menu, choose Control Panels and then Memory. Under "RAM Disk", select the Off radio button. Restart your computer for the change to take effect.
Disk Cache: While a large Disk Cache can improve overall system performance, it's important to balance that with the amount of RAM built in. In some cases, such as with Adobe Photoshop, performance will actually suffer if Disk Cache is set too high. To adjust Disk Cache, from the Apple menu, choose Control Panels and then Memory. Under "RAM Disk", select the Default setting radio button. If you don't have this option, use the up and down arrows to adjust the size to between 256K and 1024K. Restart your computer for the change to take effect.
Desktop pictures: Often, a desktop picture can use up RAM and slow the overall system performance. Particularly if you are using a third-party desktop picture utility, system memory usage may increase by as much as 2MB. While the memory requirements to place a picture on the desktop using Apple's Appearance control panel are not as bad, significant slow-downs will occur as the computer approaches memory capacity. To remove the desktop picture, from the Apple menu, choose Control Panels and then Appearance. Click Desktop. If a desktop picture is in use, click the Remove Picture button. Click Set Desktop to remove the desktop picture. Restart your computer to free the memory previously used by the picture.
32-bit addressing: To access more than 8MB of RAM, your computer must be capable of 32-bit addressing. Without 32-bit addressing, you will notice the system software has been assigned all of the memory beyond 8MB. Depending on what kind of computer you have and what version of the OS you are using, there are a number of ways to fix this problem:
- If you are running any version of System 6, you will have to upgrade to at least System 7.0 to make use of more than 8MB of RAM. This is because System 6 is only capable of 24-bit addressing.
- If you have a Macintosh II, IIx, IIcx, or SE/30, your computer has a "dirty ROM". Before you can use 32-bit addressing, you must install Mode32, available from Apple's FTP site as well as most online Mac OS software archives. Make sure that you get Mode32 7.5, because earlier versions of Mode32 were incompatible with Mac OS 7.5.
- If you are running System 7 (and have Mode32 installed if your computer requires it), make sure that 32-bit addressing is turned on. You can do this from the Memory control panel.
Note: All Mac OS computers with PowerPC chips, AV Macintoshes, the PowerBook 500 series, and computers running Mac OS 7.5 and higher always have 32-bit addressing turned on.