The CONTEXT mechanism
or how to avoid use of QUERY
Sorry, this document is yet to be
The way A-code is currently implemented, has one substantial disadvantage.
The two input directives (INPUT and QUERY) expect execution to proceed from
the the point where they have been called, which is fine if the game
executable takes care of its own main loop. However, in the single turn (CGI)
and library modes, this is not the case. Input is obtained by other
controlling software (e.g. by a PHP script, in the single turn mode) and there
is no mechanism to return the game to the point at which an input directive
is used. The problem is aggravated by the fact that it may make perfect sense
to have multiple calls for input in a single pass through the main loop (e.g.
get general command, if this command is SAVE without a name to save the game
under, get the name; if there is already such a saved game, query whether it
should be overwritten).
In Platt's original A-code implementation as a virtual machine this would
not have been a problem. One simply saved the state of the VM before returning
control to the external software. Once the game code is re-invoked and
supplied with the next command, the VM state is restored and executaion
proceeds as normal. Things are noweher near as simple when a game is simply an
executable (or a library) built from C sources.
There are possible solutions, of course. If the game is split into two
processes (one in charge of player communication and the other running the
The CONTEXT meschanism was my solution to this problem when implementing
the original CGI-based implementation of Adv770 to be used by game's
beta-testers. The idea is a simple one