Reference STATE OF THE ART Stan Augarten

ISBN 0-89919-195-9

Photo of
The First 256-Bit Static RAM
The 4100


Computers store information in two places: internally, on memory chips like RAMs and ROMs, and externally, on magnetic discs and tapes. Just before the invention of semiconductor ICs, however, computers used a form of internal storage know as a magnetic-core memory. Cores were tiny rings of ferro-magnetic material, each about a sixteenth of an inch in diameter, that were strung up on grids of fine wires suspended on small screens inside computers. Magnetized one way, a core represented a one; magnetized the other, it stood for a zero.

  Cores were random-access devices, which meant that a single core could be accessed directly, without disturbing any of the other cores on the grid. Cores were rather fast - it took as little as a millionth of a second to read a bit stored in a core - and retained their contents even after the power had been turned off. But they were expensive, bulky, and destructive, in that the simple act of reading a core erased the data stored in it. It was therefore necessary to install circuits to restore the data as soon as it had been extracted.

  Then, in 1970, Fairchild produced the first relatively capacious semiconductor memory, the 256-bit static RAM on the opposite page. (The charges in static RAMs do not leak away, freeing such chips from the need for periodic refreshing; hence the term static.) Designed by H. T. Chua, this chip was able to retain, in the space of a single core, many times the amount of information. It was nondestructive an much faster and cheaper (although not at first). But, unlike a core, it lost its contents after the power was shut off, a condition known as volatility and one that is characteristic of most RAMs.

  Primitive by today's standards, Fairchild's 256-bit RAM went into the ILLIAC IV computer, a powerful machine made for NASA by the Burroughs Corporation in the late 1960s, and one of the first mainframe computers to have an internal memory composed entirely of chips. With magnetic-core memories, the capacity and performance of computers were severely limited, but the arrival of memory ICs like the one on the right led the way to the creation of today's sophisticated, powerful, miniaturized computers.

Reading or writing a bit into any of the 4100's two hundred and fifty-six memory cells (the T-shaped features) requires only seventy billionths of a second. The circuits on the bottom are column decoders; those on the right are row decoders, amplifiers, and input/output elements. A signal sent simultaneously to both the row and column decoders activates one cell only. Actual size: 0.104 x .0134 inches. Photo of


©Copyright Stan Augarten
This book is provided for general reference. The National Museum of American History and the Smithsonian Institution make no claims as to the accuracy or completeness of this work.

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