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IMP Core Environment Standard
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Section 9: Asynchronous Communication
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$p Two procedures are provided so that a program can communicate asynchronously
with its environment (i.e$. out of step with its normal flow of execution).
Because IMP is essentially a synchronous language, these facilities are
provided as normal procedure calls which the program can issue when convenient;
the asynchronous communication takes place at the instigation of the external
environment. This normally takes the form of an explicit request by an
interactive terminal user: many operating systems include standard 'commands' in
the form of special control characters for the purposes of asynchronous
communication with a running program.
The two procedures provided correspond to two kinds of asynchronous request
which are commonly provided by operating systems: a status-requesting
communication to which programs conventionally respond with a message
describing their current operational state and an attention-requesting
communication which programs conventionally interpret as meaning that an
interactive user wishes to interrupt their processing in favour of some
direct communication.

$p Under operating systems which do not provide facilities sophisticated enough
to support the procedures described here, the procedures are defined to act as
if no communication  had taken place.

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$b4$v5$t0 * $t %routine DOING ( _s_t_r_i_n_g(255) What ) $b0
$p The parameter WHAT is a statement by the program of its current activity. For
example, a compiler might issue a call to DOING at the beginning of each of its
major phases. When a status-requesting asynchronous communication is made with
the program, the string supplied to the most recent call on DOING is made
available in response. 

$v5$p Under operating systems which do not support this form of asynchronous
communication, the procedure DOING is defined to have no effect.
Some operating systems provide only a limited capability in this respect, a
common limitation being on the length of the WHAT string. On these systems,
the string supplied to DOING is truncated to a number of characters acceptable
to the system environment.

$v5$p An implementation is required to define (DEF0011; DOING facility)
whether or not this facility is available and if so how to invoke it
and the form of response to expect.


$b4$v5$t0 * $t %predicate INTERRUPTED$b0
$p This predicate is %true if an attention-requesting communication has occurred
since the last call on INTERRUPTED.  Note that this implies that INTERRUPTED
will be %true at most once for each attention-requesting communication.

$v5$p An implementation is required to define (DEF0012; INTERRUPTED facility)
whether or not this facility is available and if so how to invoke it.