Will-Do Information Services: RAM information

• SIMM (Single Inline Memory Module): An electronic chip that a computer uses to make up its temporary workspace memory, "working memory", known as RAM.
  
  
• DIMM (Dual Inline Memory Module): These 168-pin memory chips are used in PowerMacs: 7200, 7500, 8500, 9500. In the 7500, 8500, and 9500 these chips, even though they may installed one at a time, will give the computer a speed boost if they are installed in pairs. These have a 64-bit BUS width.

Curently, MicroWarehouse sells the 30-pin SIMMs, 72-pin SIMMs, and 168-pin DIMMs. The number of pins must match the specifications of the machine it is to be used in. They are not interchangeable.

Capacity is normally measured in Kilobytes (KB) or Megabytes (MB). MB is the main unit you deal with today in terms of memory, but it is still noted in the computer as KB for example 4000 KB would mean the computer has 4MB.

SIMM speeds are measured in nanoseconds (ns), 9 decimals, 1 billionth of a second. Speed is the time the computer takes to access one byte of information. The smaller the number the faster the RAM operates. 80ns is faster than 150ns; 1MB is faster at 80ns than 1MB at 100ns.

SIMMs are put into the computer in special slots called SIMM Slots, which are arranged in groups called Banks. The BUS width of a computer will determine how many SIMMs will make up a bank. A 16-bit BUS will have a different configuration than a 64-bit BUS.

Banks may consist of 1, 2, or 4 SIMM Slots.

Banks which have only one SIMM Slot would be referred to as Independent Slots. Computers with more than one Independent Slot can be upgraded with SIMM chips in any combination; the SIMMs which are added do not have to be the same size in megabytes.

Computers with Banks which consist of 2 or 4 SIMM Slots must have all the slots filled in within the Bank with all the same SIMM chips, and all the SIMMS must be the same speed.

One aspect of memory design involves ensuring the integrity of the data stored in memory. Currently there are two methods to check for errors:

Parity has been the most common method used to date. This process adds 1 additional bit to every 8 bits (1 byte) of data.

Error Correction Code (ECC) is a more comprehensive method of error checking that can detect and correct single bit errors. Because of the higher quality of memory components and to make SIMMs more affordable, it has become more common among PC manufacturers not to use data integrity checking (Non-Parity).

SIMMs are specified in terms of depth and width, which indicate the SIMM's capacity and whether or not it supports parity. Here are some examples of popular 30- and 72-pin SIMMs. Note that the parity SIMMs are distinguished by the X9 or X36 format specification.

Video RAM (VRAM) is RAM specifically designed to upgrade the video capabilities of the computer. This is accomplished by adding RAM directly to the motherboard as in some Macintosh models, or by adding RAM to a video card installed in the computer.

A cache is a small, higher speed memory system which stores the most recently used instructions or data from a larger but slower memory system. Because programs frequently use a subset of instructions or data repeatedly, the cache is a cost effective method of enhancing the memory system in a "statistical" method, without having to resort to the expense of making ALL of the memory system (RAM) faster.

1. The larger the cache, the more instructions and data the cache can store, and the more likely it is for the requested item to be in the cache.

An n-th level cache is n-1 levels away from the CPU. A 1st Level cache, (also called an internal, or L1 cache) is usually built into the processor chip itself, while 2nd level caches (also called external, or L2 caches) and higher level caches that are usually outside the processor chip.

• Each cache level away from the processor (larger value of n) is typically larger, but slower, than levels closer to the processor.

A large cache will always outperform a smaller cache in general, but due to budget constraints, not everyone can afford a cache of the maximum size offered. The following are a few indicators which will help you determine how much cache you need.

Using L2 256 and 1024 Cache cards resulted in the following speed increases compared to using the same machine with no cache installed:

• Computationally intensive applications such as 3-D rendering.

If you are performing operations such as 3-D renders, which take hours to perform, an investment in a 1024KByte cache will quickly pay for itself. For example, Strata Inc. has tested our 1024K cache vs. a 256K cache in a 7100/80 using Strata Studio Pro 1.75, and found that rendering times decreased by as much as 25% with the 1024 KByte cache.

• GAMES, particularly 3-D types such as Marathon and Descent.