Bahasa Assembler :
Alamat Variable
|
There is LEA (Load Effective Address) instruction and
alternative OFFSET operator. Both OFFSET and LEA can
be used to get the offset address of the variable.
LEA is more
powerful because it also allows you to get the address of an indexed
variables. Getting the address of the variable can be very useful in some
situations, for example when you need to pass parameters to a procedure.
Reminder:
In order to tell the compiler about
data type,
these prefixes should be used:
BYTE PTR - for
byte.
WORD PTR - for word (two bytes).
For example:
BYTE PTR [BX] ; byte access.
or
WORD PTR [BX] ; word access.
Emu8086 supports shorter prefixes as
well:
b. - for BYTE PTR
w. - for WORD
PTR
sometimes compiler can calculate the data type
automatically, but you may not and should not rely on that when one of the
operands is an immediate value.
Here is first example:
ORG 100h
MOV AL, VAR1 ; check value of VAR1 by moving it to AL.
LEA BX, VAR1 ; get address of VAR1 in BX.
MOV BYTE PTR [BX], 44h ; modify the contents of VAR1.
MOV AL, VAR1 ; check value of VAR1 by moving it to AL.
RET
VAR1 DB 22h
END
Here is another example,
that uses OFFSET instead of LEA:
ORG 100h
MOV AL, VAR1 ; check value of VAR1 by moving it to AL.
MOV BX, OFFSET VAR1 ; get address of VAR1 in BX.
MOV BYTE PTR [BX], 44h ; modify the contents of VAR1.
MOV AL, VAR1 ; check value of VAR1 by moving it to AL.
RET
VAR1 DB 22h
END
Both examples have the same
functionality.
These lines:
LEA BX,
VAR1
MOV BX, OFFSET VAR1
are even compiled into the same
machine code: MOV BX, num
num is a 16
bit value of the variable offset.
Please note that only these
registers can be used inside square brackets (as memory pointers): BX,
SI, DI, BP!
(see previous part of the tutorial).
|