• Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint
Share this Page URL

Chapter 4. Memory > The Types of Memory

The Types of Memory

Memory comes in all sorts of configurations and packages, and you want to make sure you are referring to and using the appropriate kind of memory to ensure everything works right. Here's a listing of the types of memory in your Macintosh, with basic descriptions of their function and purpose.

In Mac OS 9 and earlier, you have quite a bit of control over how your system and applications use memory. Mac OS X handles memory quite differently and gives you much less control over what to do with it. So just about all the information I cover in this chapter about memory settings and allocations pertain to Mac OS 9 and earlier. If a bit of information applies to Mac OS X too, I say so.

About Mac OS X and Memory

Here's a brief overview of how Mac OS X's memory handling differs from that of previous Mac OS versions.

Protected Memory

Unlike previous Mac OS versions, Mac OS X has protected memory, which means that a mis-behaving program cannot write to other memory areas—an action that could crash the system or other applications. Although a program can still crash, it should not affect other running programs or the OS. With Mac OS X you can force-quit a frozen or crashed program and not have to restart the entire system.

Automatic Allocation of Virtual Memory

Mac OS X (like Unix and Windows) automatically and dynamically handles virtual-memory allocation to programs. This means the user doesn't have to adjust RAM allocations for native Mac OS X programs, as is the case with previous Mac OSs. It's one less thing for the user to mess with (possibly causing problems such as allocating too little or too much RAM to a program).

If you're not upgrading to Mac OS X yet, never fear. Current versions of the Mac OS will be supported and useful for many years to come, because software developers know it's the OS used by the vast majority of Mac owners (their customers). In my opinion, Mac OS X will take years to approach the installed base of users running earlier versions. And as with any new OS release (especially one as radically different as Mac OS X), it will be some time before Mac OS X has all the functionality you enjoy with Mac OS 9 and earlier software.

DRAM (Dynamic Random Access Memory)

Most references to memory pertain to the main system RAM that your programs and system software use. (When you see the term RAM used here, I'm referring to DRAM—the memory modules that you can install in your Mac.) It's called Random Access because individual pieces (locations) of RAM can be read or changed. It is volatile, which means it retains its contents only while power is on (or while the system is in sleep mode). The contents of RAM are maintained by refreshing (powering) the memory in repeating cycles. If you forget to save your work before shutting down the computer or if a power outage occurs, the contents in RAM (the main system memory where programs and the OS are loaded) are gone. (Operating system and applications code can be reloaded from disk, but any changes to data files you were working on that were not saved to disk will be lost.)

With the current cost of a UPS (uninterruptible power supply) relatively low, I recommend that you buy one for protection from power outages, line surges, and lightning strikes.

Many users overlook the importance of adequate RAM; instead, installing CPU or other upgrades but forgetting the primary role that memory plays in performance and stability. In one lab I know, a low-cost RAM upgrade for a slew of 32 MB iMacs resulted in a dramatic reduction in system lockups and support costs, with better performance and user productivity. In the not-so-distant past, 128 MB was a lot of RAM, but current Mac OS versions and popular applications use a lot more memory, and many users run several programs simultaneously. With memory prices at an all-time low, if you own a modern Mac with less than 64 MB of RAM, this should be your first upgrade, unless you use only one program at a time. And if you are using Mac OS X, 128 MB of RAM should be your minimum installation.

DRAM is normally added via small circuit boards called DIMMs (Dual Inline Memory Modules, used in Power Macs and slot CD drive iMacs), SIMMs (Single Inline Memory Modules, used in older Macs), or SODIMMs (Small Outline DIMMs, used in modern PowerBooks, iBooks, and early iMacs). I cover the types and sizes in more detail later in this chapter. (Most older PowerBooks use nonstandard memory modules, which are too varied to cover here.)

How do I find out which type of RAM my Mac uses?

The type and size of compatible memory vary by Mac model. I provide details later in this chapter.

How do I determine how much RAM I have?

To see how much memory you have installed in your system, switch to the Finder, and from the Apple menu select About This Computer in Mac OS 9 and earlier and About This Mac in Mac OS X. The amount of physical RAM installed is listed as Built-in Memory in Mac OS 9 and earlier (Figure 4.1) and as Memory in Mac OS X.

Figure 4.1. About This Computer in Mac OS 9 and About This Mac in Mac OS X tell you how much RAM you have installed; check the figure next to Built-in Memory.

In Mac OS 9, the Virtual Memory size (if enabled) is listed underneath. As the name implies, virtual memory is not really physical RAM (I cover it later in this chapter).

How do I make the most of the RAM I have?

In Mac OS 9 and earlier, the first step is to minimize the amount of RAM your system is using. (Because of the way Mac OS X handles memory for each application, you have much less control over how to make the best use of RAM.) Enabled control panels and extensions consume memory, so first use the Extensions Manager control panel to create a backup set (I call it Trimmed) just in case you want to revert back to your original set. Then disable any extensions you don't need or use in the Trimmed extensions set. Later OS versions install a lot of extensions I don't need (in Mac OS 9, for instance, I always disable the Multiple Users extension and control panel, Web Sharing, and so on, because I never use them). Trimming extensions can reduce RAM use by the system as well as save CPU cycles, and that improves performance. Some applications install extensions you may not need (Toast 4.x, for instance, installs USB and FireWire extensions, which you don't need if your CD-Rewritable drive is not USB or FireWire). I also get rid of the myriad of printer drivers installed that I don't need. I avoid running screen savers or appearance add-ons—these consume RAM and CPU resources. (Modern monitors don't need screen savers to prevent burn-in. For LCD, or flat-panel, displays, use the Energy Saver control panel to turn off the display instead because that extends the life of the display backlight.) Decreasing the disk cache can also save RAM, but very low settings will degrade disk performance to such an extent that it is noticeable even in the Finder.

If you're confused about what all those extensions in your System Folder are for, or you need a little help managing them, you might want to give Extension Overload a try ($25, single user; www.extensionoverload.com). This shareware includes descriptions of more than 4,000 extensions, control panels, and Control Strip modules from System 7 to Mac OS 9.2.1.

When you're trying to manage memory, don't arbitrarily increase memory allocation to applications that don't need it. Consider which applications you use most heavily before making any changes. For instance, I typically have many windows open in my Web browsers, so I increase their RAM allocation a bit for improved stability. Don't go overboard, however. If you allocate more RAM to a program than it needs, that's memory you won't have available for other uses. If the program you're using doesn't allow you to open another document, freezes often, or displays a low-memory warning, you need to increase its allocation. I'll go over how to check and change a program's memory use in more detail later in this chapter.

If you own a very old Mac that already has the maximum RAM installed, or memory upgrades are no longer available (or horribly expensive), consider using a RAM compression utility such as Connectix's RAM Doubler ($29; www.connectix.com). With older Macs, you should carefully evaluate your OS upgrades—they usually require more RAM and may not add any functionality you need. OS upgrades can also introduce conflicts with older software, third-party drivers, and utilities. If your Mac doesn't have enough RAM for a high-end program, consider alternatives that may require much less memory. Instead of an expensive commercial program, you may find a shareware or alternative one that uses less RAM, runs better on older machines, and still does the tasks you need. VersionTracker.com (www.versiontracker.com) is a great source for shareware programs or demo versions you can try before you buy.

How do I change the memory allocated to applications?

Make sure you've quit the program, because you can't change the memory allocation while the program is active. From the Finder, select the program's icon (not an alias of it) and then press or choose Get Info in the Finder's File menu. In the Get Info dialog box, select Memory from the Show pop-up menu (Figure 4.2). You'll see three fields with memory amounts listed: Suggested Size, Minimum Size, and Preferred Size. The Suggested Size is the software author's recommendation of how much RAM to allocate to the application for general use; the Minimum Size is the smallest recommended setting. The Preferred Size usually defaults to the Suggested Size and is the field you should use to allocate more memory to the program. Using the Preferred field rather than the Minimum field lets the program use more if it's available but still retain the ability to run in less memory (the Minimum Size). You can reduce the Minimum Size, but you may experience problems if you do.

Figure 4.2. In the Get Info dialog box you can change a program's memory allocation.

How much RAM should I allocate to applications?

First, you never want to allocate more than your installed physical RAM to any single application, even with virtual memory enabled. In many cases, increasing the default allocations can help stability and performance with programs using large files or many open windows. Don't go overboard and waste precious RAM, however. If you allocate more memory than the program needs, the excess won't be available for other programs or system use. One way I check this is, while running the program with my typical work files loaded, I switch back to the Finder and select the About item in the Apple menu (Figure 4.3). This shows each running application, the size of its allocation (in megabytes), and how much of the allocated memory is actually being used. If you see that the program is not using most of its allocation (its bar is only partially filled in), you can reduce the program's Preferred Size setting. (You must quit the program to change its memory settings.) Use caution in reducing the Minimum Size setting, however, because that is more likely to cause problems. As I discuss in more detail later in this chapter, enabling virtual memory will also reduce program (and system) memory requirements.

Figure 4.3. About This Computer shows how memory is put to use—the bars show how much of its allocation each running application is using.

In Mac OS X, you can watch the percent of memory applications and processes are using in the ProcessViewer application. It's located in the Utilities folder inside the Applications folder. It doesn't clearly label all applications. TextEdit is listed as TextEdit, for example, but iTunes is shown as LaunchCFMApp. And you can't change memory allocations; instead, you can watch the system do it for you.

The memory information shown in Figure 4.3 indicates that most applications are using only a fraction of their partition size. (I intentionally overallocated memory for some applications for illustrative purposes.) Notice that Netscape Communicator is using almost all of its allocation, however, so I would consider increasing its amount. BBEdit Lite and Fetch don't normally deal with large data files, so their allocations are wasting a lot of RAM and should be reduced to about a quarter of the current amount, based on the information shown. Remember, though, that memory use will vary depending on such factors as the number of open windows in the application and the size of data files loaded. Check usage during worst-case conditions after extended use and with your largest work files loaded before making final determinations on what changes, if any, are needed. If you are having problems opening large files or additional windows, or you are experiencing freezes, these are sure signs that you need to increase the memory allocated to that program.

My Microsoft Word, Outlook Express, and Internet Explorer memory allocations are larger than usual because I often use these programs with many windows open and with several large files. I've found that these programs are more reliable with extra memory allocated for my particular work pattern and needs. With 256 MB of RAM installed in my computer, the benefits of stability are worth the extra memory use.

For best results with Adobe Photoshop, an old rule of thumb is to allocate memory to the program up to five times the size of the image you will be working on (assuming you have more than that in physical RAM in your Mac). This helps avoid Photoshop's using its own virtual memory (called a swap file). Once Photoshop hits the swap file, performance takes a nosedive. The performance benefit from Photoshop's running at 100 percent efficiency can be as great as adding a faster hard drive or CPU in some cases, depending on the image size and system.

For 3D games, avoid a common cause of problems: Don't allocate most of your installed RAM to the game, especially with virtual memory off. Remember that the game, operating system, and OpenGL all consume memory. I'd suggest your Mac have a minimum of 256 MB of RAM installed if you want to run OpenGL games such as Quake III or Unreal Tournament. Mac OS 9 can use 40 MB to 70 MB of RAM on a 256 MB system, depending on whether virtual memory is enabled and how many applications are running. The more applications you run simultaneously, the more RAM the OS will consume. Enabling virtual memory helps, but that has other tradeoffs.

What is memory fragmentation? How can I avoid it?

Memory fragmentation occurs when you run several programs and then quit one or more. Let's say you have 128 MB of RAM free (after the Mac OS's usage) and launch programs A, B, and C. Program A uses 32 MB, Program B uses 28 MB, and Program C uses 40 MB (total usage is 100 MB, with 28 MB available). Let's say you then quit program B, which leaves a hole in memory that is now free but may not be enough to run another program, depending on its requirements. Even though you now have 56 MB of RAM (total) available, the largest contiguous block of available RAM is only 28 MB; if a program's minimum requirements are more than 28 MB, you won't be able to launch it. The solution is to exit program C. (You could exit and restart program C to eliminate the hole in memory left by exiting program B, making all free memory contiguous.)

To minimize fragmentation, I launch the largest programs (and those I use for the longest duration) first. That way I'm less likely to exit those programs, and I have the maximum possible contiguous memory available.

How much RAM do I need?

That depends on the types and number of applications you want to run simultaneously. As I noted earlier, serious Photoshop users should consider adding as much RAM as possible, but even today's 3D games require large amounts of RAM (often 100 MB or more for the game alone). If you're a casual user on Mac OS 9 and earlier just running applications such as AppleWorks and a Web browser, 64 MB is the absolute minimum I would suggest—preferably 128 MB. If you are running Mac OS X, 128 MB is the minimum. If your Mac is a G3 or later model (that uses SDRAM memory modules), this is affordable now. For 3D applications or Photoshop (even for nonprofessional use) on a Mac OS 9 system, I like to have at least 256 MB of RAM installed. In my Power Mac G4, on which I run LightWave 3D and other professional applications, I have the luxury of 1 GB of RAM. As of mid-2001, the two 512 MB DIMMs cost less than $100 each. Unfortunately, Fast-Page Mode RAM (which I detail later in this chapter) for older Macs is much more expensive (typically more than $100 for a 64 MB module).

Cache Memory

Since the advent of personal computers, CPUs have outpaced memory technology in operating speeds. High-speed memory, called cache, is used between the CPU and main system memory to minimize the time the CPU waits when reading or writing data. Cache memory allows the CPU to run faster and more efficiently without being a slave to the speed of main memory. The CPU's caches store data and instructions that the CPU is working on at the time or is predicted to need (a technique called branch prediction, in which the CPU prefetches data or code into the cache based on educated guesses of what will be needed next). Having code or data in the cache allows the programs to execute in many fewer CPU clock cycles than if it had to be fetched from main memory. Cache is also used to store data written from the CPU to avoid waiting on much slower main memory. In general, the larger the cache the better, since it can hold more program code and data for high-speed access by the CPU. If the CPU can't find the data it needs in the cache, it has to wait for it to be fetched from main memory (called wait states). The more often the CPU finds the data it needs in the cache, the faster the program will execute.

Cache memory is referred to by levels, generally denoting how close the cache memory is to the CPU. For instance, Level 1 cache (often called L1 cache) is normally a part of the CPU chip itself and is very high speed. Level 2 (L2) cache most often resides external to the CPU, although the latest Mac models use new PowerPC designs such as the PowerPC 750CX (G3) and PowerPC 7450 (G4), which have on-chip 256 Kbyte L2 caches running at the full processor speed. The higher-end 2001 Power Mac G4 models with PowerPC 7450 (G4) CPUs also have a Level 3 (or L3) cache external to the processor. External L2 cache is located on the CPU module or main logic board (for models that do not have the CPU on a separate module/daughtercard).

L1 cache runs at the full CPU clock speed. L2 cache (except for the PowerPC 750CX and 7450 chips) normally runs at a ratio or divisor of the CPU speed (such as 1:2 or 1:4) for G3 and G4 Macs or at the system bus speed in pre-G3 models. The Level 3 cache in the 2001 G4 Macs with PowerPC 7450 CPUs runs at one-fourth the processor clock speed. [Whew. —Ed.]

The L3 cache chips in the PowerPC 7450-equipped Macs are double-data-rate (DDR), which means the effective rate is twice the actual clock speed.

The so-called Mach 5 or Kansas Power Mac 8600/250, 8600/300, 9600/300, and 9600/350 systems use a 604ev processor with an inline L2 cache on the CPU card running at 100 MHz, twice their main system bus speed of 50 MHz. These Mach 5 CPU cards are not usable in any other Mac model.

If you're upgrading the CPU of a pre-G3 (PowerPC 604/603/601 CPU) series Power Mac with a PowerPC G3 or G4 CPU upgrade card, it's usually best to remove the L2 cache DIMM on the logic board (motherboard), if present. Often the faster bus speeds or different timing of the CPU upgrade card can cause problems with the original system's L2 cache DIMM. Some Power Macs, such as the 9500 and 9600 (except the Mach 5 models just noted), and Mac OS clones, such as the Umax S900, have soldered-in L2 cache that you cannot remove. However, the soldered-in cache is usually not a problem for most CPU upgrades. For a guide to getting at the cache DIMM on the 8500, the most difficult-to-access Mac model, see www.xlr8yourmac.com/8500cache.html.

Related Links

For tests of L2 cache–size effects on performance, see:

G4/350 CPU upgrade—1 MB versus 2 MB L2 tests at www.xlr8yourmac.com/G4CARDS/G4_1MBvs2MB.html.

G3 CPU upgrade—512 Kbyte versus 1 MB L2 cache Photoshop tests at www.xlr8yourmac.com/G3CARDS/g3cachetest.html.

Power Macintosh—L1 and L2 cache explained (does not cover 2001 systems as of this writing) at http://til.info.apple.com/techinfo.nsf/artnum/n14750.

The disk cache.

The other type of cache your Mac uses is the system disk cache. The system disk cache is allocated from main memory (RAM) to increase performance of disk operations by storing portions of the hard drive's contents in RAM for faster access. When you first access the drive to load a file or program, the contents are held in memory for quick retrieval should they be needed again. RAM has access times much faster than mechanical devices such as hard drives. (RAM access time is measured in nanoseconds; disk-drive access times are in milliseconds.) In Mac OS 9 and earlier, you adjust the system disk-cache size in the Memory control panel (Figure 4.4). In later Mac OS versions, the disk-cache size defaults to a percentage of the installed RAM (up to a maximum of about 8 MB). You can override this setting and designate your own preferred size by selecting the “Custom setting” option; however, I've seen no real benefit to doing so except for video-capture applications with pre-G3 series Macs. (A tip for better video-capture performance with older Macs is to run the smallest possible disk cache size.)

Figure 4.4. The Memory control panel lets you manipulate disk-cache settings.

In general use for Mac OS 9 and earlier, the larger the disk-cache setting, the better for performance, with diminishing returns once sizes exceed about 4 MB. Simple disk caches can't help for random data access patterns (for example, searching a huge database or searching for files across an entire disk) since the cache can't hold the entire contents of the disk. If you use video-capture programs with older (pre-G3) Mac models, set the disk cache to the smallest size possible for higher capture rates. (Older Macs' RAM write speeds seem to be a bottleneck and writing the captured data directly to the hard drive usually results in higher capture rates. Some capture-card owners note that this is unnecessary for the latest Mac models.) Using a 1998 G3 series PowerBook I got approximately three times the EditDV capture rate with the disk cache set to the minimum size versus the default setting, so you may want to experiment with different cache sizes. Small disk-cache settings have also been useful for Photoshop, but I don't see any need to lower the disk-cache settings if you have allocated enough RAM to Photoshop that it doesn't use its swap file.

More on Disk Cache

For an article evaluating performance with various disk-cache sizes in an older Mac, see www.xlr8yourmac.com/tips/cachestudy.html.

Virtual Memory

Virtual memory, as the name implies, is not actually RAM at all. It's the term for assigning part of the hard drive's disk space for use as if it were physical RAM. Power Macs (those with PowerPC CPUs) have better virtual-memory capabilities and performance than the early 680X0-based Macs. PowerPC Macs use a virtual-memory feature called file mapping that reduces the amount of physical RAM required to run an application.

In Mac OS 9 and earlier, the Get Info dialog box for an application usually shows the reduction of RAM requirements from enabling virtual memory.

Without virtual memory enabled, the program's temporary data storage (called the stack), dynamically allocated storage (called the heap), and all executable code must load into the application's partition (set in the memory section of the Get Info dialog box). With virtual memory enabled, only the program's stack and heap load into the partition, and this will reduce program load times as well as memory requirements. Sometimes the savings can be substantial—for instance, Adobe GoLive 5.0 uses 4.7 MB less RAM with virtual memory enabled.

There are negatives to using virtual memory, however; the most universal is its effect on audio. Several parts of Mac OS 9's sound software are hindered by enabling virtual memory, causing delays in sound effects, for instance. Virtual memory can also affect overall performance, most noticeably in older Macs with slower hard drives or in cases where there is little physical memory installed.

I don't recommend using virtual memory for pro audio applications or for video capture. I've also seen negative effects even in modern Macs with plenty of RAM while importing digital video into iMovie. With virtual memory enabled on a PowerBook G3/500 FireWire model, the movie's audio was distorted and useless. Disabling virtual memory and reimporting the video solved the problem. (Restarting is required for any changes in virtual memory to take effect.)

Virtual Memory Pros and Cons
Pros Cons
Reduces application RAM requirements Has negative effects on audio and video capture
Program load times are decreased Consumes disk space
Mac OS RAM use is reduced (especially when running multiple programs simultaneously) No substitute for physical memory for applications that require a large amount of RAM
 Performance is reduced
  May cause delayed sound effects in games

In Mac OS 9 and earlier, turn on and adjust virtual memory in the Memory control panel (Figure 4.5). If you have one drive that is much faster than another, you can select that drive to hold the virtual-memory swap file using the Select Hard Disk pop-up menu. Any changes take effect only after restarting. Best overall performance is usually obtained by setting virtual memory to 1 MB more than the installed RAM size (which is the default setting).

Figure 4.5. Set your virtual-memory configuration in the Memory control panel.

Connectix's RAM Doubler is an alternative to Apple's virtual memory. With more than 1 million copies sold over the years, it's clearly been popular. However, if you own a G3 series or later Power Mac, I'd recommend getting more RAM instead. (As of this writing, 128 MB of SDRAM sells for under $30.) For older Macs with more expensive RAM, RAM Doubler may be worth a look. RAM Doubler uses a compression scheme and other tricks to increase the amount of apparent RAM in your system by up to a factor of three. Since the CPU has to handle the compression, performance is affected to some extent. Performance degrades further if you allocate more RAM to an application than you have physically installed in your Mac. RAM Doubler doesn't have to use the hard drive, and this can increase battery life for portables, compared with conventional virtual memory. For more information on RAM Doubler, including important compatibility information, see www.connectix.com/products/rd9.html.

Related Links

For a technical discussion of how virtual memory differs in Power Macs versus older 680X0 CPU-based models, see http://til.info.apple.com/techinfo.nsf/artnum/n15854.

Video RAM

Often called VRAM for short, video RAM is memory dedicated for use by the graphics card or onboard graphics chip. (If your Mac doesn't have a graphics card installed, it has a graphics chip on the logic board—called onboard video.) The graphics chip uses VRAM to store the display's frame buffer, which holds the contents of the monitor's screen images. The amount of memory required primarily depends on the display mode and color depth set in the Monitors control panel. The more VRAM the better, in general, as higher resolutions and color depths increase the amount of VRAM required. Each pixel of the display mode uses varying amounts of VRAM, depending on the color depth. In 256-color mode, each pixel uses 1 byte of memory; thousands-of-colors mode uses 2 bytes per pixel; and millions-of-colors uses 4 bytes. Three-dimensional software and games also use additional amounts for buffering, texture storage, and display-page flipping (holding a second screen already calculated in VRAM to allow rapid changing of the display for animation, for instance).

You cannot increase the amount of VRAM on graphics cards, in all but rare cases. Modern graphics cards for the Mac include anywhere from 8 MB to 64 MB of VRAM. Many onboard-video Macs, such as the Power Mac 7300/7500/7600/8500/8600 series, have VRAM slots on the logic board. For those models you can add VRAM up to a maximum of 4 MB via special SIMMs available from Apple dealers or through mail order. Check prices, however—you may be able to add more performance and functionality by adding a graphics card rather than upgrading the onboard VRAM. Adding VRAM won't add 3D graphics acceleration to the onboard-video chip or make it faster; it will only allow your computer to run higher resolutions and color depths. If you have a second monitor handy, connecting it to the graphics card will allow you to run dual displays (using the onboard video for one monitor and the graphics card for the other). The Mac OS will treat both monitors as one wide desktop area.

Related Links

For reviews and performance tests of Macintosh graphics cards, and related articles, see www.xlr8yourmac.com/video.html. If you have a PCI or AGP (Accelerated Graphics Port) slot model, you can see how your Mac may perform with a new graphics card by searching the systems/video-card performance database at http://forums.xlr8yourmac.com/fpsdb/.


Your Mac stores settings for such system elements as the startup disk, AppleTalk, and the Chooser in a special type of memory called NVRAM (Non-Volatile RAM). NVRAM chips are used to store settings in the PRAM (Parameter RAM) on the Mac. (Don't confuse PRAM, the name for the area where your system settings are stored, with NVRAM, which is a chip type used.) This small amount of memory has its contents preserved, even when the computer power is off, by means of the logic-board battery. The battery is typically a nicad or lithium-ion type good for five years or more. If you ever notice that your Mac doesn't retain settings such as the date when powered off, it's time to replace the battery. Apple dealers and electronics stores such as Radio Shack usually carry replacements.

The PRAM (Parameter RAM) contents include the status of AppleTalk, date, serial-port configuration and port definition, alarm-clock setting, application font, serial-printer location, keyboard repeat rate and delay, speaker volume, alert sound, double-click rate, cursor-blink time (insertion-point rate), mouse scaling (mouse speed), startup disk, menu-blink count, monitor depth, 32-bit addressing (only on older 680X0 CPU Macs), virtual-memory settings, RAM-disk settings, and disk-cache settings.

Clearing, or zapping, the PRAM.

Sometimes you may want to reset the PRAM contents when you're troubleshooting a problem to make sure no corrupted settings are stored there. (This technique can also help if you have problems after replacing a graphics card, for instance.) The common term for this is zapping the PRAM. To do this, press the system power button and then quickly hold down the key combination. You must press this key combination before the gray screen appears. For best results, hold down the keys until you hear the Mac's startup bong repeat three times. This will clear the startup-disk selection, so you may see a flashing question mark, but after some delay the system disk will be found and the Mac will start up. Remember to reset your control panels, such as Startup Disk, Date, and AppleTalk, to their preferred settings.

Another way to clear the PRAM settings is to use the logic-board reset button (called the CUDA on pre-G3 Macs). This is also recommended when upgrading older Macs with a different type of CPU card, such as a PowerPC G3 or G4 upgrade, or in extreme troubleshooting cases in which zapping the PRAM does not fully clear contents. (The instruction manual for the CPU upgrade usually shows the location of the reset button, often near the CPU card slot.) In some severe cases, removing the logic-board battery overnight is another way to clear the NVRAM/PRAM contents.


When a computer is powered on, the main memory of the system contains no data or code. To allow the system to start up (or boot), access the hard drive for loading the operating system, and perform other low-level system functions, instructions are permanently stored in memory called ROM, or firmware. (On PCs this is often called the BIOS—basic input/output system.) It's called read-only memory because it cannot be written to or modified, since erasing or changing this basic low-level code and instructions could render the system inoperable. The Blue and White Power Mac G3 and later Macs can have the ROM updated with special Apple system software (and button-press sequences) to allow firmware updates that address bugs and increase performance or to provide for things such as future OS support. The system's boot ROM is not to be confused with the Mac OS ROM file that is in later versions of the System Folder. Without the boot ROM, the computer would not be able to access the higher-level Mac OS ROM file stored on the hard drive. (On modern Macs, Apple System Profiler—found in the Apple Menu in Mac OS 9 and earlier and in the Utilities folder inside the Applications folder in Mac OS X—will list your firmware version, Mac OS ROM file version, and boot ROM version if available.)

  • Creative Edge
  • Create BookmarkCreate Bookmark
  • Create Note or TagCreate Note or Tag
  • PrintPrint