INTRODUCTION:
Assembly languages are low-level languages for programming computers, microprocessors, microcontrollers, and other IC. They implement a symbolic representation of the numeric machine Codes and other constants needed to program a particular CPU architecture. This representation is usually defined by the hardware manufacturer, and is based on abbreviations that help the programmer to remember individual instructions, registers. An assembler directive is a statement to give direction to the assembler to perform task of the assembly process.
It control the organization if the program and provide necessary information to the assembler to understand the assembly language programs to generate necessary machine codes. They indicate how an operand or a section of the program is to be processed by the assembler.
An assembler supports directives to define data, to organise segments to control procedure, to define macros. It consists of two types of statements: instructions and directives. The instructions are translated to the machine code by the assembler whereas directives are not translated to the machine codes.
Assembler Directives of the 8086 Microprocessor
(a) The DB directive
(b) The DW directive
(c) The DD directive
(d) The STRUCT (or STRUC) and ENDS directives (counted as one)
(e)The EQU Directive
(f)The COMMENT directive
(g)ASSUME
(h) EXTERN
(i) GLOBAL
(j) SEGMENT
(k)OFFSET
(l) PROC
(m)GROUP
(n) INCLUDE
Data declaration directives:
1. DB – The DB directive is used to declare a BYTE -2-BYTE variable – A BYTE is made up of 8 bits.
Declaration examples:
Byte1 DB 10h
Byte2 DB 255 ; 0FFh, the max. possible for a BYTE
CRLF DB 0Dh, 0Ah, 24h ;Carriage Return, terminator BYTE
2. DW – The DW directive is used to declare a WORD type variable – A WORD occupies 16 bits or (2 BYTE).
Declaration examples:
Word DW 1234h
Word2 DW 65535; 0FFFFh, (the max. possible for a WORD)
3. DD – The DD directive is used to declare a DWORD – A DWORD double word is made up of 32 bits =2 Word’s or 4 BYTE.
Declaration examples:
Dword1 DW 12345678h
Dword2 DW 4294967295 ;0FFFFFFFFh.
4. STRUCT and ENDS directives to define a structure template for grouping data items.
(1) The STRUCT directive tells the assembler that a user defined uninitialized data structure follows. The uninitialized data structure consists of a combination of the three supported data types. DB, DW, and DD. The labels serve as zero-based offsets into the structure. The first element’s offset for any structure is 0. A structure element is referenced with the base “+” operator before the element’s name.
A Structure ends by using the ENDS directive meaning END of Structure.
Syntax:
STRUCT
Structure_element_name element_data_type?
. . .
. . .
. . .
ENDS
(OR)
STRUC
Structure_element_name element_data_type?
. . .
. . .
. . .
ENDS
DECLARATION:
STRUCT
Byte1 DB?
Byte2 DB?
Word1 DW?
Word2 DW?
Dword1DW?
Dword2 DW?
ENDS
Use OF STRUCT:
The STRUCT directive enables us to change the order of items in the structure when, we reform a file header and shuffle the data. Shuffle the data items in the file header and reformat the sequence of data declaration in the STRUCT and off you go. No change in the code we write that processes the file header is necessary unless you inserted an extra data element.
(5) The EQU Directive
The EQU directive is used to give name to some value or symbol. Each time the assembler finds the given names in the program, it will replace the name with the value or a symbol. The value can be in the range 0 through 65535 and it can be another Equate declared anywhere above or below.
The following operators can also be used to declare an Equate:
THIS BYTE
THIS WORD
THIS DWORD
A variable – declared with a DB, DW, or DD directive – has an address and has space reserved at that address for it in the .COM file. But an Equate does not have an address or space reserved for it in the .COM file.
Example:
A – Byte EQU THIS BYTE
DB 10
A_ word EQU THIS WORD
DW 1000
A_ dword EQU THIS DWORD
DD 4294967295
Buffer Size EQU 1024
Buffer DB 1024 DUP (0)
Buffed_ ptr EQU $ ; actually points to the next byte after the; 1024th byte in buffer.
(6) Extern:
It is used to tell the assembler that the name or label following the directive are I some other assembly module. For example: if you call a procedure which is in program module assembled at a different time from that which contains the CALL instructions ,you must tell the assembler that the procedure is external the assembler will put information in the object code file so that the linker can connect the two module together.
Example:
PROCEDURE -HERE SEGMENT
EXTERN SMART-DIVIDE: FAR ; found in the segment; PROCEDURES-HERE
PROCEDURES-HERE ENDS
(7) GLOBAL:
The GLOBAL directive can be used in place of PUBLIC directive .for a name defined in the current assembly module; the GLOBAL directive is used to make the symbol available to the other modules. Example:
GLOBAL DIVISOR:
WORD tells the assembler that DIVISOR is a variable of type of word which is in another assembly module or EXTERN.
(8) SEGMENT:
It is used to indicate the start of a logical segment. It is the name given to the the segment. Example: the code segment is used to indicate to the assembler the start of logical segment.
(9) PROC: (PROCEDURE)
It is used to identify the start of a procedure. It follows a name we give the procedure.
After the procedure the term NEAR and FAR is used to specify the procedure Example: SMART-DIVIDE PROC FAR identifies the start of procedure named SMART-DIVIDE and tells the assembler that the procedure is far.
(10) NAME:
It is used to give a specific name to each assembly module when program consists of several modules.
Example: PC-BOARD used to name an assembly module which contains the instructions for controlling a printed circuit board.
(11) INCLUDE:
It is used to tell the assembler to insert a block of source code from the named file into the current source module. This shortens the source module. An alternative is use of editor block command to cop the file into the current source module.
(12) OFFSET:
It is an operator which tells the assembler to determine the offset or displacement of a named data item from the start of the segment which contains it. It is used to load the offset of a variable into a register so that variable can be accessed with one of the addressed modes. Example: when the assembler read MOV BX.OFFSET PRICES, it will determine the offset of the prices.
(13) GROUP:
It can be used to tell the assembler to group the logical segments named after the directive into one logical group. This allows the contents of all he segments to be accessed from the same group. Example: SMALL-SYSTEM GROUP CODE, DATA, STACK-SEG.