stored in binary sign/magnitude format. Single-precision floating-point numbers have a magnitude sign, an 8-bit excess-128 exponent and a 27-bit magnitude. Alphanumeric characters were usually 6-bit BCD, packed six to a word. The instruction set implicitly subdivides the data format into the same fields as type A instructions: prefix, decrement, tag and address. Instructions exist to modify each of these fields in a data word without changing the remainder of the word, though the Store Tag instruction was not implemented on the IBM 704. The original implementation of Lisp uses the address and decrement fields to store the head and tail of a linked list respectively. The primitive functions car ("contents of the address part of register") and cdr ("contents of the decrement part of register") were named after these fields. Memory and peripherals Controls are included in the 704 for: one 711 Punched Card Reader, one 716 Alphabetic Printer, one 721 Punched Card Recorder, five 727 Magnetic Tape Units and one 753 Tape Control Unit, one 733 Magnetic Drum Reader and Recorder, and one 737 Magnetic Core Storage Unit. Total mass was about . The 704 itself came with a control console having 36 assorted control switches or buttons and 36 data-input switches, one for each bit in a register. The control console essentially allows only setting the binary values of the registers with switches and seeing the binary state of the registers displayed in the pattern of many small neon lamps, appearing much like modern LEDs. For human interaction with the computer, programs would be entered on punched cards initially rather than at the console, and human-readable output would be directed to the printer. The IBM 740 Cathode Ray Tube Output Recorder was also available, which is a 21-inch vector display with a very long phosphor persistence time of 20 seconds for human viewing, together with a 7-inch display receiving the same signal as the larger display but with a fast-decaying phosphor designed to be photographed with an attached camera. The 737 Magnetic Core Storage Unit serves as RAM and provides 4,096 36-bit words, the equivalent of 18,432 bytes. The 727 Magnetic Tape Units store over 5 million 6-bit characters per reel. Reliability In its day, the 704 was an exceptionally reliable machine. Being a vacuum-tube machine, however, the IBM 704 had very poor reliability by today's standards. On average, the machine failed around every 8 hours, which limited