The memory in a Mac is divided between Read Only Memory (ROM), fixed at
manufacture, and Random Access Memory (RAM), used as a working store for
software and documents.
Read Only Memory
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The Mac’s ROM contains data used by the system, including a Toolbox of standard routines. Its contents can’t be changed and remain indefinitely.
ROM size has increased as the system has developed. The original Mac 128 and Mac 512 used a 64 K ROM. With the 512e and the Plus this increased to 128K, with the SE, SE/30, II, IIcx and Portable it rose to 256K and with the IIci and IIfx it reached 512 K. Later 680x0 machines use 1 M ROMs — some PowerMac clones use 4 M ROMs with 64-bit addressing.
The ROM in an SE/30, IIx or IIfx is a SIMM (see below). Some models use sockets, allowing you to upgrade the ROM — but in others they’re soldered to the motherboard. Other Macs include an extra ROM upgrade SIMM slot for upgrading to a Quadra or for adding extra capabilities. Later models have a ROM in the form of a DIMM (see below).
Random Access Memory
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The contents of RAM is lost on Shut Down — but not in PowerBook models that use the main battery as backup. A separate lithium cell is used for the clock and PRAM.
Memory Logic
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Modern Macs have a Paged Memory Management Unit (PMMU) that gives access to a large amount of RAM organised in pages. This reduces the need for a wide address bus and allows part of the hard disk to be used as RAM Disk or Virtual Memory on a 32-bit Mac.
Most machines use a Direct Memory Access Controller (DMAC) to gives the processor easy access to memory and other devices. A General Logic Unit (GLUE) handles the Mac’s processor clock and RAM timing.
Hardware Cache
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Some Macs incorporate an intelligent hardware cache in addition to the software Disk Cache in the Memory control panel. A hardware cache acts as a buffer for rapid data flows and saves valuable memory space — the Mac speeds up as it learns which data to retain.
Some models have a built-in cache, others need a cache card. The cache in 68040 Macs can upset older software — the Cache Switch control panel can be used to disable it.
PowerMacs use two caches — a fixed 32 K Level 1 Cache and a Level 2 Cache in a 256 K,
512 K or 1 M memory module. Early models use a Single In-line Memory Module (SIMM) cache card but PCI-based machines use a Dual In-line Memory Modules (DIMM). In some models the DIMM is soldered in place, preventing any upgrading of cache size.
Battery Backup
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Some PowerBooks use their main battery to backup the data in RAM. Typically, they use several rechargeable (secondary) cells, usually of the nickel-cadmium (NiCd) type, in a sealed package. For reliable operation they must be charged and discharged regularly. In most models (not the PowerBook 100) they should be discharged every month — just leave the Mac running until it shuts itself down. A full day should be allowed for recharging.
The Mac’s time-of-day clock and PRAM are driven by one or two 3.6 volt lithium batteries — they’re non-rechargeable and have an expected life of three to five years. If you have clock problems or lose control panel settings, they may need replacing. Sometimes they’re easily removed but in other models they’re soldered to the motherboard. One of the batteries may be used for startup — if it fails you can’t start the computer!
In some models the lithium battery is supplemented by a trickle of mains power — even when the Mac is turned off! In these machines the battery isn’t used until the computer has been unplugged from the wall for 10 minutes or more.
Each NuBus slot in a Mac has its own PRAM which stores the control panel settings for the appropriate card. This slot PRAM can be cleared by restarting the Mac with the relevant card removed. The computer should then be Shut Down and the card replaced.
Fitting RAM
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  Mac RAM comes in the form of plug-in Single In-line Memory Modules (SIMMs) or
Dual In-line Memory Modules (DIMMs), each containing a number of RAM chips.
Before upgrading the RAM in any Mac you really must refer to the Macintosh Memory Guide (Connectix) and the GURU (Newer Technology). Both are free and provide vital information that’s difficult to find anywhere else!
Static Electricity
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When installing RAM you must always take precautions against static electricity. One small spark can wipe a module for good! Just follow these simple rules:-
‡ Ensure the computer is earthed. If your model has a cover that simply unclips you
can simply leave the mains cable plugged in with the machine turned off. For
other models you’ll need to make up a special mains extension lead with only the
earth circuit connected.
‡ Make sure you or the computer aren’t standing on a nylon carpet or similar
static-inducing surface.
‡ Touch the metalwork frame of the computer, or the case of its disk drive, before
removing the RAM from its packaging or touching the Mac’s motherboard.
‡ Avoid walking around the room during installation of RAM — you’ll create static!
‡ Always store RAM modules in their special non-static packaging.
Module Types and Sizes
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The different modules used in various Macs are outlined in the following sections. Some models come with a minimum amount of memory already soldered to the motherboard. The tables show RAM capacities for 32-Bit addressing — double these values if you’re using RAM Doubler!
Computers used as file servers require self-checking parity RAM modules to provide a high level of data security. They’re more expensive than ordinary SIMMs or DIMMs and don’t do anything extra in a standard machine!
° Memory modules may not be marked to indicate their size!
The only way to find its size of such a module is to put it into a computer! You can then mark the module with a fine spirit pen — preferably not on top of its circuit tracks.
SIMMs
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  SIMMs are used in 680x0 Macs and early PowerMacs. They’re often fitted into
two or more banks of sockets within the machine, each with 1, 2 or 4 sockets.
The rules for installing SIMMs are:-
‡ The speed of any SIMM must be as good as your Mac or better
Faster SIMMs don’t improve performance and slower SIMMs won’t work!
Typical speeds are 70, 80, 100, 120 and 150 nanosecond (ns) — they’re often
marked as 7, 8 etc.
‡ Avoid mixing different speed SIMMs within a machine
You should certainly never mix them within a single bank.
‡ Don’t mix sizes in a single bank
‡ Don’t partially fill a bank
‡ Bank A must be filled first
‡ Bank A should usually contain the largest SIMMs
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These SIMMs are used in the earliest Macs — they can’t be used in newer machines that employ the 64 or 72 pin versions.
/ The standard sizes are 256 K, 512 K, 1, 2, 4, 8 and 16 M.
Å 256 K SIMMs can’t be used in newer models.
512 K SIMMs can only be used in the Mac IIsi or IIci.
2 M SIMMs can only be used in the LC, LCII, IIsi, IIci, Performer 400 or 600.
Desktop Macs
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Model Speed (ns) Max RAM (M) SIMMs
Plus 150 4 • 256 K,1,4 M
SE 150 4 • 256 K,1,4 M
Classic 150 4 • 256 K,1,4 M
Classic II 120 10 1,4 M
Clr Classic 120 10 1,4 M
LC, LCII 100 10 1,2,4 M
SE/30 120 128 * 256 K,1,4,8,16 M
• 32-bit addressing is not supported
The Plus and SE require link and resistor changes for extra ROM
* MODE32 software required for 32-bit addressing
II Series
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Model Speed (ns) Max RAM (M) SIMMs
II 100 68 * 256 K,1,4,8,16 M °
IIx, IIcx 100 128 * 256 K,1,4,8,16 M °
IIsi 100 65 256 K,512 K,1,2,4,8,16 M
IIvx 100 65 256 K,1,4,8,16 M
IIci 80 128 ∆ 256 K,512 K,1,2,4,8,16 M
* MODE32 software required for 32-bit addressing
° PAL SIMMs required for 4, 8 or 16 M in a II or IIx
∆ Limited to 20 M if on-board video is used
Quadra/Centris Series
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Model Speed (ns) Max RAM (M) SIMMs
700 80 68 1,4,8,16 M #
900 80 256 1,4,8,16 M #
950 80 256 1,4,8,16 M #
# Low profile 16 M SIMMs required.
Performa Series
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Model Speed (ns) Max RAM (M) SIMMs
200 80 10 1,2,4 M
400/405 80 10 1,2,4 M
410/430 80 10 1,2,4 M
The following variations of 30-pin SIMMs may be encountered:-
Parity SIMMs
These modules contain 9 chips, rather than the usual 8, and are used for data security in file server computers. They don’t have any advantages in a standard Mac, athough they can be used. These modules shouldn’t be confused with PAL SIMMs (see below).
PAL SIMMs
A PAL SIMM contains 8 RAM chips plus a PAL or similar logic chip. You must use this variety for any module of 4 M or over in a Mac II or IIx, although they can also be used in other Macs that such sizes. Don’t confuse them with Parity SIMMs (see above).
1 M SIMM Variations
Modules with a capacity of 1 M can be constructed using either 8 chips, each containing one megabit or 2 chips each containing 4 megabits. The 8 chip version must be used in a Mac II or IIx. Other Macs accept both types!
64-pin qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq
These SIMMs are found in the Mac IIfx and the LaserWriter II NTX printer. They can’t be used in place of 72-pin SIMMs!
Model Speed (ns) Max RAM (M) SIMMs
IIfx 80 128 * 1,4,8,16 M
* MODE32 software required for 32-bit addressing
72-pin qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq
This variety of SIMMs are used in later 680x0 Macs and early PowerMacs.
/ The standard sizes are 1, 2, 4, 8, 16 and 32 M.
LC Series
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Model Speed (ns) Max RAM (M) SIMMs
III 80 36 1,2,4,8,16,32 M
475 80 36 1,2,4,8,16,32 M
520/550 80 36 1,2,4,8,16,32 M
575 80 36 1,2,4,8,16,32 M
630 80 36 1,2,4,8,16,32 M
Quadra/Centris Series
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Model Speed (ns) Max RAM (M) SIMMs
605 80 36 1,4,8,16,32 M
610 80 68 1,4,8,16,32 M
630 80 36 1,4,8,16,32 M
650 80 136 1,4,8,16,32 M
660av 80 68 4,8,16,32 M
800 80 136 4,8,16,32 M
840av 80 128 4,8,16,32 M
Performa Series
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Model Speed (ns) Max RAM (M) SIMMs
450 80 36 1,2,4,8,16,32 M
460/6/7 80 36 1,2,4,8,16,32 M
475/6 80 36 1,4,8,16,32 M
550/60 80 36 1,4,8,16,32 M
580 80 52 1,4,8,16,32 M
575/7/8 80 36 1,4,8,16,32 M
600 80 68 1,4,8,16,32 M
630/5/6 80 36 1,4,8,16,32 M
640 80 52 1,4,8,16,32 M
5200/15 80 64 1,4,8,16,32 M
5260 80 64 1,4,8,16,32 M
5300 80 64 1,4,8,16,32 M
6300 80 64 1,4,8,16,32 M
6400 80 136 1,4,8,16,32 M
Single-sided 16 and 32 M SIMMs must be fitted in some models
PowerMac Series
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Model Speed (ns) Max RAM (M) SIMMs
6100/60 80 72 4,8,16,32 M
7100/60 80 136 4,8,16,32 M
8100/80 80 264 4,8,16,32 M
TSOP SIMMs qqqqqqqqqqqqqqqqqqqqqqqqqqqqq
The PowerBook 100, 180 and others employ a special kind of SIMM to minimise battery consumption — the Pseudo-static Thin Small Outline Package (TSOP),
These PowerBooks accommodate up to 8 M of RAM, 2 M of which is on the motherboard. Up to 6 M can be added using a special plug-in card that can also contain a video card or internal modem — the RAM on these cards can’t be expanded at a later date.
Later computers, such as the PowerBooks 5300, use 4, 8, 16, 32 or 64 M cards.
DIMMs
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  DIMMs are used in PCI PowerMacs and PowerMac Performas. The standard sizes are
8, 16, 32 and 64 M. Top-flight machines have up to eight DIMM slots — cheaper
models have less slots which means they must be upgraded with larger DIMMs.
Some recent PowerPC-based computers, including those complying with the PowerPC Platform (PPCP) standard, use 3.3 volt DIMMs instead of the older 5 volt variety. These newer DIMMs also use Extended Data Out (EDO) technology for improved performance and have offset pins to prevent them being used in the wrong type of computer.
The following lists provide details for some Apple models.
