The Caltech Cosmic Cube was developed by Chuck Seitz et al. at Caltech providing architectural support for Actor systems. A significant difference between the CosmicCube and most other parallel processors is that this multiple instruction multiple-data machine uses message passing instead of shared variables for communication between concurrent processes. This computational model is reflected in the hardware structure and operating system, and is also the explicit message passing communication seen by the programmer. According to Seitz :
Hewitt presented a prototype Actor programming language in the sense that it directly expresses important aspects of the behavior of Actors. Messages are expressed in XML using the notation :<tag> for The semantics of the programming language are defined by defining each program construct as an Actor with its own behavior. Execution is modeled by having Eval messages passed among program constructs during execution.
Environment Actors
Each Eval message has the address of an Actor that acts as an environment with the bindings of program identifiers. Environment Actors are immutable, i.e., they do not change. When Request customer] is received by an Actor Environment, a new environment Actor is created such that when the new environment Actor receives Request customer’] then if identifier is the same as identifier’ send customer’Returned, else send Environment Request customer’]. The above builds on an Actor EmptyEnvironment which when it receives Request customer], sends customerThrownNotFound[identifier. When it receives a Bind request EmptyEnvironment acts like Environment above.
Expressions
The prototype programming language has expressions of the following kinds: ; <identifier> ; send <recipient> <communication> ;<recipient>.<message> ;receiver... <pattern>i <expression>i... ; behavior... <pattern>i <expression>i... ; let <identifier> = <expression>valuein <expression>body ; serializer <expression>
Example program
An example program for a simple storage cell that can contain any Actor address is as follows: The above program which creates a storage cell makes use of the behavior [ReadWrite which is defined as follows: Note that the above behavior is pipelined, i.e., the behavior might still be processing a previous read or write message while it is processing a subsequent read or write message.. For example, the following expression creates a cell x with initial contents 5 and then concurrently writes to it with the values 7 and 9. The value of the above expression is 5, 7 or 9.
