Editing Atlas Autocode (section)

==Features==
AA was a [[Block (programming)|block structured]] language that featured explicitly typed [[Variable (computer science)|variables]], [[subroutine]]s, and functions. It omitted some ALGOL features such as ''[[Call by name#Call by name|passing parameters by name]]'', which in [[ALGOL 60]] means passing the [[memory address]] of a short subroutine (a ''[[thunk]]'') to recalculate a parameter each time it is mentioned.

The AA [[compiler]] could generate range-checking for [[Array data structure|array]] accesses, and allowed an array to have dimensions that were determined at [[Runtime (program lifecycle phase)|runtime]], i.e., an array could be declared as <code><u>integer</u> <u>array</u> Thing (i:j)</code>, where <code>i</code> and <code>j</code> were calculated values.

AA high-level routines could include [[machine code]], either to make an [[inner loop]] more efficient or to effect some operation which otherwise cannot be done easily.<ref name="AA"/>

AA included a <code><u>complex</u></code> data type<ref name="AA"/> to represent [[complex number]]s, partly because of pressure from the [[electrical engineering]] department, as complex numbers are used to represent the behavior of [[alternating current]]. The [[imaginary unit]] square root of -1 was represented by <code><u>i</u></code>, which was treated as a fixed complex constant = ''i''.

The <code><u>complex</u></code> data type was dropped when Atlas Autocode later evolved into the language [[Edinburgh IMP]]. IMP was an extension of AA and was used to write the [[Edinburgh Multiple Access System]] (EMAS) [[operating system]].

In addition to being notable as the progenitor of IMP and EMAS, AA is noted for having had many of the features of the original ''[[Compiler-compiler|Compiler Compiler]]''. A variant of the AA compiler included run-time support for a top-down [[recursive descent parser]].  The style of [[parser]] used in the Compiler Compiler was in use continuously at Edinburgh from the 60's until almost the year 2000.

Other [[Autocode]]s were developed for the [[Titan (1963 computer)|Titan computer]], a prototype Atlas 2 at Cambridge, and the [[Ferranti]] Mercury.