Intel IA-32 - Manual (Page 243)

Vol. 3A 6-1
CHAPTER 6
TASK MANAGEMENT
This chapter describes the IA-32 architecture’s task management facilities. These facilities are
only available when the processor is running in protected mode.
This chapter focuses on 32-bit tasks and the 32-bit TSS structure. For information on 16-bit
tasks and the 16-bit TSS structure, see Section 6.6, “16-Bit Task-State Segment (TSS).” For
information specific to task management in 64-bit mode, see Section 6.7, “Task Management in
64-bit Mode.”
6.1 TASK MANAGEMENT OVERVIEW
A task is a unit of work that a processor can dispatch, execute, and suspend. It can be used to
execute a program, a task or process, an operating-system service utility, an interrupt or excep-
tion handler, or a kernel or executive utility.
The IA-32 architecture provides a mechanism for saving the state of a task, for dispatching tasks
for execution, and for switching from one task to another. When operating in protected mode,
all processor execution takes place from within a task. Even simple systems must define at least
one task. More complex systems can use the processor’s task management facilities to support
multitasking applications.
6.1.1 Task Structure
A task is made up of two parts: a task execution space and a task-state segment (TSS). The task
execution space consists of a code segment, a stack segment, and one or more data segments
(see Figure 6-1). If an operating system or executive uses the processor’s privilege-level protec-
tion mechanism, the task execution space also provides a separate stack for each privilege level.
The TSS specifies the segments that make up the task execution space and provides a storage
place for task state information. In multitasking systems, the TSS also provides a mechanism for
linking tasks.
A task is identified by the segment selector for its TSS. When a task is loaded into the processor
for execution, the segment selector, base address, limit, and segment descriptor attributes for the
TSS are loaded into the task register (see Section 2.4.4, “Task Register (TR)”).
If paging is implemented for the task, the base address of the page directory used by the task is
loaded into control register CR3.