Context Switching: Difference between revisions

The hardware context switching mechanism (called Hardware Task Switching in the CPU manuals) can be used to change all of the CPU's state except for the FPU/MMX and SSE state. To use the hardware mechanism you need to tell the CPU where to save the existing CPU state, and where to load the new CPU state. The CPU state is always stored in a special data structure called a TSS (Task State Segment).

 

 

The CPU has a register called the "TR" (or Task Register) which tells which TSS will receive the old CPU state. When the TR register is loaded with an "LTR" instruction the CPU looks at the GDT entry (specified with LTR) and loads the visible part of TR with the GDT entry, and the hidden part with the base and limit of the GDT entry. When the CPU state is saved the hidden part of TR is used.