Assembler Directives of 8086/8088

ASSUME This directive tells the assembler the name of the logical segment it should use for a specified segment. For example ASSUME CS:CODE, tells the assembler that the instructions for a program are in a logical segment named CODE. The 8086 works directly with only 4 physical segments: a Code segment, a data segment, a stack segment, and an extra segment.

DB – Define Byte This directive is used to declare a byte type variable or to store a byte in memory location. For example NAME DB ‘THOMAS’; VALUES DB ‘0’,’1’,’2’,’3’;

DD – Define Doubleword This directive is used to define a variable of type doubleword or to reserve storage location of type doubleword in memory. For example POINTER DD 12341234H

DQ – Define Quadword This directive is used to define a variable of type quadword or to reserve storage location of type quadword in memory. For example POINTER DQ  1234123412341234H

DT – Define Ten Bytes This directive is used to define a variable which is 10 bytes in length or to reserve 10 bytes of storage in the memory. For example POINTER DT 11223344556677889900

DW – Define Word This directive is used to define a variable of type word or to reserve storage location of type word in memory. For example ARRAY DW 100DUP(0) ; defines an array of size 100 all initialized to 0. MULTIPLIER DW 1234h; defines a variable multiplier of value 1234h

ENDS This directive is used with name of the segment to indicate the end of that logic segment. For example CODE SEGMENT ; this statement starts the segment CODE ENDS ; this statement ends the segment

EQU This directive is used to give a name to some value or to a symbol. Each time the assembler finds the name in the program, it will replace the name with the value or symbol you given to that name. for example FACTOR EQU 03H ; this equates factor to 03h can be used as ADD AL, FACTOR ; When it codes this instruction the assembler will code it as ADDAL, 03H ;The advantage of using EQU in this manner is, if FACTOR is used many no of times in a program and you want to change the value, all you had to do is change the EQU statement at beginning, it will changes the rest of all.

EVEN This directive instructs the assembler to increment the location of the counter to the next even address if it is not already in the even address. If the word is at even address 8086 can read a memory in 1 bus cycle. If the word starts at an odd address, the 8086 will take 2 bus cycles to get the data. A series of words can be read much more quickly if they are at even address. When EVEN is used the location counter will simply incremented to next address and NOP instruction is inserted in that incremented location. For example DATA1 SEGMENT ; Location counter will point to 0009 after assembler reads next statement SALES DB 9 DUP(?) ;declare an array of 9 bytes EVEN ; increment location counter to 000AH RECORD DW 100 DUP(0) ;Array of 100 words will start from an even address for quicker read DATA1 ENDS

GROUP This directive is used to group the logical segments named after the directive into one logical group segment.

INCLUDE This directive is used to insert a block of source code from the named file into the current source module.

Rest will be added with time……

Various addressing modes of 8086/8088

1)      Register Addressing mode

2)      Immediate Addressing mode

3)      Register Indirect Addressing mode

4)      Direct Addressing mode

5)      Indexed Addressing mode

6)      Base Relative Addressing mode

7)      Base Indexed Addressing mode

Register Addressing Mode Data transfer using registers is called register addressing mode. Here operand value is present in register. For example MOV AL,BL; MOV AX,BX;

Immediate Addressing mode When data is stored in code segment instead of data segment immediate addressing mode is used. Here operand value is present in the instruction. For example MOV AX, 12345;

Direct Addressing mode When direct memory address is supplied as part of the instruction is called direct addressing mode. Operand offset value with respect to data segment is given in instruction. For example MOV AX, [1234]; ADD AX, [1234];

Register Indirect Addressing mode Here operand offset is given in a cpu register. Register used are BX, SI(source index), DI(destination index),  or BP(base pointer). BP holds offset w.r.t Stack segment, but SI,DI and BX refer to data segment. For example MOV [BX],AX; ADD AX, [SI];

Indexed Addressing mode Here operand offset is given by a sum of a value held in either SI, or DI register and a constant displacement specified as an operand. For example Lets take arrays as an example. This is very efficient way of accessing arrays. My_array  DB ‘1’, ‘2’, ‘3’,’4,’5’; MOV SI, 3; MOV AL, My_array[3]; So AL holds value 4.

Base Relative Addressing mode Operand offset given by a sum of a value held either in BP, or BX and a constant offset sepecified as an operand. For example MOV AX,[BP+1]; JMP [BX+1];

Base Indexed Here operand offset is given by sum of either BX or BP with either SI or DI. For example MOV AX, [BX+SI]  JMP [BP+DI]