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Process Context Description

Process Context Description As seen in Figure 6, the process control block (struct pcb), contains the 386-specific per-process information. This is broken down into hardware-dependent fields and software-related fields. The process control block is place at the front of the user structure so that the information can be reloaded from the address of the user structure and force active a previously inactive process. The user structure address is recorded in the process table. Each entry describes global information about a process. The 386's hardware context switch facility can be used to switch from process to process. By placing the hardware-dependent information at the beginning of the process control block, in the form of the 386's Task Switch State (TSS) data structure, it is possible to switch from one process to another with a single intersegment ljmp instruction to the appropriate task gate selector. While this feature has been implemented in 386BSD, it is not used at this time for switching between processes due to performance considerations. However, it can be used in other cases, such as exception handling, and we may elect to use it for process switching in the future. We view this as one of those rare "have your cake and eat it too" decisions. In 386BSD, not all hardware context is switched in this manner, because some processes never access the large amount of state information (108 bytes) used by the numeric coprocessor. We allow for this with the pcb_fpusav structure. Other fields correspond to some implementation demands specific to Berkeley UNIX, including simulating VAX hardware constructs invoked by the virtual memory system not existing on the 386. Fortunately, this was a small amount of code. It is a tribute to the concept of UNIX that the machine-dependent portion of the system is as small as it is. Copyright©1994 Willaim & Lynne Jolitz