Editing DEC Alpha (section)

==Data types==
In the Alpha architecture, a ''[[byte]]'' is defined as an [[8-bit computing|8-bit]] [[Data (computing)|datum]] (octet), a ''[[Word (data type)|word]]'' as a [[16-bit computing|16-bit]] datum, a ''[[longword]]'' as a [[32-bit computing|32-bit]] datum, a ''[[quadword]]'' as a [[64-bit computing|64-bit]] datum, and an ''[[octaword]]'' as a [[128-bit computing|128-bit]] datum.

The Alpha architecture originally defined six data types:

* Quadword (64-bit) integer
* Longword (32-bit) integer
* IEEE T-floating-point (double precision, 64-bit)
* IEEE S-floating-point (single precision, 32-bit)

To maintain a level of compatibility with the [[VAX]], the 32-bit architecture that preceded the Alpha, two other floating-point data types are included:

* VAX G-floating point (double precision, 64-bit)
* VAX F-floating point (single precision, 32-bit)

VAX H-floating point (quad precision, 128-bit) was not supported,<ref>{{cite magazine
|magazine=DEC Professional  |page=62 |title=Alpha Bits – Migrating To The Future
|quote=H floating datatypes are unavailable on Alpha |date=August 1992}}</ref> but another 128-bit floating-point option, X-floating point, is available on Alpha, but not VAX.<ref>{{cite web
|url=https://www0.mi.infn.it/~calcolo/OpenVMS/ssb71/6459/6459p002.htm
|title=Migrating an Application from OpenVMS VAX to OpenVMS Alpha
|at=2.5.2 Data Types}}</ref><br>H and X have been described as similar, but not identical. Software emulation for H-floating is available from DEC, as is a source-code level converter named DECmigrate.