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addrmods<br />
@ P. Klimo<br />
October 2005<br />
Base Relative:<br />
Operand offset given by a sum of a value held in either<br />
BX or BP registers and a constant offset specified as an<br />
operand.<br />
A typical application would be to access various structures containing a variable size<br />
elements, using the register value as the pointer to the start of the structure and the<br />
displacement to pint at the start of the element within the structure.<br />
Example:<br />
; Constant definitions<br />
name EQU 0<br />
height EQU 20<br />
age EQU 30<br />
; Structure Definition with initial data<br />
my_details 20 DB "Paul Klimo" ; name (element)<br />
10 DB "2.3 m " ; height (element)<br />
5 DB '2', '1' ; age (element)<br />
; Data for other structures (may be not initialized)<br />
his_details 35 DB ??<br />
her_details 35 DB ??<br />
; This code retrieves the age of the person as a string of 2 ASCII letters held in AX<br />
MOV BX, my_details<br />
CALL get_age<br />
;point to base of structure<br />
;call subroutine that<br />
;returns with ASCII age<br />
;string in AX<br />
..... ; do something with it<br />
ADD BX, 35<br />
CALL get_age<br />
...<br />
; next customer, please<br />
; Subroutine (procedure) get_age<br />
; Get two bytes at offset age into the start of array pointed by BX<br />
get_age MOV AL, [BX + age+1] ; AL has low ASCII of age<br />
MOV AH, [BX + age] ; AH has high ASCII of age<br />
RET<br />
2
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