SIMM's

SIMM's
SIMM modules were first made in 8 bit editions. They were small cards with 1, 2 or 4 MB RAM.
They were connected to the system board with a 30 pin edge connector. The modules were 8
bit wide. This meant that 16 bit processors (286 and 386SX) needed 2 SIMM's 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 SIMM's in a bank, since their
banks are 32 bit wide. So, on a typical 1st generation 486 system board, 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 system board 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 system
board 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: