Assembler Directives of 8086 Microprocessor

Assembler Directives of 8086 MicroprocessorINTRODUCTIONAssembly languages are low-level languages for programme 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 ordinarily placed by the hardware manufacturer, and is based on abbreviations that help the programmer to remember individual instructions, memorializes. An assembler leading is a statement to progress to 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 interpret 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. I t 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) planetary (j) SEGMENT (k)OFFSET (l) PROC (m)GROUP(n) INCLUDEData declaration directives1. DB The DB directive is utilise to declare a BYTE -2-BYTE shifting A BYTE is made up of 8 bits. resolve examples Byte1 DB 10hByte2 DB 255 0FFh, the max. possible for a BYTECRLF DB 0Dh, 0Ah, 24h Carriage Return, terminator BYTE2. DW The DW directive is used to declare a WORD type variable A WORD occupies 16 bits or (2 BYTE).Declaration examples Word DW 1234hWord2 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 Words or 4 BYTE.Declaration examplesDword1 DW 12345678hDword2 DW 4294967295 0FFFFFFFFh. 4. STRUCT and ENDS directives to define a anatomical 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 elements offset for any structure is 0. A structure element is indite with the base + operator before the elements 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? . . . . . . . . . ENDSDECLARATION STRUCTByte1 DB?Byte2 DB?Word1 DW?Word2 DW?Dword1DW?Dword2 DW?ENDS theatrical role OF STRUCT The STRUCT directive enables us to cha nge the order of items in the structure when, we reform a agitate header and shuffle the data. Shuffle the data items in the saddle 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 duplicate data element.(5) The EQU DirectiveThe EQU directive is used to give name to some value or symbol. Each while the assembler finds the apt(p) 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 BYTETHIS WORDTHIS DWORDA 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 generate an address or space reserved for it in the .COM file. practice A Byte EQU THIS BYTE DB 10 A_ word EQU THIS WORD DW 1000 A_ dword EQU THIS DWORD DD 4294967295 buffer storage Size EQU 1024 Buffer DB 1024 DUP (0) Buffed_ ptr EQU $ actually points to the next byte after the 1024th byte in buffer.(6) ExternIt is used to tell the assembler that the name or label following the directive are I some other assembly staff. For example if you call a procedure which is in program mental faculty 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 SEGMENTEXTERN SMART-DIVIDE FAR found in the segment PROCEDURES-HEREPROCEDURES-HERE ENDS(7) GLOBAL The GLOBAL directive can be used in place of PUBLIC directive .for a name defined in the latest 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) build 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) OFFSETIt is an operator which tells the assembler to subside 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) GROUPIt 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.