Memory is used by a computer’s processor to retain data whilst it performs other operations. The contents of the read-only memory (ROM) is fixed at manufacture and is usually concerned with basic elements of the operating system (OS) that are related to the machine’s hardware. Random access memory (RAM), on the other hand, contains temporary data, including parts of the system that were loaded from the hard disk at startup or any documents that are currently in use.


In a Mac OS computer, some parts of the operating system, including the Toolbox of Classic Mac OS routines, are kept in ROM. Since this software can’t be changed it’s often known as firmware. The ROM used in Mac OS computers has expanded with the system: the original Mac 128 had a 64 KB ROM, although later 680x0-based models employed 128 KB, 256 KB, 512 KB or 1 MB devices. Recent PowerPC-based machines have a 4 MB ROM and employ 64-bit addressing.

In a modern PowerPC, the ROM is in a dual in-line memory module (DIMM), although older machines contain a single in-line memory module (SIMM). In some models the ROM is in a ROM socket, allowing later upgrades, whilst in others it’s soldered to the motherboard. Older machines sometimes have an extra SIMM slot for a ROM upgrade, intended for adding extra OS capabilities. Fortunately, Apple never adopted hardware ROM upgrades, instead updating the OS by loading software from the hard disk at startup and then using this to ‘patch’ the system. In fact, later machines employ an OpenFirmware ROM, which can be updated using the appropriate software, whilst more of the OS is kept on hard disk.


The main RAM stores parts of the Mac OS, copied from the hard disk to speed up the system. It also retains computer code from currently-running applications and data from open documents. The contents of the RAM is usually lost when you shut down a computer, although some portable machines retain data in RAM by continuing to power it from the main battery.

Modern computers use dynamic RAM (DRAM) or synchronous DRAM (SDRAM), with each block of memory supplied as a DIMM or SIMM component. Recent Macs contain the same kind of SDRAM as found in a modern PC, whilst older portable machines contain special devices that conserve battery life, albeit with a compromise in speed. For example, the original Mac Portable uses static RAM (SRAM) whilst older PowerBook models contain psuedo-static RAM (PSRAM).

All recent computers have another area of RAM, known as video RAM (VRAM), which is used to store the picture that’s currently shown on the computer’s display. The DIMMs and SIMMs used for video RAM are frequently fitted onto a video card and are usually physically and electrically different from those employed for the main RAM.

Memory Logic

Some computers use a paged memory management unit (PMMU) to gain access to a large amount of RAM, which is organised in pages. This reduces the need for a wide address bus, allowing part of the hard disk to be used as a RAM disk or as virtual memory (VM), whilst some ‘classic’ models use a direct memory access controller (DMAC), to give easy access to memory and other devices. In older machines a general logic unit (GLUE) is used to handle the computer’s processor clock and RAM timing.

Battery Backup

Some older portable computers use the rechargeable main battery to save RAM data, even when switched off. The usual nickel-cadmium (NiCd), NiMH (Type 1, Type II or Type III) or lithium-ion (Li-ion) battery packs each contain several cells.

Don’t try to open batteries, as they contain hazardous chemicals and can catch fire if mistreated.

Power Management

Modern computers, including Apple’s G4 series of machines, employ a Power Management Unit (PMU). This appears to retain its own special information, which can only be cleared by actuating the PMU Reset button: this can be found on the computer’s motherboard. If your machine simply refuses to start under any circumstances, disconnect the mains supply, press the PMU Reset button once, wait ten seconds, reconnect the power and then press the button.

Fitting Extra RAM

You can add RAM to a Mac by plugging in extra modules or changing those already fitted.

Opening up your machine to replace or add RAM modules may invalidate the guarantee.

The location of RAM varies with each model of computer. The picture below shows the position of the RAM sockets in a now-outdated Mac Performa 6400:-

Whenever upgrading RAM you should observe the following guidelines:-


Do not upgrade the RAM in a computer that contains an integral VDU screen unless you’re experienced with such equipment. These models contain very high voltages that are still present long after the computer has been switched off. If in doubt, refer the job to a qualified electronics engineer.

You’re also advised to refer to the Apple Memory Guide, Connectix’s Macintosh Memory Guide or Newer Technology’s Guide to RAM Updates, also known as GURU. You should read all three if there’s any doubt or confusion, since these free electronic publications provide vital information that you can’t find anywhere else. Remember, although dealers ought to give you the correct information some are obviously more interested in sales than in providing useful advice.

Static Electricity

When installing RAM you must avoid creating static electricity. One small spark can easily write off a module or your entire machine. You should follow these precautions:-

You should also connect the computer’s metalwork to earth as follows:-

Before touching modules or any of the electronics in your computer you should touch your hand on part of the machine’s metalwork. You should repeat this operation if you have to walk away and then return to the machine.

A conductive wrist band can be used, although this kind of device makes the author nervous about electrocution. They’re really not necessary if you follow the above steps.

Special tools can help in the insertion or removal of RAM, such as the SIMM extraction tool shown below, although DIMMs can usually be extracted using a pair of watchmaker’s screwdrivers.

RAM Type and Size

RAM usually comes in a dual in-line memory module (DIMM) or single in-line memory module (SIMM). Several modules may be required to make up the desired amount of RAM.


Desktop computers usually have banks of sockets on the motherboard that can accommodate extra RAM. However, some machines come with a basic amount of RAM, provided by chips that are soldered to the motherboard. This kind of RAM can’t be removed, although you can supplement it by adding modules to any vacant sockets.

The banks are often given letters from A upwards and are usually organised in pairs, such as Bank A and Bank B in the first pair followed by Bank C and Bank D in the second pair.

The following rules apply to many computers, especially older models:-

Recent G5 models also require RAM chips to be added in pairs, so a 1 GB upgrade would force you to install two 512 MB modules. Machines of this type have eight slots, at least two of which are filled at the time of manufacture.

The following rules apply to the speed of modules:-

  1. The speed of any module must be as good as the original RAM or better

    Remember, using modules that are faster than the computer won’t improve its performance, whilst using slower modules can stop it from working. Typical speeds include 60, 70, 80, 100, 120 and 150 nanosecond (ns). The chips on a module are often marked with 6, 7 or 8 to show the speed. Most older machines are happy with 70 ns modules.

  2. Avoid mixing different speed modules

    You should certainly never mix the speed of modules within a single bank.

Dual In-line Memory Module (DIMM)

Modern computers, including PowerPC-based models and those incorporating a Peripheral Component Interconnect (PCI) bus or using 64-bit addressing, are usually fitted with RAM in the form of DIMMs, often of the 168-pin variety.

The most common types of DIMM are:-

Error Correction Code RAM (ECC-RAM)

This form of DDR-SDRAM, usually of the PC3200 variety, incorporates a mechanism that prevents memory errors from crashing the computer, and is used in Apple’s G5-based servers. Such devices have 184 pins and can run at a bus clock rate of 400 MHz.

Double Data Rate SDRAM (DDR-SDRAM)

This form of RAM, used in almost all modern computers, is a development of normal SDRAM technology (see below). Common sizes include 128, 256 and 512 MB, although larger capacities are also available. Varieties include the PC266, which runs 20 to 30% faster than a PC100 or PC133 (see below), and the more recent PC1600 (100 MHz), PC2100 (133 MHz) and PC2700 (167 MHz) modules, all effectively running at twice the given rate. Such devices are commonly known by other names: for example, the PC2100 is also referred to as a DDR266, since it really operates at 266 MHz. All of these RAM modules operate at 2.5 volts, making them incompatible with hardware that’s designed for older 3.3 volt SDRAM devices.

Synchronous DRAM (SDRAM)

This kind of RAM is used in many computers. The original PC133 (133 MHz) type is used in older Mac G4 machines and is available in 64, 128, 256 and 512 MB sizes, while the older PC100 (100 MHz) device, as used in some iMacs, G3 and G4 models, comes in 32, 64, 128 and 256 MB versions. The much earlier PC66 (66 MHz) variety is only found in older PCs.

Small Outline DIMM (SO-DIMM)

This variety is smaller than a standard module, with only 144 pins instead of 168, and can be found in PC133, PC100, PC66 and PC2100 varieties. Older devices in iMac, PowerBook G3 and iBook models come in 32, 64, 128 and 256 MB sizes, although larger capacities are used in later machines.

Extended Data Out RAM (EDO-RAM) 5V

This is the oldest form of EDO device, as used in the PowerMac 7300 to 9600 series and other models. This type of RAM is buffered, operates at 5 volts and has a 2 K refresh rate, offering a 15% speed improvement over any FPM-RAM (see below) with the same speed rating. Some computers equipped with FPM-RAM also accept EDO-RAM, although in some models this doesn’t give an improvement in performance. This kind of RAM is supplied in 16, 32, 64 and 128 MB sizes.

Extended Data Out RAM (EDO-RAM) 3.3 V

A later proprietary form of EDO-RAM, as used in the PowerMac 4400 series and in Mac ‘clones’ such as the Motorola Starmax. This type of RAM is unbuffered, operates at 3.3 volts and has a 1 K or 2 K refresh rate, giving an improved speed performance. The device has offset pins, preventing use in a machine designed for 5 volt DIMMs. This kind of RAM is supplied in 16, 32, 64 and 128 MB sizes.

Fast Paging Mode RAM (FPM-RAM)

A fast 5 volt device, as used in older Apple PCI-based PowerPC models, such as the Performa 5400 to 7200 series. This kind of RAM is supplied in 16, 32, 64 and 128 MB sizes.

Single In-line Memory Module (SIMM)

SIMMs are used in 680x0-based Macs and older PowerPC-based models. There are three basic varieties of SIMM, so you must check you have the correct type for your particular machine.

The numbers on the SIMM’s ‘chips’ can suggest its size. Typically, 256A, 1000A, 1024A, 4000A, 4400A, 8000A or 8192A shows the bit capacity of each chip. If the SIMM has 8 or 9 chips the total RAM could be 256 KB, 1 MB, 4 MB or 8 MB respectively.

The most common types of SIMM are:-

72-pin SIMM

This kind of SIMM is used in later 680x0 and early PowerPC machines and is designed for 32-bit or 36-bit addressing, although they can be ‘paired’ for 64-bit addressing in a PowerPC-based machine. Unfortunately, these devices can’t be used in older machines that need a 30-pin or 64-pin SIMM.

30-pin SIMM

As used in early Macs that employ 8-bit addressing, this device typically contains two to eight ‘chips’. In later models these SIMMs are used in pairs for a 16-bit addressing or in groups of 4 for a 32-bit addressing. Sadly, they can’t be used in newer machines that require 64-pin or 72-pin SIMMs.

The smaller sizes are only used in very old machines, as shown below:-

256 KBMac Plus and others
512 KBIIsi, IIci
2 MBLC, LCII, IIsi, IIci, ​Performa 400/600

30-pin SIMMs come in a number of confusing variations:-

Size (MB)ParityNon-parity
11 × 91 × 8
22 × 92 × 8
41 × 361 × 32
82 × 362 × 32
164 × 364 × 32
328 × 368 × 32

64-pin SIMM

A special type of SIMM, as used in the Mac IIfx computer and LaserWriter II NTX printer. It can’t be used in other machines that require a 30-pin or 72-pin SIMM. In the case of a Mac IIfx computer, you must use MODE32 software or Mac OS 7.6.1 to address more than 8 MB of RAM.

Thin Small Outline Package (TSOP) SIMM

Another special SIMM, used in older 680x0-based PowerBook computers, containing psuedo-static RAM (PSRAM) that’s intended to minimise battery consumption. Such modules are often designed for a particular PowerBook model. Typical sizes include 4, 8, 16, 32 and 64 MB.

Video RAM (VRAM)

A computer’s video hardware usually includes a special random access memory (RAM) known as video RAM (VRAM). This stores the image currently displayed on the computer’s screen, its size setting the maximum pixel count of the display.

Operation and Location

The oldest Macs, as well as some Intel-based PCs and odd graphics cards, use part of the main RAM as VRAM. This gives faster graphics with a PCI graphics card, since it avoids bottlenecks caused by the slow PCI bus, a problem encountered in older PowerPC-based Mac OS machines. Despite this, most modern video hardware has a separate video RAM.

Computers with a built-in video output have their VRAM on the motherboard. In other machines it’s on the video card or graphics processor card, which is installed in an expansion slot or a dedicated video card slot.


Higher resolutions and pixel counts always require more VRAM. For example, the original Mac Plus with its tiny one-bit monochrome screen, uses 21 KB of memory that’s ‘borrowed’ from the main RAM. In comparison, a video card needs a least 256 KB of independent VRAM, whilst most modern cards having a capacity of between 16 MB and 256 MB.

The modest ATI Rage 128 video card comes with 16 MB of RAM, although more sophisticated cards have proportionally more RAM, usually in the form of DDR-SDRAM. Examples include the ATI Radeon 7500 card, with 32 MB, the NVidia GEForce4 MX, with 64MB, and the later ATI Radeon and NVidia GeForce products, with 128 MB or 256 MB.

This table shows the colour depths available with various amounts of RAM and pixel counts:-

Pixel ▼ ​Count256 ​KB512 ​KB768 ​KB1 ​MB2 ​MB4 ​MB
640 × ​4804816243232
832 × ​624488163232
1024 × ​768-4881632
1152 × ​870-4481632
1600 × ​1200---4816

RAM Modules

The video RAM in a video card or computer is often in the form of a single in-line memory module (SIMM), dual in-line memory module (DIMM) or small outline DIMM (SO-DIMM) and is usually different to the devices used for a computer’s main RAM. Before buying any type of video RAM you should double-check that it’s suitable for your video hardware.

The basic types of video RAM are:-

Double Data Rate SDRAM (DDR-RAM)

A very fast form of SDRAM, as used in modern hardware and usually in 2.5 volt form.

Synchronous DRAM (SDRAM)

A fast type of DRAM, usually supplied as a 3.3 volt EDO chip in a DIMM package. Such devices have offset pins to stop you putting them into hardware designed for a 5 volt chip.

Synchronous Graphics RAM (SGRAM)

A fast form of RAM, as used in the video card of some Apple G3 models.

Dynamic RAM (DRAM)

A slightly slower and cheaper form of RAM, usually supplied as a 5 volt Extended Data Output (EDO) chip in a DIMM package. This kind of RAM can’t be used in modern machines.

Video RAM (VRAM)

A fast form of RAM specifically designed for video hardware of an older vintage. In 680x0-based Macs this is usually in the form of a 256 KB or 512 KB SIMM but in some older PowerPC-based models it’s supplied as a 1 MB or 2 MB DIMM.

Fortunately, some video cards have 120-way sockets that allow you to install several different types of DIMM, such as 5 volt EDO DRAM, 3.3 volt SDRAM or 3.3 volt SGRAM.

Parameter RAM (PRAM)

The PRAM is a small area of battery-protected RAM, independent of the main RAM, that a Mac OS computer employs for storing special information. This includes any data that needs to be retained between startups, such as your choice of mouse speed, startup drive or date and time settings. The hardware used by the PRAM can’t be expanded or upgraded.

The lower 128 bytes of PRAM contain basic settings, such as boot services, date of manufacture and hours of use, whilst the upper 128 bytes retain some control panel settings. When you zap the PRAM, usually by pressing -Option-P-R whilst starting up your computer, only the upper bytes are cleared. Unfortunately, some third-party software also uses the lower PRAM, which can cause problems, requiring you to use a special utility, such as TechTool or TechTool Pro, to clear it.

The PRAM Battery

The data in PRAM is protected by one or two 3.6 volt non-rechargeable lithium (Li) or rechargeable lithium-ion (Li-ion) batteries. Fortunately, even the non-rechargeable variety have a life of three to five years, and often last much longer.

Some older PowerBooks use a lithium PRAM battery that’s supplemented by a trickle of mains power, even when the Mac is turned off but plugged into the mains. Power isn’t taken from the battery until the computer has been unplugged from the wall for 10 minutes or more. The rechargeable Li-ion PRAM battery in a modern PowerBook is automatically charged from any available source of power, usually the computer’s main Li-ion battery or an external power unit.

Signs of a Failing Battery

If the voltage of a PRAM battery falls below 2.8 V, or it fails completely, all settings in the PRAM are lost. Symptoms vary according to the version of Mac OS that’s in use, although any of the problems listed below can indicate a failure. The details shown here refer specifically to the Classic Mac OS.

Whilst your battery is faulty, you must adjust the appropriate control panels each time the machine is started. However, if you’ve previously saved your PRAM settings to disk using TechTool or TechTool Lite, you can employ this application to restore the former settings.

Replacing the PRAM Battery

To replace a faulty battery you’ll need to open up the computer. In most instances, the battery is inside a battery holder on the motherboard. A typical 3.6 V battery in its holder is illustrated below:-

First of all, you’ll need to remove the upper part of the holder from its base, preferably using a non-metallic object such as a Biro pen cap, thereby avoiding any possible damage to the motherboard. Before removing the faulty battery you should take note of the end that’s marked by a + sign. Fortunately, a matching sign is often provided on the holder or motherboard.

The diagram below shows how you can remove the battery itself:-

Replacement batteries of this type are available from Apple (part number 742-0011) or, in the UK, at Maplin Electronics (part number GS99H). Other equivalents include Hitachi Maxell ER35 TC, Tadiran TL5101 or TL2150, Electrochem 3B26, Ternacell T04/41, Varta VL1/2AA and Radio Shack 23-026.

Other types of PRAM batteries are often connected via a plug and socket, allowing you to use a separate battery external to the motherboard, assuming space is available. This should provide 4.5 V or less, or exactly 4.5 V if the computer originally had a 4.5 V battery. It can be provided by connecting three 1.5 V alkaline cells in series, ideally by using a three-cell battery holder. Failing this, you can use a four-cell holder, although you’ll have to fit a piece of metal in place of the fourth cell.

Having replaced the battery, settings can be restored manually or by using TechTool.


MacWorld magazine (UK), IDG Communications, 2002-2004

Replacing A Flat Battery, Frank Brook, Macintosh & Apple Computer Users Society [MaCus]

©Ray White 2004.