Intel 80386 manual System Registers, Multitasking

CHAPTER 3

SYSTEM ARCHITECTURE

The purpose of a system architecture is to support operating systems, but operating systems are quite diverse in their needs. In response, the 80386 provides an array of resources that operat- ing system designers and implementors can selectively employ. In effect, the 80386 system architecture can be configured to fit the needs of the operating system under development.

3.1 System Registers

In addition to the registers described in the preceding chapter, an operating system some- times uses the 80386 registers shown in Figure 3-1. (Later sections of this chapter sometimes refer to these registers, so they are shown here for reference.) In the main, it is the 80386 that uses the system registers; the operating system ini- tializes the system registers and then ignores them during normal operation. The operating system may, however, use a system register to handle an exception. For example, when a page fault occurs, the processor loads the faulting address into CR2; the operating system's page fault handler uses the address to find the asso- ciated page table entry. The system registers are normally inaccessible to application programs,

since only privileged instructions can operate on them. (Exceptions, page faults, and privileged instructions are explained later in this chapter.)

3.2 Multitasking

Many of the 80386's system architecture facilities directly support multitasking operating systems, though, of course, the 80386 can be used in demanding single-task applications. Multitasking is a technique for managing a computer system's work when that work consists of multiple activi- ties; three such activities might be editing one file, compiling another, and transmitting a third to another computer. In a multitasking system, each activity that can proceed in parallel with other activities is represented by a task. (In this introduction, the term "task" is considered equiv- alent to the term "process.") Each task executes a program consisting of instructions and initial data values. More than one task can execute the same program; for example, in a timesharing multitasking system several tasks (each corre- sponding to a user) commonly execute the same compiler or editor. Programs and tasks are related in somewhat the same way that sheet music and musical performances are related: a

47	15
BASE ADDRESS	LIMIT	GDTR: GLOBAL DESCRIPTOR TABLE
63		IDTR: INTERRUPT DESCRIPTOR TABLE
I SELECTOR		LDTR: LOCAL DESCRIPTOR TABLE

I	TR: RUNNING TASK'STSS
	SYSTEM ADDRESS REGISTERS
	31
	CRO: OPTIONS (FOR EXAMPLE, PAGING)
	~--------------~CR1: (RESERVED BY INTEL)
	CR2: PAGE FAULT LINEAR ADDRESS
	CR3: PAGE DIRECTORY BASE ADDRESS
	SYSTEM CONTROL REGISTERS
	Figure 3-1. System Registers

3-1