Identifying RAM

Lecture Notes

Identifying RAM

Let us suppose we come across various RAM memory modules. To know anything useful about the RAM we need to identify the module type, chip type, amount of memory and data transfer speed.

Module Type

Memory chips are usually mounted on a separate printed circuit board (PCB) called a memory bank or module that can be plugged into a connector on the motherboard. There are two main forms of modules, SIMMs and DIMM's.

SIMMs - Single Inline Memory Modules (older type)

DIMM's - Dual Inline Memory Modules (more modern type)

30 pin SIMMS

If you look at the connection edge of a 30 pin SIMMS module, you will see the number 1 printed on the left and 30 printed on the right.

72 pin SIMMS

Look at the connection edge of each module. The 72 pin SIMMs module has a small semicircular indentation nearly half way across the connection edge. Also, if you look at the connection edge you will see the number 1 printed on the left and 72 printed on the right.

168 pin DIMMS

Look at the connection edge of the DIMMS module. It has 2 small semicircular indentations on the connection edge. Also, if you look at the connection edge you will see the number 1 printed on the left and 84 printed on the right. Why is it 84 and not 168? Turn the module around and look at the back of it.

Note: --

In a lot of old computers, you often had to install SIMMs in pairs each with the same capacity and speed. This is because the board would have a 16 bit bus but each 8 MB module could only take 8 bits at a time. So pairs of SIMMS were needed to handle 16 bits from the bus

The newer DIMMs with a 168-pin connector ranged in capacity from 8 MB to 128 MB per module. DiMMs can be installed singly instead of in pairs.

RAM Chip Types

There are different types of memory chips. The table below lists the common types of RAM chips.

SRAM

Static random access memory - primarily used for cache memory.

DRAM

Dynamic random access memory - accesses one block of data at a time. The memory addresses in DRAM need to be refreshed many times each second. This causes a greater amount of electricity to be used by your RAM and also slows it down when compared to SRAM. The advantage to using DRAM is the price; DRAM is cheaper than SRAM.

FPM DRAM

Fast page mode dynamic random access memory - Normal memory access requires that a row and column address, which takes time. Paging allows faster access to all the data within a given row of memory by keeping the row address the same and changing the column.

EDO DRAM

Extended data-out dynamic random access memory - unlike DRAM, can fetch a block of data while simultaneously sending a block of data to the CPU. So it is faster than conventional DRAM and FPM RAM. Unlike regular DRAM which can allow access to only one byte of information at a time, EDO allows an entire block of memory to be moved into the internal cache for quicker access by the CPU. Can only be used on a bus speed of up 66 MHz

SDRAM

Synchronous dynamic random access memory - Faster than EDO RAM. SDRAM delivers information in very high speed bursts and run in synchronization with the memory bus. SDRAM is capable of running at 133 MHz.

RDRAM

Rambus dynamic random access memory - a type of memory designed by Rambus. Very different architecture to SDRAM. RDRAM uses a special high-speed data bus. RDRAM chips are mounted on a Rambus in-line memory module (RIMM), which is similar to a standard DIMM.

RDRAM memory chips work in parallel to achieve a data rate of 800 MHz.

DDR SDRAM

Double Data Rate SDRAM - output operations on the chip occur on both the rising and falling edge of the clock. Currently, with SDRAM only the rising edge signals an event to occur, so the DDR SDRAM design can effectively double the speed of operation up to 200 MHz (PC1600) or 266 MHz (PC2100) or 333 MHz (PC2700).

DDR modules have 184 pins

The type of RAM you need for your system depends on the motherboard. In general...

An old 486 or Pentium I will accept 30 pin SIMMS DRAM

A Pentium I will have slots for 72 pin SIMMS DRAM.

Newer motherboards accept 168 pin DIMMs SDRAM

A modern motherboard bought in 2002 (e.g. Pentium 4 or Athlon Thunderbird) will have slots for the new DDR SDRAM DIMMS

RAM Serial Numbers

What do the markings on your memory chips mean?

The memory module itself may have a part number as well as the memory chips, (which may have different part numbers).

The chip part number (not the module part number) is the most important in helping us determine the memory capacity and other details.

The part number on memory chips tend to have 2 or 3 lines of text on them that include a part number, speed, and date code. Most part numbers start with a two or three character abbreviation that identifies the manufacturer, such as M5M (Mitsubishi), HM (Hitachi), MT (Micron Technology) or TMS (Texas Instruments).

The exact part number is manufacturer-specific. The table below shows the codes for various manufacturers.

Hyundai	HY	Micron	MT	Mitsubishi	M5M
Motorola	MCM	NEC	UPD	Oki	MSM
Samsung	KM	Texas Inst	TI	Toshiba	TC
Fujitso	MB	Goldstar	GM	Hitachi	HM

To find information about a particular memory chip, just carry out an Internet search including the first few digits of the part number as a keyword.

Or you could try going to the chip manufacturer's web site and searching for the part number there.

Note: --

You may see memory specified as 1Mx16 or 4Mx4 say.

1Mx16 means this chip has a capacity of 1 x 16 = 16 megabits. (16 bits are addressable at any one time)

4Mx4 means this chip has a capacity of 4 x 4 = 16 megabits. (4 bits are addressable at any one time)

To get the total amount of memory on a memory module, multiply the chip capacity by the number of chips (ignoring any parity chips) and divide by eight to convert it to bytes.

Example:   A module with 2 4Mx4 chips has a module capacity of

16 x2 / 8 =4 megabytes

Identifying RAM - Summary

Let us go back to the original question of how we obtain useful information about mysterious RAM. We wanted to identify the module type, chip type, amount of memory and data transfer speed.

We can identify module type by looking at the connector edge and size of the module.

We can identify chip type by carrying out an Internet search using the chip serial numbers. A successful search will give us information on chip type, chip memory size and data transfer speed.