A technique for process pre-emption in the transputer

Abstract

Transputer hardware (the T8 series) allows a process to be interrupted momentarily but not pre-empted and saved for later execution. The latter implies that in order to implement pre-emption (by software) the context of the pre-empted process must be completely extracted from the system. There is difficulty in doing so with the T8 transputer because parts of the context of a pre-empted process are not directly accessible. This paper presents a technique, which has been successfully incorporated in several versions of a scheduler, that can circumvent the problem by forcing a process to save the context by itself before giving up the CPU. Although the technique takes five context switches, the time (referred to as the scheduler overhead) turns out to be rather small - less than 50 μs for a 25 MHz transputer. A method for adding a process control block to a transputer process is also presented, which can be used to hold the saved context of a pre-empted process. This requires solving the 'floating workspace pointer' problem. © 1995.