Memory chips are usually used in what is called a module. In early computers, instead of modules, RAM was part of the motherboard. This was a bad idea because it made upgrading of RAM nearly impossible. So modules came along as a way of connecting chunks of RAM to the rest of the computer. A modular contains any number of chips, with almost any number of pins, like 30, 72, 168, and is responsible for either 16 bits, 32 bits or 64 bits of bandwidth. So with all of these variables, how does it work.

A basic memory chip on a module is responsible for 1 bit of information. This isn't really that much, considering the CPU probably wants more than that. So multiple chips are used to store multiple bits.

In this example, if one byte (or 8 bits) needs to be stored, 8 chips each with the ability to store one bit each could be used. All of the chips could collectively store the information. This is the principle behind a memory modular works. Modules are used in configurations so that there are always as many bits able to be stored at one time as there are to be transferred through the memory bus. This is the most efficient arrangement. In the previous example, the memory bus would have only been 8 bits wide. Some chips are able to store up to 8 bits by themselves. The way that this works is that they have internally 8 different sections which would act like separate chips. The 8 bit chip would have the same number of address pins, but have more input and output pins. This technique is used to make modules of higher density and using fewer chips.

Today's memory buses are mostly either 16-bits, 32-bits or 64-bits in width, so memory modules are made so that they can properly fill this bandwidth. For example, 486 computers use a 32bit system bus, so they naturally would have memory which is capable of 32-bit access. Newer computers, Pentium and up, all use a 64-bit system bus, so they memory which is capable of 64-bit access.

The details of SIMM's, DIMM's, and RIMM's are similar, but slightly different in operation.

SIMM is an acronym for Single Inline Memory Module. SIMMS came in a number of configurations. One of the first designs was the 30-pin SIMM which was capable of 8-bit access. This means that to be used in a 32-bit system, 4 modules would need to be used to fill up the bus. This module was one of the first to be designed, and it was small in size and used chips like the 16 k 1 bit that I used as an example for RAM pinouts. This meant to have 8-bit access, there were 8 chips on each module.

Next came the 72-pin SIMM. This mostly was used in 486 systems, because only one module needed to be used to fill the 32bit bus. These modules were capable of more storage because they used newer chips that could store 4 bits a piece, so instead of needing 32 chips on each module to create 32bit access, only 8 were needed. Some 72pin SIMM's used 8-bit chips, which meant only 4 chips were needed on one SIMM. Even still, some used 16-bit chips, so only 2 chips were on each SIMM. This was usually not done because it would make chip addressing a difficult task for the SIMM engineers, because each chip would have 16 DataIn pins and 16 DataOut pins. That is a lot to keep track of.

DIMM is an acronym for Dual Inline Memory Module. These are referred to as "dual" because they were double sided. Instead of chips on only one side, chips were used on both sides, in separate configurations. You can think of these as if you had 2 SIMMS, and you glued them back to back. DIMM's were needed in the transition from a 32-bit to 64-bit system bus which that the Pentium used. This of course meant that the pin could also rose, up to 168 pins. Either DIMM's or pairs of 72-pin SIMM's can be used in today's systems, although motherboard manufactures have lost support for SIMM's because they could never reach the speed that DIMM's could.

RIMM is an acronym for Rambus Inline Memory Module. Instead of having only one channel for data to travel through, RIMM's have a series of smaller, high speed channels that offer dedicated bandwidth. Each of these channels is able to supply memory bandwidth to different devices simultaniously.

NORMAL RAM                                                         RAMBUS

This means if a hard drive needs to access memory, along with the CPU and video card, all 3 can access at the same time. This major change in functionality nessesitates a different system set up to handle these dedicated channels. Each RIMM uses the same DRAM chips, but instead of being parallel like DRAM, RIMM's use them in series, on parallel channels.

Chip Operation | SIMM, DIMM, & RIMM