Intel 210200-002 manual Mydata Segment Ends Myextra Alpha Beta Gamma Mystack

ARCHITECTURE AND INSTRUCTIONS

First, intrasegment jump and call instructions require only the offset (16-bits) of the new location. Intersegment jump and call instruc- tions require the segment (another 16-bits) in addition to the offset.

Second, data-accessing instructions that use the current data segment and current stack segment in the manner most optimal for the 8088 architecture contain only the offset (l6-bits) of the data location. Any other instruction that accesses a data location within one of the four currently-addressable segments must contain a segment-overriding prefix (another 8-bits) in addition to the

offset. Here, current refers to when the instruction is executed, not assembled.

Therefore, to assemble the correct object code, the assembler must know the segment structure of the program and which segments will be addressable - pointed at by segment registers - when various instructions are executed. This information is supplied by the ASSUME directive.

The following example shows how the SEGMENT, ENDS, and ASSUME direc- tives can be used to define a code, data, extra, and stack segment:

MY_DATA	SEGMENT
X	DB	?
Y	OW	?
Z	DO	?
MY_DATA	ENDS
MY_EXTRA	SEGMENT
ALPHA	DB	?
BETA	OW	?
GAMMA	DO	?
MY_EXTRA	ENDS
MY_STACK	SEGMENT
	OW	100 DUP (?)	;this is the stack
TOP	EQU	THIS WORD
MY_STACK	ENDS
MY_CODE	SEGMENT
	ASSUME	CS:MY_CODE,DS:MY_DATA
	ASSUME	ES:MY_EXTRA,SS:MY_STACK
START:	MOV	AX,MY_DATA	;initializes DS
	MOV	DS,AX
	MOV	AX,MY_EXTRA	;initializes ES
	MOV	ES,AX
	MOV	AX,MY_STACK	;initializes SS
	MOV	SS,AX
	MOV	SP,OFFSET TOP	;initializes SP
	ENDS
	END	START

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