Quiz -Memory Addressing

addrmods
@ P. Klimo
October 2005
Addressing Modes – 8086 Examples
Register:
Examples:
Operand value(s) in registers.
MOV AL, BL ( no operand only opcode)
ADD AX, BX
Immediate :
Examples:
Operand value present in the instruction.
MOV AX, 63 ( 16 bit operand)
ADD AL, 63 (8 bit operand)
Direct:
Examples:
Operand offset address (with respect to DS) given in the
instruction.
MOV AX,[13AF], ADD AX,[87FD] (all operands are 16 bit addresses)
Register Indirect:
Operand offset given in a CPU register.
Registers used are BX (B register) , SI (source index),
DI( destination index) or BP (base pointer).
BP holds offset with respect to SS, but SI, DI and BX
refer to DS.
Examples: MOV [BX], AX , ADD AX, [SI] ( no operands, only opcodes)
Direct Indexed Address:
Operand offset given by a sum of a value held in either
SI or DI registers and a constant displacement specified
as an operand.
Examples:
MOV AX, [SI + 064F], ADD AX, [DI + 23EF]
A typical application would be to access arrays using displacement as the pointer to
the start of the array and the register value as an array index :
my_array
10 DB 'I', 'N', 'T', 'E', 'L', ' ', '8', '0', '8', '6'
MOV SI, 7
MOV AL, [SI + my_array];
AL holds my_array[7] which is '0'
1

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

addrmods
@ P. Klimo
October 2005
Base Indexed:
Operand offset given by the sum of BX or BP with
either SI or DI plus a constant displacement.
Typical application would be accessing two dimensional arrays:
; This extract reads the numbers stored in the specified row
; variable declaration
row DB 3 ; read data from this row
column DB 2 ; and column
; Data for a 4x3 array
my_array DB 1, 2, 3, 4
DB 5, 6, 7, 8
DB 9, A, B, C
; the code
; Calculates offset to my_array : 4*(row-1) + (column-1) and
; loads the byte it finds there into AL
access_my_array MOV BX, row ; BX will hold row offset
MOV DI, column ; DI column
DEC DI
; column-1
DEC BX ; row -1
SHL BX
; 2*(row-1)
SHL BX
; 2(*2*(row-1))
MOV AL, [my_array+ DI + BX] ; AL holds value A
.....
3