Editing IBM 704 (section)

==Instruction and data formats==
There are two instruction formats, referred to as "Type&nbsp;A" and "Type&nbsp;B".<ref>{{Cite web | title=From the IBM 704 to the IBM 7094 | author=John Savard | url=http://www.quadibloc.com/comp/cp0309.htm | access-date=2009-11-15}}</ref> Most instructions were of type&nbsp;B.

Type A instructions have, in sequence, a 3-bit ''prefix'' (instruction code), a 15-bit ''decrement'' field, a 3-bit ''tag'' field, and a 15-bit ''address'' field. There are conditional jump operations based on the values in the index registers specified in the ''tag'' field. Some instructions also subtract the ''decrement'' field from the contents of the index registers. The implementation requires that the second two bits of the instruction code be non-zero, giving a total of six possible type&nbsp;A instructions. One (STR, instruction code binary 101) was not implemented until the [[IBM&nbsp;709]].

Type B instructions have, in sequence, a 12-bit instruction code (with bits 2 and 3 set to 0 to distinguish them from type&nbsp;A instructions), a 2-bit ''flag'' field, four unused bits, a 3-bit ''tag'' field, and a 15-bit ''address'' field.

* Fixed-point numbers are stored in binary [[Computer numbering formats|sign/magnitude format]].
* Single-precision [[floating-point]] numbers have a magnitude sign, an 8-bit excess-128 exponent and a 27-bit fraction (no hidden bit).
* Alphanumeric characters were usually 6-bit [[Binary-coded decimal#IBM|BCD]], packed six to a word.

The instruction set implicitly subdivides the data format into the same fields as type&nbsp;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&nbsp;704.

The original implementation of [[Lisp (programming language)|Lisp]] uses the ''address'' and ''decrement'' fields to store the head and tail of a [[linked list]] respectively. The primitive functions ''[[CAR and CDR|car]]'' ("contents of the address part of register") and ''[[CAR and CDR|cdr]]'' ("contents of the decrement part of register") were named after these fields.<ref>{{Cite web |title=Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I |first=John |last=McCarthy |url=http://www-formal.stanford.edu/jmc/recursive.html |year=1960 |access-date=2009-02-14 |archive-url=https://web.archive.org/web/20131004215327/http://www-formal.stanford.edu/jmc/recursive.html |archive-date=2013-10-04 |url-status=dead }} p.&nbsp;28.</ref>