Intel ARCHITECTURE IA-32 User Manual download pdf (Page 250)

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6-8 Vol. 3A

TASK MANAGEMENT

The base, limit, and DPL fields and the granularity and present flags have functions similar to

their use in data-segment descriptors (see Section 3.4.5, “Segment Descriptors”). When the

G flag is 0 in a TSS descriptor for a 32-bit TSS, the limit field must have a value equal to or

greater than 67H, one byte less than the minimum size of a TSS. Attempting to switch to a task

whose TSS descriptor has a limit less than 67H generates an invalid-TSS exception (#TS). A

larger limit is required if an I/O permission bit map is included or if the operating system stores

additional data. The processor does not check for a limit greater than 67H on a task switch;

however, it does check when accessing the I/O permission bit map or interrupt redirection bit

map.

Any program or procedure with access to a TSS descriptor (that is, whose CPL is numerically

equal to or less than the DPL of the TSS descriptor) can dispatch the task with a call or a jump.

In most systems, the DPLs of TSS descriptors are set to values less than 3, so that only privileged

software can perform task switching. However, in multitasking applications, DPLs for some

TSS descriptors may be set to 3 to allow task switching at the application (or user) privilege

level.

6.2.3 TSS Descriptor in 64-bit mode

In 64-bit mode, task switching is not supported, but TSS descriptors still exist. The format of a

64-bit TSS is described in Section 6.7.

In 64-bit mode, the TSS descriptor is expanded to 16 bytes (see Figure 6-4 ). This expansion also

applies to an LDT descriptor in 64-bit mode. Table 3-2 provides the encoding information for

the segment type field.

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