Intel 210200-002 manual ~~~11~~ks

INTRODUCTION

sor. Within the general register group there are eight 16-bit registers. Four of these can be referenced alternately as either 16-bit or as eight 8-bit registers. All of these registers are available to the programmer for general pur- pose activities.

In addition to the general registers, there are two 16-bit control registers and four 16-bit segment registers. The function of all 8088 registers is described in more detail in the following paragraphs.

Data Registers

The data group registers which, in their 16-bit form, are the AX, BX, ex and DX registers (Fig. 1-7). For 8-bit operations they are broken up into a high byte and low byte. AH is the high byte of the AX register, AL is the low byte of the AX register, and so on. As

,mentioned, these registers have general usage for simple arithmetic and logical operations.

Some registers have additional special func-

tions which are performed in the execution of certain instructions. For example, the ex register is frequently used to contain a count value during repetitive instructions. The BX

register is used as a base register in some of the more powerful addressing modes.

Pointer and Index Registers

Figure 1-8 shows the pointer and index regis- ters. The BP and SP registers both point to the 8088's stack, a linear array in the 8088's memory used for subroutine parameters, subroutine return addresses, or other data temporarily saved during execution of an 8088 program.

Most microprocessors have a single stack pointer register called the SP. The 8088 has an additional pointer into the stack called the BP or the base pointer register. While the SP is used similar to stack pointers in other machines (for pointing to subroutine and

AHAL AX

BHBL BX

CHCL CX

DHDL OX

f INDEX			GENERAL	Figure 1-7. Data Group Registers
	~
			REGISTERS
f POINTER	~			BP & SP FOR
				STACK PARAMETER ffip
f CONTROL ~ }				PASSING	SP
			~~~11~~ks
B}				SI& 01 FOR	1
		'SEGMENT		STRING MANIP. &	88
				DATA STRUCTURES	01
SEGMENT
			REGISTERS
				THESE CAN ALSO BE USED AS GENERAL
				REGISTERS
Figure 1-6. 8088 Register Set				Figure 1-8. Base and Index Registers

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