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If you're familiar with other autocodes, you might infer from the name that Atlas Autocode was a somewhat low-level language, but that's not the case - it's an extremely nice high-level language invented by Tony Brooker to redress some of the design problems with Algol60. (Related topic: the design of the MU5 was influenced by the Atlas and the AA compilers)

Atlas Autocode was adopted by Edinburgh for our first serious computer, the KDF-9, and after the initial port to that machine, Edinburgh took the language over as its own and called subsequent versions "Imp". In fact the earliest Imp manual I've seen was titled "Imp(AA)".

The main changes to AA when it became Imp were:

The internal character code was changed to ISO.
Bitwise logical operators were added to the language (&, !, !!, \).
The additional declarators %byte %integer and %short %integer were introduced.
Structured data objects - %records - were introduced.
Text handling features were added specifically to aid writing of command interpreters (%string, and the string resolution operator)
Pointer variables (%name) and additional features were added to enable programmers to operate on storage areas outwith the compiler controlled stack.

Any Imp programmer will instantly feel at home with the original Atlas language and it is extremely close to the early Imp implementations such as Hamish Dewar's Imp for the PDP-9, or David Rees' Skimp which was developed into a full Imp for the PDP-11.

One of the idiosyncrasies of AA was that the language was defined using the character set of the only output device they had at the time, the Flexowriter, so for example could be used as meaning "x/2".

We have a scan of the original manual, which was printed on the Flexowriter and beautifully bound with a gold-leaf title. They just don't make them like that any more. One of our correspondents, Diarmuid Pigott (who maintains the fascinating History of Programming Languages web site) took the .png scans of the manual and converted them into a .pdf file. Finally, here is the source of the very first Atlas Autocode-derived Imp compiler for the KDF-9 written at Edinburgh!