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Mac Memory Explained

All you ever wanted to know about buying memory for you Macintosh

Click here to link to the ultimate guide to installing memory for your Macintosh

Confused about the different types of memory that are being used in
current Apple products? What is meant by FPM, EDO, SDRAM, and SGRAM? Which Power Macintosh computers support these different memory types?
Fast-Page Mode (FPM), Extended-Data Out (EDO), Synchronous Dynamic Random Access Memory (SDRAM), and Synchronous Graphic Random Access Memory (SGRAM) are different types of memory used in various Power Macintosh computers.


*NOTE: The new G3 Macs use SDRAM (3.3-volt, unbuffered, 64-bit wide, 168-pin, running at more than 100 MHZ, 10-nanosecond cycle time) memory.
Also the Macintosh Performa 6400/200 computer that includes an internal Zip drive supports EDO memory.
Fast-Paged Mode Memory

A specific location in a memory chip is identified by the row and the
column addresses. Each time memory is accessed, the memory controller first supplies the chip with the row address and then the column address. After the information obtained from these locations is validated, the column deactivates and gets ready for the next cycle. This introduces a wait state because nothing is happening while the column is deactivating. The processor must wait for the memory to complete the cycle.
The Fast-Paged Memory (FPM) chip decreased the time required to read these addresses by allowing the memory controller to select a particular row and then access the corresponding column addresses for that row. This process works under the assumption that the next piece of data needed is in the memory location adjacent to the previous piece. Because the row address is only set up once and only the column address changes, this saves time when reading or writing information to or from memory.
Using FPM memory in Power Macintosh computers.

Most Power Macintosh computers support Fast-Paged Mode Memory. The only exceptions are the Power Macintosh 4400, 5500, and 6500 series computers.
EDO Memory

EDO (Extended Data Out) DRAM is a subset of FPM memory that saves the memory controller even more time by allowing it to begin locating the row and column for the next address, while reading the data at the first address. It is able to do this because EDO memory keeps the output buffer on while preparing for the next read operation. By keeping the buffer on, EDO eliminates wait states. This speeds up data transfer rates since EDO RAM can access data continuously without waiting for addresses to be located. This reduces the time of the read cycles by approximately 10%. However, during write cycles, the system behaves exactly as an FPM chip would behave.
Although EDO devices improve timing efficiency to main memory by approximately 10%, it does not necessarily mean programs will execute 10% faster. The processor often gets instructions and data from cached memory, for example, L1 cache within the PowerPC microprocessor and or L2 cache on the logic board.
Using EDO memory in Power Macintosh computers

Because EDO DRAM is a subset of FPM memory EDO DRAM can typically be used in place of Fast Page Mode DRAM. However, unless the memory controller is designed to use the faster EDO timing, the memory performance will be the same as Fast Page Mode.
There are three categories of Power Macintosh computers based on their level of EDO memory support. In some Power Macintosh computers, you canuse EDO memory and get some potential performance boosts. In others, you can use EDO memory even though you will not derive any benefits from doing so. Finally, there are some Power Macintosh computers in which Apple does not recommend using EDO memory because doing so may damage your computer.
Power Macintosh Computers that Support EDO Memory

The following Power Macintosh computers fully support EDO memory and may experience increased performance:
- Power Macintosh 4400 series

- Power Macintosh 5500 series

- Power Macintosh 6500 series

- Macintosh Performa 6400/200 with internal Zip drive
All memory installed must be EDO to take advantage of the benefits.
If you mix FPM and EDO RAM, the EDO modules will perform as if they were FPM.
IMPORTANT: There are two types of EDO memory--5 Volt and 3.3 Volt. The Power Macintosh 4400 series requires 3.3 Volt EDO memory whereas the Power Macintosh 5500, 6500, and 6400 computers require 5 Volt EDO. The two types of EDO memory are NOT interchangeable. Most 5 V and 3.3 V EDO memory are physically keyed differently to prevent you from using the wrong type, but even if the DIMM can be installed, verify that you are installing the correct type.
Additionally, EDO memory can also be used as video memory in the Power Macintosh 4400 ONLY. However, when using EDO memory as video memory, the Power Macintosh 4400 uses 5 Volt EDO memory rather than the 3.3 Volt used as DRAM.
Power Macintosh Computers that Can Use EDO memory

Although you will not derive any performance benefits by using EDO
DIMMs, you can use them in the following Power Macintosh computers:
- Power Macintosh 6100 series

- Power Macintosh 7100 series

- Power Macintosh 8100 series

- Power Macintosh 5200 series

- Power Macintosh 5300 series

- Power Macintosh 5400 series

- Power Macintosh 6200 series

- Power Macintosh 6300 series

- Power Macintosh 6400 series

- Power Macintosh 7300 series

- Power Macintosh 7500 series

- Power Macintosh 7600 series

- Power Macintosh 8500 series

- Power Macintosh 8600 series

- Power Macintosh 9500 series

- Power Macintosh 9600 series

- Macintosh Performa 6360 series

- Some Macintosh Performa 6400 series

 
Power Macintosh Computers that CANNOT use EDO memory-

You cannot use EDO DIMMs in the Power Macintosh 7200 computer. Using EDO memory in the Power Macintosh 7200 computer can cause damage to the logic board and to the DIMMs. Because of this, Apple does not support using EDO memory in the Power Macintosh 7200 computer. Any damage incurred from using EDO memory in the Power Macintosh 7200 computer may not be covered under Apple Computer's limited hardware warranty. EDO RAM has not been tested, and has not been certified or is supported by Apple in the Workgroup Server 9150/xx series computers. SDRAM Memory

The processes performed by a computer are coordinated by an internal clock, but memory access has traditionally used its own fixed timers for reading and writing data. Rather than synchronizing its actions with those of the internal clock, memory access had set times for reading and writing data regardless of the actual time the processes required. This would sometimes result in periods of wait cycles where nothing was happening. Because of this, memory was considered to be "asynchronous".
However, Synchronous Dynamic Random Access Memory (SDRAM) eliminates this difference between memory speed and processor speed because SDRAM has a clock synchronized with the computer's central processing clock.
Thus, SDRAM uses only the time required to read/write data which increases data transfer rates by eliminating non-productive periods of waiting. The clock coordinates with a computer's central processor's clock so that data can be delivered continuously to the microprocessor. The timing coordination between memory, the microprocessor, and other support chips permits more efficient memory access and eliminates wait states. This results in memory access speeds of up to 20% faster than EDO.

 
Using SDRAM memory in Power Macintosh computers.

Only the Power Macintosh 4400/200 series support the use of SDRAM.
Additionally, SDRAM is supported in these computers ONLY for video memory.
You cannot use SDRAM as the primary memory devices on the logic board.
The Power Macintosh 4400 series includes 2 MB of EDO memory for video memory, but it supports up to 4 MB of SDRAM or SGRAM. SGRAM Memory

Synchronous Graphics Random Access Memory (SGRAM) functions similarly to SDRAM except that it has added graphics support. Graphics support is provided by adding block write and masked write (or write-per-bit) functionality. Block write enables the graphics engine to do block transfers of graphical data, such as tiling, and to interpret these larger data packets. Block write is often used in 3-D operations to clear the buffers or to prepare them for new rendering. With the block write function in the graphics memory, the graphics engine is free to do other tasks which increases performance. Masked write simplifies changing selected bits in a block of data. Masked write increases graphics performance with tasks such as color management of the display.


Using SGRAM memory in Power Macintosh computers

SGRAM is supported in the Power Macintosh 4400, 5500, and 6500 series.
SGRAM is supported in these computers for video memory ONLY; you cannot use SGRAM as the primary memory devices on the logic board.
The Power Macintosh 4400 series includes 2 MB of EDO memory for video memory, but it supports up to 4 MB of SDRAM or SGRAM. However, the Power Macintosh 5500 and 6500 series computers include 2 MB of SGRAM, which is NOT expandable.

Compatibility
=============
The Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers use JEDEC-standard 168-pin DIMMs (dual inline memory module) DRAM cards rather than the 72-pin SIMM DRAM cards used in the Power Macintosh 5200 and 6200 computers. The DIMMs should be 64-bit-wide, 168-pin fast-paged mode, 70ns RAM access time or faster, and 2K refresh rate. Although you can use EDO memory in the Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers, you will not experience any benefit from doing so. SIMMs from older Macintosh computers are not compatible with your computer and should not be used.

Additionally, you cannot necessarily use the same DIMMs as you can in other PCI-based Power Macintosh computers such as the 7200, 7500, 7600, 8500, and 9500 series computers. These other Power Macintosh computers can support DIMMs with either a 2K or 4K refresh rate. However, the Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers can only support DIMMs with a maximum of a 2K refresh rate*. DIMMs incorporating 4K refresh parts will function in these computers but at half the memory density. For example, the computers will only recognize 16 MB of a 32 MB DIMM, and so on.

To prevent compatibility problems, Apple has contacted major RAM developers in several ways. In the Developer Notes for the Power Macintosh 5400/120 and for the Macintosh Performa 6400 series, the following note was added:

"DRAM DIMM developers should note that the PSX memory controller on the main logic board of the Power Macintosh 5400 computer does not provide support for 4 M by 4 bits (12 by 10 addressing) or 1 M by 16 bits (12 by 8 addressing) DRAM devices."

Additionally, Apple sent a message to specific vendors which stated:

"The PSX memory controller does not support 4Mx4 DRAM devices with 12 by 10 addressing nor 1Mx16 devices with 12 by 8 addressing. Other 16 MBit devices are compatible, for a full list refer to the Developer Note."

The following RAM developers received this note:
* Vikking
* Techworks
* SQP
* Southland
* PNY
* Newer Technology
* Lifetime, Kingston
* Delta Lu, Comtech
* Apple Japan

DIMMs available from Apple Computer are 2K refresh rate only so you can safely use them in your Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers.

*Notes:
- Refresh rates lower than 2K, such as 256K or 1K are also supported.
- The only memory devices with a 4K refresh rate which is supported on the Power Macintosh 5400/120 and Macintosh Performa 6400 series are 2M x 8 devices with 12 x 9 addressing.

Expansion
=========
The Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers have two RAM expansion slots on the logic board. You can use any DRAM configuration with DIMMs of these sizes: 8, 16, 32, or 64 MB. The exact configuration depends on the density of the DRAM chips that are mounted on the DIMMs.

You can increase your computer's DRAM to up to a maximum of 136 MB (the 8 MB that comes on your computer's logic board, plus an additional 128 MB for a total 136 MB).

DIMMs can be installed one or more at a time. The Power Macintosh 5400 and 6400; Macintosh Performa 6400 series; and Macintosh Performa 6360/160 computers support only linear memory organization. Therefore, no performance gains are seen when two DIMMs of the same size are installed. Any size DIMM can be installed in either DIMM slot, and the combined memory of all of the DIMMs installed will be configured as a contiguous memory space.


Some information provided here was obtained from Apple's Technical Information
Library.