Intel 80286, 80287 manual General Registers

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80286 BASE ARCHITECTURE

2.3 REGISTERS

The 80286 contains a total of fourteen registers that are of interest to the application programmer. (Five additional registers used by system programmers are covered in section 10.1.) As shown in figure 2-4, these registers may be grouped into four basic categories:

General registers. These eight 16-bit general-purpos~registers are used primarily to contain operands for arithmetic and logical operations.

Segment registers. These four special-purpose registers determine, at any given time, which segments of memory are currently addressable.

Status and Control registers. These three special-purpose registers are used to record and alter certain aspects of the 80286 processor state.

2.3.1 General Registers

The general registers of the 80286 are the 16-bit registers AX, BX, CX, DX, SP, BP, SI, and DI. These registers are used interchangeably to contain the operands of logical and arithmetic operations.

Some instructions and addressing modes (see section 2.4), however, dedicate certain general registers to specific uses. BX and BP are often used to contain the base address of data structures in memory (for example, the starting address of an array); for this reason, they are often referred to as the base registers. Similarly, SI and D1 are often used to contain an index value that will be incremented to step through a data structure; these two registers are called the index registers. Finally, SP and BP are used for stack manipulation. Both SP and BP normally contain offsets into the current stack. SP gener- ally contains the offset of the top of the stack and BP contains the offset or base address of the current

	16-BIT				SPECIAL
(S-BIT REGISTER					REGISTER
	NAME				FUNCTIONS
,~			07	o			15	o
	AX	AH	AL			CS		CODE SEGMENT SELECTOR
ADDRESSABLE				MULTIPLYI DIVIDE			1 ------- 1
	OX	DH	DL	1/0 INSTRUCTIONS		OS		DATA SEGMENT SELECTOR
				1
REGISTER	CX	CH	CL		LOOP ISHIFTI	SS		STACK SEGMENT SELECTOR
					REPEAT COUNT
NAMES
							t ------- f
SHOWN)
	BX	BH	BL	}BASE REGISTERS		ES		EXTRA SEGMENT SELECTOR
	BP						SEGMENT REGISTERS
	SI						15	0
	01			}) INDEX REGISTERS
						F§FLAGS
	SP			STACK POINTER		IP		INSTRUCTION POINTER
	15			o		MSW·		MACHINE STATUS WORD
			GENERAL

REGISTERS	STATUS AND CONTROL
	REGISTERS
	G3010B

Figure 2-4.80286 Base Architecture Register Set

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