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Chapter 4. Memory > RAM Memory Types

RAM Memory Types

Macs through the years have used various types and sizes of RAM. Here's a technical rundown of the types of memory modules used in Macs, through the mid-2001 models.


SDRAM (Synchronous Dynamic Random Access Memory) is the most common type of RAM manufactured as well as the most widely used and lowest in cost (due to high production volumes). Starting with the first Power Mac G3 (often called the Beige G3 or Platinum G3), Apple began using SDRAM in all models. SDRAM was required for the faster main system-bus speeds, which contributed to better overall performance. All Mac-compatible SDRAM modules must be 3.3-volt, unbuffered, 8-byte, nonparity, non-ECC types. (These are actually the most common.) Parity DIMMs use an extra bit as an error check and are not compatible, nor are ECC (Error Correcting Code) DIMMs. (ECC RAM is often used in non-Mac servers or workstations.)

In addition to the memory chips, SDRAM modules contain a small chip that is basically a ROM with the module's characteristics and timing. The system reads this information to identify the module type. This chip is called the SPD (Serial Presence Detect). Information contained in the SPD includes the speed rating (PC133, PC100) and the memory timing parameters, such as CAS (Column Address Strobe) timing and latency. Most common or lower-cost PC133 and PC100 DIMMs have CAS timings of 3-2-2, although the lower-latency 2-2-2 DIMMs are available often for the same price (at least in the PC100 variety) if you shop around. The slowest SDRAM timing made currently is 3-3-3.

CAS timing is often referred to as a “CL” rating. For instance, you may see 2-2-2 SDRAM referred to as CL2 and 3-3-3 referred to as CL3. The Apple System Profiler on 2001 PowerPC systems, for instance, will report CL2 or CL3 instead of the 222 or 333 ratings it shows on earlier models.

SDRAM is made up of four banks; each has its own internal clock synchronized with the system-memory bus. When one bank is active and switches to the adjacent bank for input/output, there is a switching delay before continuing I/O operations. For the slowest speed code (3-3-3) SDRAM, this means that when switching from one bank to another, the CPU has to wait three clock cycles. With the fastest speed code (2-2-2) SDRAM, the CPU only has to wait for two clock cycles.

Although 2-2-2 is the lowest-latency (or the shorter-delay) SDRAM, the difference in real-world performance between 2-2-2 and 3-2-2 SDRAM that I have seen in testing is small—about 2 percent in Photoshop tests on a PC100-based system with a 10 MB image file. The difference between 2-2-2 and 3-3-3 would be greater, however, and those insisting on maximum efficiency will want to buy only 2-2-2 SDRAM. The good news is, if you shop around, it doesn't always cost more than the more common 3-2-2 types.

By the way, the memory controller in the Beige and Blue and White Power Mac G3s runs all DIMMs at the CAS timing of the slowest installed module. I have not seen test results on the later models to determine if the controller in the later Core99 or Uni-N chipset operates the same way.

For owners of the iMac 350 MHz and up; iBook; PowerBooks with FireWire; PowerBook G4; and Power Mac G4s with AGP, Apple's System Profiler will report your memory's CAS timing. Figure 4.7 shows a clipping from the Apple System Profiler (under the Apple menu), taken from a PowerBook FireWire model. To the right of the “Built-in memory” size is “PC100-222S,” which denotes that PC100 modules with 2-2-2 timing (fastest) are installed.

Figure 4.7. For Macs made since 1999, Apple System Profiler will show your memory's rating.

Apple System Profiler shows CAS timing on 2001 Power Mac G4 models as a single CL2 or CL3 rating rather than the full 222, 322, or 333 ratings shown with the Power Mac G4 Cube, pre-2001 Power Mac G4s, PowerBook G3 (FireWire), and slot-loading iMacs. Only 1999 and later model Macs will show memory timing in Apple System Profiler.

SDRAM is available in two physical sizes: 168-pin DIMMs for desktops; and the smaller, 144-pin SODIMMs (Small Outline DIMMs) for the PowerBook G3 series, early iMacs (233 MHz through 333 MHz), iBooks, and the 2001 PowerBook G4.

There are several grades of SDRAM named by their bus-speed rating (see the “Mac Models and Compatible SDRAM Types” section):


As the name implies, these modules are designed for computers with a 133 MHz system bus. They are 168-pin DIMMs commonly available in 64 MB, 128 MB, 256 MB, and 512 MB sizes. The 2001 Power Mac G4s are the first Macs to use PC133 RAM. Apple refers to the early 2001 Power Mac G4s as Power Mac G4 (Digital Audio) to differentiate them from previous Power Mac G4 models of 1999 and 2000, which used PC100 memory and 100 MHz bus speeds. The Power Mac G4 (QuickSilver) models introduced in the summer of 2001 also use PC133 RAM.


Rated for 100 MHz system buses. They are available in 168-pin DIMMs for desktops and 144-pin SODIMMs for notebooks (and early iMacs). In systems that use PC100 RAM, faster types such as PC133 can be used, with the exceptions I note in the listing of systems that follow. Installing PC133 RAM in a PC100-based system won't make the RAM run faster, however, since memory bus speeds are controlled by the Mac's logic board. An analogy would be putting 133 mph–rated tires on a car has a maximum speed of 100 mph. The higher-rated tires can't make the car go faster. One benefit of buying PC133 is that it's usable in the 2001 Power Mac G4s, should you buy a new system later.


Rated for 66 MHz system buses. Available in 168-pin DIMM and 144-pin SODIMM modules. PC66 memory is rarely made now, as systems moved to faster bus speeds years ago. (See below for compatibility notes on using PC100 or PC133 RAM in Macs originally designed for PC66 memory.)

Mac Models and Compatible SDRAM Types

Power Mac G4 (QuickSilver).

Introduced in July 2001, the QuickSilver version of the Power Mac G4 use the same PC133 RAM Apple first used in the Power Mac G4 (Digital Audio) models. These machines have three DIMM slots and can accept DIMM sizes of 128 MB, 256 MB, and 512 MB, for a maximum of 1.5 GB of RAM.

Power Mac G4 (Digital Audio).

In January 2001, Apple announced a new line of Power Mac G4 systems using a 133 MHz system bus and requiring PC133 RAM. The Digital Audio G4s have three DIMM slots, as opposed to four in the previous G4 towers. Compatible DIMM sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB. The system has a maximum of 1.5 GB of RAM. For more information on the 2001 Power Mac G4 RAM specifications, see http://til.info.apple.com/techinfo.nsf/artnum/n58763.

Power Mac G4 AGP Graphics.

In fall 1999, Apple released its first systems with an AGP graphics-card slot. In summer 2000 it made Gigabit Ethernet standard, replacing the 10/100-Mbps Ethernet of the original models. You can tell if your Power Mac G4 is a Gigabit model by the small silver heat sink on the networking chip on the logic board. These systems have four DIMM slots for PC100 RAM. Compatible DIMM sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB. The maximum addressable RAM (in Mac OS 9) is 1.5 GB.

Power Mac G4 Cube.

The G4 Cube uses a 100 MHz system bus and PC100 (or faster) SDRAM DIMMs. Compatible sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB. The G4 Cube has three DIMM slots for a maximum of 1.5 GB of RAM.

Power Mac G4 PCI Graphics.

This system, introduced in September 1999, is based on the same logic board (or motherboard) as the Blue and White Power Mac G3 and uses PC100 SDRAM. PC133 SDRAM should also work if the DIMM is composed of memory chips that are 128 Mb or less density. Compatible sizes are 32 MB, 64 MB, 128 MB, and 256 MB. (Note: 256 MB DIMMs will only be fully recognized if they are constructed of 128 Mb chips. If a 256 MB DIMM has only 8 chips, it's using 256 Mb memory chips and would not be compatible.) No 512 MB DIMMs are compatible, as they are composed of 256 Mb chips.

The Power Mac G4 PCI Graphics systems had a short production life and were discontinued after three months with the release of the first Power Mac G4 AGP Graphics systems. If you're unsure if your Power Mac G4 is the PCI or AGP model, look at the graphics-card slot (the slot closest to the middle of the logic board). If the connector on the logic board for the graphics card is white, the computer is a PCI Graphics box. If it's brown, it's an AGP Graphics model.

Power Mac G3 (Blue and White).

The Blue and White Power Mac G3 was the first Mac to use a 100 MHz system bus, three 64-bit PCI slots, and one 66 MHz 32-bit PCI slot. It uses PC100 SDRAM DIMMs, although PC133 SDRAM should also work if the DIMM is composed of memory chips with 128 Mb or less density. Compatible sizes are 32 MB, 64 MB, 128 MB, and 256 MB, with the same chip-density restrictions noted for the Power Mac G4 PCI Graphics model. The motherboard has four DIMM slots, for a maximum RAM capacity of 1 GB using compatible 256 MB DIMMs. The 512 MB DIMMs are not compatible, as they are composed of 256 Mb chips.

Power Mac G3 Desktop, Mini Tower, and All-In-One (commonly called the Beige Power Mac G3s).

These were the first SDRAM-based Macs and have a 66 MHz system bus. PC66 RAM is hard to find now, and most owners buy PC100 DIMMs. The PowerMac G3 uses the same memory controller as the Blue and White Power Mac G3 and Power Mac G4 PCI Graphics models and cannot fully address memory chips denser than 128 Mb. Compatible sizes are 32 MB, 64 MB, 128 MB, and 256 MB. (Some 256 MB DIMMs may use eight 256 Mb chips instead of sixteen 128 Mb chips. Only the latter is compatible.) No 512 MB DIMMs are compatible with these models. The motherboard has three DIMM slots, for a maximum RAM capacity of 768 MB using compatible 256 MB DIMMs.

Installing RAM in G3s

The maximum DIMM height for Power Mac G3 Desktop models is 1.15 inches. For a guide to installing RAM (or VRAM) in a Beige G3, see www.xlr8yourmac.com/G3-ZONE/G3_RAM.html.

PowerBook G4.

The low-end PowerBook G4 uses the same PC100 SODIMMs as the FireWire model but has a 1.5-inch maximum module height. The higher-end models have 133 MHz system buses and use PC133 SODIMMs. Compatible sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB. Make sure that the modules you're buying are 1.5 inches or less in height (this is especially important to verify when you are looking at 256 MB or 512 MB modules). The PowerBook G4 has two memory slots under its keyboard for a maximum of 1 GB or RAM.

PowerBook G3 (FireWire).

The FireWire model PowerBook, rolled out in February 2000, was the first Mac portable to use PC100 SODIMMs and a 100 MHz system bus. Compatible sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB. It has two available slots (one under and one on top of the CPU module). Maximum RAM is 1 GB.

PowerBook G3 series (1998–1999).

These models used PC66 SODIMMs, with the exception of the PowerBook G3/250 MHz and G3/292 MHz (1998), which used an 83 MHz bus speed and needed faster RAM. Code names for these models were Wall Street (1998 G3/233 to G3/300) and Lombard (1999 G3/333 and G3/400 with the Bronze keyboard). Since PC100 SODIMMS are the most common (and lowest in cost), it's best to buy those types. Available sizes are 32 MB, 64 MB, 128 MB, and 256 MB. Because of the memory controller used in these models (the same as the Power Mac G3 noted earlier), 512 MB modules cannot be used. The low-profile 256 MB modules I've owned use a trick of stacking 128 Mb chips to allow them to fit on the smaller SODIMM. There are two expansion slots (one under and one on top of the CPU module). Compatible sizes are 32 MB, 64 MB, 128 MB, and 256 MB (specials). Maximum RAM is 512 MB using two compatible 256 MB SODIMMs.

The bottom slot supports low-profile 1.25-inch memory modules only; the top slot supports up to 2-inch memory modules. 256 MB low-profile modules available at sources such as TransIntl.com (www.transintl.com).

PowerBook G3 first model (aka 3500/Kanga).

Unlike the later PowerBook G3s, this model uses a nonstandard memory-module design. The logic board has 32 MB of RAM soldered in and one RAM expansion slot. The maximum RAM is 160 MB (32 MB onboard plus a 128 MB module).


As of Fall 2001, the low-end iBook has a 66 MHz bus speed and requires PC66 (or faster) SDRAM SODIMMs. The higher-end iBooks have 100 MHz system buses and use PC100 SODIMMs. The iBooks have either 32 MB or 64 MB of RAM soldered on the logic board with one expansion slot. Commonly available sizes are 32 MB, 64 MB, 128 MB, and 256 MB. Although scarce at the time this was written, there are sources of 512 MB iBook compatible SODIMMs from vendors such as TransIntl.com (www.transintl.com). This high-density module is listed there as compatible with every iBook model made and allows them to exceed Apple's original maximum memory limits. (Apple's info was written before the high-density modules were available.) Since there's only one expansion slot, the maximum RAM depends on the iBook model's base RAM on the logic board. (You can add a SODIMM to complement the onboard soldered-in RAM).

iMacs—350 MHz and faster.

The iMac G3/350 and faster models use standard PC100 SDRAM DIMMs. They have two RAM slots, which are easily accessible without disassembly (unlike earlier iMac models). Compatible sizes are 32 MB, 64 MB, 128 MB, 256 MB, and 512 MB, for a maximum of 1 GB of installed RAM. (These iMacs use the same chipset as the Power Mac G4 AGP Graphics systems and can therefore use 256 Mb chip–based modules.)

iMacs—233 MHz to 333 MHz.

The original iMac series uses the same SDRAM SODIMM modules as the PowerBook G3 series and actually has a similar CPU module design. (The RAM slots are on the CPU module as in the PowerBook G3 series.) These iMacs have a 66 MHz bus and use PC66 or faster SODIMM modules. As with the PowerBook G3 series, the bottom slot supports low-profile 1.25-inch memory modules only, and the top slot supports up to 2-inch memory modules. There are 256 MB 1.25-inch modules available at sources such as TransIntl.com. Otherwise, the maximum SODIMM size is 128 MB.


The memory configurations of the pre-Power Mac G3 series Mac models are too numerous and varied to cover individually here. Apple frequently updates its Memory Guide, which lists Mac models and RAM compatibility. You can download the guide as an Adobe Acrobat PDF file at http://asu.info.apple.com/swupdates.nsf/artnum/n10084. You may notice that in some cases, such as the PowerBook G3 FireWire, I've put maximum RAM sizes higher than Apple's guide lists. This is because the guide does not reflect the use of higher-capacity modules. Another great RAM-compatibility guide is Newer Technology's GURU application. Although the company is no longer in business, GURU is still available at http://eshop.macsales.com/Tech/index.cfm?load=newertech.html.

GURU does not include information on the latest high-density memory modules, so the maximum RAM size listed may not be accurate for SDRAM-based Mac models. (The same is true of Apple's current Memory Guide also.) Its primary benefit in my opinion is for researching older Mac models.

A utility more up to date than GURU is available. Called MacTracker, it lists RAM information for all Mac models and information on Apple's displays and printers. There's even a Mac OS X version with an Aqua interface. Find MacTracker at http://plaza.powersurfr.com/mactracker/.

The Apple Memory Guide includes an index of Apple Mac models, notes on memory types and sizes, and even logic-board diagrams showing where the RAM slots are for each model. Refer to this guide before purchasing or adding RAM to your Mac.

If in doubt, consult the Apple guide—or for clones, check sites such as www.everymac.com—and a reputable dealer of Macintosh memory.

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