Monday, July 31, 2006

8 bits = ?

Prior to IBM's 1964 introduction of the System/360 family of computers [1], computer memories were generally measured and addressed using the same units in which they were accessed: words (of 8, 12, 16, 24, 29, 32, 48, or 60 bits, depending on the computer), characters (generally represented by 6 or 7 bits), or digits (BCD, represented by 4 bits).

All System/360 computers executed the same instruction set, despite their wide range of size and performance. Naturally, the larger, faster machines accessed memory in larger chunks [2], so there could no longer be a direct correspondence between the unit of addressing and the unit of access. IBM chose the unit of addressing to be 8 bits, and introduced a new set of 8-bit character codes called EBCDIC (Extended Binary Coded Decimal Interchange Code). All System/360 computers could execute instructions that operated on 8, 16, 32, or 64-bit quantities, regardless of the size of a memory access on that particular model.

IBM also chose the term byte for an 8-bit chunk of information, and called the successively larger units halfwords, words, and doublewords.

At that time, magnetic core memory was priced at about $1 per byte. Punning on the American slang of bit for an eighth of a dollar [3] and buck for a dollar, I suggested that the units should instead be called bucks, quarter-eagles, half-eagles, and eagles.

Fortunately, no one took my suggestion seriously. Following the hyperinflation of the memory exchange rate over the past 40 years, can you imagine buying a multi-GigaBuck memory card for your hundred-buck PDA, cellphone, or camera?

[1] According to the IBM Archives, "On April 7, 1964, IBM introduced the System/360, the first large 'family' of computers to use interchangeable software and peripheral equipment. It was a bold departure from the monolithic, one-size-fits-all mainframe. Fortune magazine dubbed it 'IBM's $5 billion gamble.' System/360 offered a choice of five processors and 19 combinations of power, speed and memory. A user could operate the same magnetic tape and disk products as another user with a processor 100 times more powerful."
[2] Various models in the family fetched and stored data 8, 16, 32, or 64 bits at a time.
[3] "Two bits, four bits, six bits, a dollar. All those for [name of school] stand up and holler."