Editing IBM 650 (section)

===IBM 653 Storage Unit===
The optional IBM 653 Storage Unit, was introduced on May 3, 1955, ultimately providing up to five features:<ref name=CPU-ext/>

* Magnetic tape controller (for IBM 727 Magnetic Tape units) (10 extra operation codes)
* Disk storage controller (1956 enhancement for then new IBM 355 Disk Storage Unit) (five extra operation codes)
* Sixty 10-digit words of [[core memory|magnetic core]] memory at addresses 9000 to 9059. This small ''fast memory'' had an access time of 96[[microsecond|μs]], a 26-fold improvement relative to the rotating drum. This feature added five operation codes and was needed as a buffer for tape and disk I/O. The 60 words could also be used by programs to speed up inner loops and table lookups.
* Three four-digit [[index register]]s at addresses 8005 to 8007; drum addresses were indexed by adding 2000, 4000 or 6000 to them, core addresses were indexed by adding 0200, 0400 or 0600 to them.  If the system had the 4000 word drum then indexing was by adding 4000 to the first address for index register A, adding 4000 to the second address for index register B, and by adding 4000 to each of the two addresses for index register C (the indexing for 4000-word systems only applied to the first address). The 4000-word systems required [[Transistor computer|transistorized]] read/write circuitry for the drum memory and were available before 1963. (18 extra operation codes)
* [[Floating point]] – arithmetic instructions supported an eight-digit mantissa and two-digit characteristic (offset exponent) – '''MMMMMMMMCC''', providing a range of ±0.00000001E-50 to ±0.99999999E+49. (seven extra operation codes)