We introduce here a simple concurrency kernel, showing how it may be used to provide pseudo-concurrency on a single processor, and also in the chapter we shall show how some of the concurrency constructs introduced in chapters 4 and 5 may be implemented.

We assume that the machine on which the kernel is to be implemented consists of a single processor of the conventional type with a single monolithic address space. A simple Motorola M68000 based system would be an appropriate type of system on which to run the kernel. To make the kernel more useful, it would be possible to consider hardware which provides a virtual address translation capability, in which case the various processes could be provided with distinct address spaces in which to operate. This, however, not only makes the kernel more complicated, but also makes sharing of memory more difficult. For the purposes of the simple kernel which we are considering here, we shall assume a single (shared) address space.

The implementation language will be Pascal, and for simplicity we shall assume that a hypothetical Pascal machine has been implemented on top of the basic hardware. Apart from the ability to run Pascal programs, we shall also require some additional features to support the concurrency kernel to be constructed. It is clear that much of the underlying support, both for Pascal itself and for the concurrency kernel, will have to be provided through a small amount of assembly code.