On most Macintoshes, the operating system uses more memory than the “Preferred size” value you provide in the Finder’s Get Info window for an application (typically 16K more). Finder subtracts this extra memory from the value it lists in its “About This Macintosh” display; so when “Group System Software” is on, About does the same thing. Where does the extra 16K go? It is added to the “System Software” bar. You might notice when you quit an application, “System Software” shrinks by about 16K.
Finder also adjusts its “Largest Unused Block” value to match. Since 16K more memory is required to launch an application than what you’re told about, the Finder subtracts 16K from the size of the Largest Unused Block, so you won’t think you have room to launch an application that would just fit. What’s more, the Finder subtracts another 10K from the Largest Unused Block just for good measure! About does the same thing, just to be consistent. This 26K also goes into the “System Software” bar.
If your Macintosh is 68000-based, you probably won’t see this behavior. The operating system won’t increase partition sizes if your Macintosh doesn’t support Color QuickDraw.
WHAT’S IN “SYSTEM SOFTWARE”?
So what does “System Software” actually represent? Well, it’s the partition size for the Finder, plus any background–only processes. Add in the size of the system heap, plus any allocated temporary memory. Also add in all those 16K fudge factors from every other running process, plus another 26K not reported in the Largest Unused Block.
About plays this charade until you ungroup System Software. Then, the fudge factors go away, and true process sizes are reported. The Finder and other background–only processes each get their own bars. As for the other system memory, it’s added together and placed in the Machine Info banner as “System Memory.” If you installed the Symbionts extension, About will also report the portion of system memory used by your system extensions.
If you have more than 8MB of RAM, the excess memory will be added to System Software if 32-Bit Addressing is off. When About detects this condition, it warns you that you have unuseable memory.
VIEW BY PARTITION
For the most accurate picture of your memory, ungroup System Software, show Partition Sizes, and view by Partitions (small icons may be more aesthetically pleasing). You will now see gray bars with various names separating some of your applications. If you have excluded any processes, they will reappear because About does not honor the excluded setting when using this view.
At the bottom of the list is a block named “System Heap,” which displays the size of the system heap. The system heap grows as you launch applications, but should never shrink.
Blocks named “unused” may appear between some processes. These gray bars represent the other unused blocks. One of them is the Largest Unused Block, and it’s usually right above the “System Heap” bar. Since the Largest Unused Block is now explicitly shown, About no longer displays its size in the Machine Info banner. Any of the gray bars may show their memory as a “broken bar” so they don’t dominate the display.
These unused blocks are “holes” in your memory that get formed by quitting applications not adjacent to the Largest Unused Block, or they may be chunks of temporary memory that were allocated and locked down by one of your running applications. (The Finder and File Sharing Extension both do this.) To reclaim unused memory, you must coalesce it back into the Largest Unused Block by quitting the applications that stand between the unused memory and the bottom of the list.
24-BIT VIRTUAL MEMORY
There is a special case which occurs if 24-bit addressing is on and you’re using Virtual Memory to expand your address space to more than 8 MB. (Yes, Macintosh SE/30 users, I’m talking to you!) When you do this, your address space is split into at least two parts, with the ROM occupying 1 MB of address space between them. About shows this split by inserting a gray bar named “ROM address space” into the list.
NUBUS™ CARDS AND 24-BIT VM
If any NuBus cards (or simulated NuBus cards) are occupying addresses below the upper portion of RAM, About inserts a gray bar labelled something like “NuBus 9: Macintosh SE/30 Built-In Video.” The number after the word “NuBus” is the card’s slot number. Alert readers may wonder what happens if NuBus cards are arranged with empty slots between them. In this case, the upper portion of RAM could be split into even smaller blocks, to utilize the holes in the address space formed by the empty slots. When About detects this condition, it warns you that you have an undesirable arrangement of NuBus cards. About displays bars for all NuBus cards it finds, to give you enough information to rearrange the cards into a more efficient pattern.
ABOUT CPU LOAD
When you view with large icons and show partition sizes, About displays the percentage of the CPU used by each process, updated approximately every two seconds. These percentages appear as red bars beneath the memory partition bars.
Sometimes the numbers don’t total to 100%. What was the Mac doing the rest of the time? The missing CPU time might have been used by excluded processes, or by background-only processes that you don’t see if “Group System Software” is checked. Part of the time might also be used by system extensions you have installed; they steal time from your applications to do their work. Or the remaining time might have been spent simply idling—waiting for you to click the mouse or to type.
You may notice About using a large percentage of the CPU. This is because About is designed to soak up all available idle time, and use it to monitor the other processes. But, when you want to do real work on the Mac, About takes a back seat. About’s design also causes it to be more active when it’s the frontmost application. When About is in the background, you’ll see its CPU usage drop considerably.
