Ken Shirriff writes a small program for an IBM 1401 giving some excellent history of the machine in the process. The footnotes are very much worth reading and, among other things, explain a few curious knob on the control panel.

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It's interesting to look at old computers because they do things very differently. Some of the unusual features of the IBM 1401 are that it used decimal arithmetic and 6-bit characters, it had arbitrary-length words, and additional instructions were available for a rental fee.

The IBM 1401 is based on decimal arithmetic, not binary. Of course it uses 0's and 1's internally, but numbers are stored as digits using binary coded decimal (BCD). The number 123 is stored as three characters: '1', '2', and '3'. If you add 7 and 8, you get the digit 1 and the digit 5. Addresses are in decimal, so storage is in multiples of 1000, not 1024: the system with 16K of memory stores exactly 16,000 characters. All arithmetic is done in base-10. So if you divide two numbers, the IBM 1401 does base-10 long division, in hardware.

The IBM 1401 does not use bytes. Instead, it uses 6-bit BCD storage. Every character is stored as a 4-bit BCD digit with two extra bits called "zone bits", named A and B.[7] The two extra zone bits allow upper-case letters (and a few special symbols) to be stored, as well as digits.[8] Using a byte as the unit of operation didn't become popular until later with the IBM System/360; in the early 1960s, computers often used strange word sizes such as 13, 17, 19, 22, 26, 33, 37, 41, 45, and 50 bit words.[9]

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