Stack Computers: 7.3 UNIFORMITY OF SOFTWARE INTERFACES

Chapter 7. Software Issues

7.3 UNIFORMITY OF SOFTWARE INTERFACES

A key conceptual feature of stack machines is their uniformity of interface between high level code and machine instructions. Both procedure calls and opcodes use the stack as a means of passing data. This consistent interface has several positive impacts on software development.

The source code for a program does not have to reflect in any manner which instructions are directly supported by the machine and which functions are implemented as procedures at the Forth language level. This capability suggests the use of a low-level stack language, similar to Forth, that is the target of compilation for all languages. Given an assembler from this target language to the actual machine, the user is freed from worry about how particular functions are implemented. This means that various implementations of the same architecture can be made very compatible at the stack-based source code level without actually having to provide all instructions on low-cost implementations. If the same interface is used for conventional languages as well as Forth, then combinations of C code, Forth code, and code from other languages can be intermingled without problems.

In microcoded machines, such as the RTX 32P, this interface can be exploited one step further. Application specific microcode can be used to replace critical sequences of instructions in the application program, and common compiler generated code sequences transparently to the user. In fact, a common method of application code development on a microcoded stack machine is to first write the entire application in high level code, then go back to microcode the critical loops. The rewriting of subroutines from high level language to microcode is invisible to the rest of the program, except for the speed increase. This speed increase is a speedup factor of approximately two for many applications.

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Phil Koopman -- koopman@cmu.edu