Category Archives: Forth

Stack Computers, Forth, RISC, CISC, etc.

I finished reading Stack Computers. This class of processors is a bit different from the mainstream today — neither is it like the classic RISC load/store architectures (MIPS, SPARC, PPC, etc…), nor is it like the surviving (x86) or dead (VAX) CISC architectures. In a way, it’s slightly reminiscent of the high-level language architectures of the 1970s which never even saw commercial application. They were processors designed intended to implement much of the semantics of a high-level language in hardware — quite the opposite of the RISC approach. (The difference being that RISC architectures provide a simple, orthogonal set of operations and a large register file to make it possible for modern compilers to compile high level languages with high performance results, while these high-level language architectures aimed to make the task of compilation for a particular language trivial.)

The analogy here is that most Stack Computer architectures (at the very least) have a close match to the execution model of the Forth languages, and often have opcodes which directly correspond to some common Forth primitives. However, I think the reason stack architectures are not entirely without commercial success is that most of those forth primitives are very low-level operations — it doesn’t require a lot of silicon to implement one of these stack CPUs, and it can be a very reasonable choice for an embedded system.

One downside of some of these stack CPUs is that while they all provide at least two hardware stacks (data stack and return stack), some of them do not provide a high-performance means of loading and storing from a base-pointer-plus-offset, making them poor targets for the C language since it generally accesses function arguments and local variables that way. (A few do extend the processor with enough operations to make them a decent target for C and similar languages.)

Besides reading that, I’ve been slowly trying to beat Forth into my head — the basics are incredibly simple, but doing serious projects in it requires a change of perspective. It seems that there are a number of techniques people have developed to provide abstraction and work with particular problems using domain-specific “little languages”, but it still seems like one unavoidable part of Forth is that it’s essentially untyped.

I’m still aiming to develop a small computer in an FPGA with Forth as the OS and programming language. Not for practical reasons, exactly, but as a way to learn two fairly big things at once.

Furthering my Forth education

Forth [wikipedia] is a programing language that at first seems like a fairly simple language, no more high-level than C, but then as you look at some of the abstractions that arise in code produced by experienced forth programmers, you realize that you’re going to have to expand your mental model of the language to properly understand these (very useful) abstractions. It’s similar to looking at the Lisp written by experienced Lisp programmers.

I’ve been thinking about Forth a little more since reading parts of Stack Computers.

This comment on Lambda the Ultimate finally made some of this return-stack trickery more understandable to me.

Also, the tutorial pointed to is interesting:

I’ve been collecting Forth-related links for a while on my