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About SIMM RAM

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About SIMMs
Number of chips per module
Buswidth 32 bit

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SIMMs

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SIMM (Single Inline Memory Modules) were first made in 8 bit editions. They were small cards with 1, 2 or 4 MB RAM. They were connected to the motherboard with a 30 pin edge connector. The modules were 8 bit wide. This meant that 16 bit processors (286 and 386SX) needed 2 SIMMs in a pair. Thus, there was room for two modules in what is called a bank.

32 bit processors (386DX and 486) need 4 of the small 8 bit SIMMs in a bank, since their banks are 32 bit wide. So, on a typical 1st generation 486 motherboard, you could install 4 X 1 MB, 4 X 2 MB, or 4 X 4 MB in each bank. If you only had one bank (with room for 4 modules), it was expensive to increase the RAM, because you had to discard the old modules.

32 bit modules

With the advent of the 486 processor, demand increased for more RAM. Then the larger 32 bit modules came into use. A 486 motherboard could still have 4 SIMM sockets, but when the modules were 32 bit wide, they could be installed one at a time. This was quite ingenious.

You could add different types of modules and still use the old ones. Also, since the 486 motherboard ran at only 33 MHz on the system bus, the RAM module quality was not so critical. You could mix 60 ns and 70 ns modules of different brands without problems.

Here you see a couple of SIMM modules. On top is a 64 bit module (168 pins - don't try to count them). Next is a 32 bit module with a 72 pin connector. Below is an 8 bit module with a 30 pin connector:

64 bit SDRAM:

32 bit DRAM:
and
16 bit DRAM:

Number of chips per module

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Some SIMMs have more chips on the module than others. Looking at just the 32 bit modules, we find modules with 2, 4, 8 or chips on each side. SIMMs with 2 MB, 8 MB and 32 MB are double sided. There are chips on both sides of the module, and all these chips are 16 Mbit ones.

The newest DIMM-modules holds 64 Mbit RAM chips. This way a 32 MB module is made of only 4 chips since 4 X 64 / 8 = 32.

Pentium motherboard with SIMMs

On the Pentium motherboard, the system bus is 64 bit wide. Therefore, the 32 bit SIMMs are installed in pairs. Since the standard motherboard only has two banks with a total of four SIMM sockets, RAM expansion possibilities are limited. NOTE: never use different speed RAM modules on the Pentium motherboard. All modules must have the same speed. Here you see a few configurations on an old Pentium motherboard with four SIMM sockets:

Bank 1 Bank 2 Total RAM
16 MB + 16 MB - 32 MB
16 MB + 16 MB 32 MB + 32 MB 96 MB
32 MB + 32 MB 32 Mb + 32 MB 128 MB

Certain motherboards (like TYAN) have 6 or 8 SIMM sockets. That provides more RAM expansion flexibility.

Learn more
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An illustrated Guide to CPUs from 8086 to the Pentium-III: Module 3.

Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

Read module 5b about AGP and module 5c about Firewire.

Read module 7a about monitors, and 7b on graphics card.

Read module 7c about sound cards, and 7d on digital sound and music.

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Bank 1	Bank 2	Total RAM
16 MB + 16 MB	-	32 MB
16 MB + 16 MB	32 MB + 32 MB	96 MB
32 MB + 32 MB	32 Mb + 32 MB	128 MB