Memory

Subject: Memory
From: Richard Mateosian <srm -at- C2 -dot- ORG>
Date: Mon, 8 Apr 1996 22:15:26 -0700

>Referring to the hard disk as memory annoys me as well.

The first computer I ever programmed was an IBM 650. Its main memory was a
2000-word drum. Part of programming it was figuring out how far it would
turn in the time required to execute an instruction so you could position
the next instruction at an optimal drum location.

By the mid-sixties, almost all computers used memory modules composed of
three-dimensional arrays of little metal tori, called cores. Wires passing
through their centers provided an addressing mechanism and a means of
changing them between two magnetic states. Core memory was non-volatile,
that is, it retained its state more or less indefinitely with or without
power. A large computer might have as much as 500K bytes of core memory.

We referred to information in the main memory as being "in core." A few
folks, mostly european, referred to it as the computer's "main store," as
opposed to disk, tape, etc, which they called "backing store."

We got well into the era of semiconductor memories before we broke ourselves
of the habit of referring to the computer's main memory as "core."

As the tradeoffs between access speed and cost of memory became clearer,
someone invented caches, and a whole theory of memory (or storage)
hierarchies developed. A modern PC has a small cache (64K or so) on the
processor chip; an external cache (256K or 512K of fast static RAM,
typically) that the processor can access with no wait states, probably using
block transfers; a slower "main" memory of at least 8 megabytes of dynamic
RAM; a hard disk of at least 500 megabytes capacity; and some sort of backup
storage.

All of it, collectively, is the computer's memory hierarchy. No piece of it,
not even that big block of dynamic RAM, has a special claim to the title of
THE memory. ...RM

Richard Mateosian Freelance Technical Writer
srm -at- c2 -dot- org Copyright 1996 Review Editor, IEEE Micro
http://www.c2.org/~srm/ All rights reserved President, Berkeley STC


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