Uniprocessor operating system design facilitating fast context switching

Abstract

A task stack and a context pointer in a task control block (TCB) are implemented to provide more efficient context switching. Additionally, multiple routines each of which saves or restores a certain combination of volatile registers is implemented. A task can store in its task control block a routine identifier to select from the multiple routines a set of routines for saving and restoring volatile registers during context switching. On the occurrence of an event that may lead to a context switch a scheduler selects based on the routine identifier a routine that only saves registers used by the task, thereby, reducing execution overhead. The registers are saved on the task stack and a context pointer to the registers is saved in the TCB. In the event a context switch is necessary, it is not necessary to copy the registers to the TCB because the context pointer is in the TCB. A non-volatile register indicator that indicates whether non-volatile registers are used is stored in the task control block. The non-volatile registers are only saved if used, also reducing execution overhead. Furthermore, a storage area is implemented for saving task context for the task when it is interrupted during context switch in. Upon the occurrence of an interrupt the task context is saved in the storage area. If the interrupt leads to a second task which has a higher priority being made ready, the second task is switched in without first returning to switch-in the task.