How NESL evaluates expressions

 

The NESL system is interactive: the current implementation is built on top of Common Lisp  and implements a similar read-eval-print loop. Expressions are typed at the NESL prompt and terminated with a semicolon and a carriage return. For example:

<Nesl> 2 + 3;

Compiling..Writing..Loading..Running..
Exiting..Reading..
it = 5 : int

Expressions are compiled dynamically into an intermediate language called VCODE , which is then interpreted by a subprocess. The phases of executing an expression are:

• Compiling: Compiles the expression into VCODE.

• Writing: Writes the compiled VCODE program out to a file.

• Loading: Starts up a subprocess for the VCODE interpreter and loads the VCODE program.

• Running: The subprocess executes the VCODE program.

• Exiting: The subprocess writes the results to a file.

• Reading: Reads the results back into the NESL system.

This setup makes it relatively easy to run code on remote machines, since the VCODE interpreter can be run remotely, communicating with the NESL system through a shared file system or through calls to rsh and rcp. This is how the CM-2, CM-5, Cray, Maspar, and MPI implementations work.