Some implications of shared variables

Abstract

Recognition ofshared variables as the fundamental means of communication between concurrently operating processes has important implications for the design of systems and programming languages.As a consequence, current APL systems, such as APLSV, embody two shared variable facilities: one for establishing temporary interfaces between system components, and the other one for defining parts of the permanent interface between an APL program and the underlying processor.These are treated in depth in this paper.More general implications for language and system design, system programming, and system performance are briefly noted.It is observed, for example, that languages that attempt to communicate with the environment without the explicit use of shared variables do not have a true communication facility, and therefore tend to absorb more and more of the environment within their own formal structure.

The application of APL to control more efficient processes

“…In RUN2 [3] the variable DATA is sent to the invoked process. Then in RUN [4] the result returned by this process is stored in the variable RESULT. The invoked process terminates after outputting all of its response through channel CH [2].…”

“…V + OpO [2] I + 255 [3] NXT: T ÷ ~QAPS'SUM I THRESH P' [4] V + (T-1 +V),V [5] ÷ (O<I÷I-I)/NXT This function computes the frequency of occurrence of each element of the APS image P and stores the result in the vector V, i .e., V + +/(~255)°.=,P…”

“…© 1979--ACM 0-89791-005--2/79/0500--0096 $00.75 these schemes is shared variables [4,53 in which an APL variable in a workspace may be shared concurrently with a specially designed processor, including another active APL workspace. This scheme is not easily implemented on some systems.…”

“…In RUN1[3] a shell is invoked, which in turn, invokes the process named in PROCESS, providing input from IFILE and creating an output OFILE. When the named process and the shell have terminated RUNI[4] is executed and a channel to OFILE is established. The READALL function then copies the contents of OFILE into RESULT and closes channel CH.…”

Multisystem processing with APL

“…The shared-variable processor in general offers a basis of communication between any two or more concurrently active processors [3]. It follows that the workspace-control functions described here can be used as models for extension of multi-processing techniques, including development of system functions, to make this level of control available in APL systems to control networks of concurrently active processors both within and across system boundaries.…”