What is the difference between a PAL,PLA,GAL& PROM?

Question

What is the difference between a PAL,PLA,GAL& PROM?

Answer 1

Programmable Array Logic (PAL) and Programmable Logic Array (PLA) are the same thing.  A Gate Array Logic (GAL) is pretty similar.

a Programmable Read Only Memory is a special kind of computer memory chip.

PLAs, PALs, and GALs have various digital functions inside them, but the functions are unprogrammed.  Using a special programmer box, you "burn" connections inside the chip to make it act like a whole host of other chips.  But being rather small, one PAL, PLA, or GAL takes up a lot less room and a lot less power than does the same circuit built out of discrete logic chips.  Depending on how complex the PAL is, you might gat a whole board woth of chips into one small PAL!

A PROM is just like any other computer memory chip, except that it NEVER forgets, even when the power is turned off.

Your PC contain a PROM (possibly 2 or 4) that contain teh BOOTSTRAP LOADER program.  this PROM contains just enough "smarts" to allow the computer to read the "boot sector" of the hard disk, which contain a prgoram to load everything else.  becasue of this PROM, your computer always knows how to "boot up" everytime you turn on the power.

In the old days (like 1970) they didn't have PROMs.  Every time you wanted to turn on your computer you had to use toggle switches to hand-load a special binary "bootstrap" program into the computer's memory. That progrm then read the rest of the program in from a cassette tape recorder. It could easily take 1/2 an hour to type in this program -- everytime you wanted to turn it on.

In 1977, Radio Shack came up with the TRS-80 computer, which had the whole operating system in PROM!  When you turned on teh computer it was ready to go in about 3 seconds.

BUT . . . the problem with PROMS is that once you "burn" them, it is very difficult to change them.  So when you Got TRS-80 Basic Level 1 -- that was ALL you got, unless you had the Radio Shack Service Center take the unit apart and put in new PROMs.