Intel architecture ia-32 User Manual

Vol. 3A 6-23

In 64-bit mode, task structure and task state are similar to those in protected mode. However, the
task switching mechanism available in protected mode is not supported in 64-bit mode. Task
management and switching must be performed by software. The processor issues a general-
protection exception (#GP) if the following is attempted in 64-bit mode:

•

Control transfer to a TSS or a task gate using JMP, CALL, INTn, or interrupt.

•

An IRET with EFLAGS.NT (nested task) set to 1.

Although hardware task-switching is not supported in 64-bit mode, a 64-bit task state segment
(TSS) must exist. Figure 6-11 shows the format of a 64-bit TSS. The TSS holds information
important to 64-bit mode and that is not directly related to the task-switch mechanism. This
information includes:

•

RSPn — The full 64-bit canonical forms of the stack pointers (RSP) for privilege levels
0-2.

•

ISTn — The full 64-bit canonical forms of the interrupt stack table (IST) pointers.

•

I/O map base address — The 16-bit offset to the I/O permission bit map from the 64-bit
TSS base.

The operating system must create at least one 64-bit TSS after activating IA-32e mode. It must
execute the LTR instruction (in 64-bit mode) to load the TR register with a pointer to the 64-bit
TSS responsible for both 64-bit-mode programs and compatibility-mode programs.