PDP-8 C Compiler

Every year or so for a while now, the topic of a PDP8 C compiler has come up in one or another discussion forum. The PDP-8 is not very suited for C -- there's no stack and consequently no recursion. There are no addressing modes suitable for accessing a stack even if you create one, and the conveniently addressable address space is only 4096 words long. Folks have argued about whether it was even possible, and how lame would it be if one did get it working.

Recently, I decided to have a go at creating one, thereby creating a data point for the discussion. My C implementation is based on the following tools:

Small-C was written by Ron Cain and modified by James E. Hendrix. I got the copy I started with from Toby Thain. The compiler is quite small, contained mostly in 4 modest source files, with cc4.c being the one most concerned with code generation. The compiler understands a simple subset of C, having only "char", "int", pointers to char and int, and functions.

The Small-C compiler generates code for a virtual machine "middle-end". This code is peephole optimized and then translated into assembler for the target machine. A large part of the PDP-8 implementation work is actually concerned with creating a "VM" on which to do the operations being requested by the peephole optimizer. Each virtual operation is then typically implemented by an indirect JMS instruction through a pointer on page zero.

SMAL Assembler
The SMAL assembler is an assembler framework written by Doug Jones which understands notions of relocatability and an ability to also act as a linker. I've done some work on this to add PDP-8 instructions and change the syntax to be more like a conventional PDP-8 assembler. The result is a reasonable approximation to PAL-III, but with the ability to output a partially assembled/linked relocatable ".o" file.

SMAL Linker
The SMAL assembler is actually used again as a linker, to merge various ".o" files, resolving their references to one another and assigning final addresses to the code. The code for various C functions is typically loaded after the VM. Since OS/8 required handlers and whatnot to load in field 0, the VM and all the C code are currently loaded into field 1.

Small-C Library
The Small-C compiler also comes with a library, containing implementations of printf and other library routines typically used with C. This is the part I have done the least with. It still needs to be interfaced to OS/8 for file operations, in particular. The hope is to add some code in field 0 to handle file I/O, then interface the existing library to that.

The compiled and linked "Hello World" comes in at about 1.7K, with the VM taking an additional 660 words. (This is with "fputc" stubbed with a routine to output on the TTY.) I estimate that adding support for proper stdio-style I/O would just about use the rest of the available 4K, unless most of it can be placed in field 0.

You can download a snapshot of the code here, or browse the SVN repository here. Once you have extracted the snapshot, you can update to the bleeding edge with "svn update .".

Last updated on 04/30/12 02:07