Assemblers & Assembler Directives

To assemble a program automatically the assembler needs information in the form of assembler directives that controls the assembly or Assembler directives are the directions to the assembler which indicate how an operand or section of the program is to be processed. These are also called pseudo operations which are not executable by the microprocessor. For example, the assembler must be told at what address to start assembling the program. These assembler directives are command placed in the program by the designer that provides information to the assembler. They do not become part of the final program as they are not the part of the instruction set of the microprocessor nor did they translate into executable code. Therefore, they are also known as pseudo-instruction on false instructions.

Each assembler has its own unique pseudo instructions or assembler directives. These instructions differ from assembler to assembler but most of the assembler contains an equivalent set of pseudo instructions written in assembly language format. The various directives are explained below.

·         ASSUME: The ASSUME directive is used to inform the assembler the name of the logical segment it should use for a specified segment.

Ex: ASSUME DS: DATA tells the assembler that for any program instruction which refers to the data segment, it should use the logical segment called DATA.

·    2. DS: Another pseudo instruction, the define storage, reserves or allocates read/write memory locations for storage of temporary data. The first of the locations allocated can be referred to by an optional symbolic label. The define storage instruction has the form

opt. label: DS expression.

·         3. DB -Define byte. It is used to declare a byte variable or set aside one or more storage locations of type byte in memory. When a table of fixed data values is required, memory must also be allocated. However, unlike the DS, each memory locations must have a defined value that is assembled into it.

For example, CURRENT_VALUE DB 36H tells the assembler to reserve 1byte of memory for a variable named CURRENT_ VALUE and to put the value 36 H in that memory location when the program is loaded into RAM .

·      4. DW -Define word. It tells the assembler to define a variable of type word or to reserve storage locations of type word in memory. Define word DW instruction is similar to define byte pseudo instruction.

The only difference between the DB & DW is that expression in this define word list is evaluated to 16-bit quantity and stored as 2-bytes. It is stored with the lower order byte in the lower of the two memory locations and the higher order byte in the next higher location. This is consistent with the convention for storing 16- bit quantities in 8085A systems. Some of the assemblers use DFW for this assembler directive.

·      5. DD(define double word) :This directive is used to declare a variable of type double word or restore memory locations which can be accessed as type double word.

·         6.DQ (define quadword) :This directive is used to tell the assembler to declare a variable 4 words in length or to reserve 4 words of storage in memory .

·         7.DT (define ten bytes):It is used to inform the assembler to define a variable which is 10 bytes in length or to reserve 10 bytes of storage in memory.

·         8. SET: SET is similar to EQU assembler directive. This directive also assigns a value to the name associated it. However, the same symbol can be redefined by another SET statement later in the program. Thus, more than one SET instructions can have the same name the SET assembler directive has the form.

name SET expression.

·       9. EQU – EQU: Symbolic names, which appear in assembly language programs as labels, instructions mnemonics and operands are translated to binary values by the assembler. Equate it is used to give a name to some value or symbol. Every time the assembler finds the given name in the program, it will replace the name with the value or symbol we have equated with that name.

A symbolic operand can be a register name, an address or a data constant. Register names have predefined values. All addresses correspond to labels in the program and their values are defined. Data constants, on the other hand, are defined by the designer using an equate assembler directive. The equate instruction EQU defines symbols used in the program. Equate assembler directives usually appear as a group at the beginning of a program and have the form.

name EQU expression.

‘name’ stands for the symbolic name. The assembler evaluates the expression and equates the symbolic name to it by placing the name in its symbol table along with the value of the expression. Therefore, whenever the name appears in the program, it is replaced by the value the expression in the equate pseudo instruction.

·         10. ORG -Originate: The origin (ORG) instruction tells the assembler the address of the memory location for the next instruction or data byte should be assembled. ORG is entered at the beginning of a program. When different parts of a programme (e.g. subroutines) are to be placed in different areas of memory, an ORG pseudo instruction is used before each part of the program to specify the starting location for assembly of that part of the program. The ORG statement changes the starting offset address of the data.

It allows to set the location counter to a desired value at any point in the program. For example the statement ORG 3000H tells the assembler to set the location counter to 3000H.

·           11 .PROC- Procedures: It is used to identify the start of a procedure or subroutine.

·       12. END- End program .This directive indicates the assembler that this is the end of the program module. The assembler ignores any statements after an END directive. When an assembler scans the program to be assembled it must know, where the program ends. It cannot depend on a HLT instruction for this because some programmes don’t contain a halt instruction as the last instruction and other don’t contain a halt at all. The END statement explicitly indicates the end of the program to the assembler. If no END statement is given, then the assembler just keeps on running through all the memory. When there is more than one ORG assembler directive, then the assembly of group of instruction start at the location specified by the origin assemble directive that proceeds. But there will be only one END instruction to tell the assembler the physical end of the program.

·           13. ENDP- End procedure: It indicates the end of the procedure (subroutine) to the assembler.

·         14. ENDS-End Segment: This directive is used with the name of the segment to indicate the end of that logical segment.

Ex: CODE SEGMENT: Start of logical segment containing code

CODE ENDS: End of the segment named CODE.

Macros

Macros: Sometimes it is required that same set of instructions are to be repeated again & again. One way to simplify the problem is the use of subroutine. This increases the execution time due to overhead. The other way is the use of macros. The assemblers which have the capability to process macro instructions are called macro assemblers. The assemblers are designed such that the programmer need to write set of instruction once and then refer it many times as desired.

A macro instruction is a single instruction that the macro assemble replaces with a group of instruction whenever it appears in an assembly language program. The macro instruction and the instruction that replace it are defined by the system design only once in the program. Macros are useful when a small group of instruction must be repeated several times in a program, with only minor or no changes in each repetition.

The use of macro in ALP entails three groups:

1)      The macro definition

2)      The macro reference

3)      The macro expansion.

The macro definition defines the group of instructions equivalent to macro. Macro reference is the use of the macro instruction as an instruction in the program. A macro expansion is the replacement of the macro instruction defined by its equivalent. The first two steps are carried out by the system designer and the third by the macro assembler.

--- LOCAL label names

The specified label names are defined to have meaning only within the current macro expansion. Each time the macro is referenced and expanded; the assembler assigns each local symbol a unique symbol in the form ‘??nnnn’. The assembler assigns ‘??0001’ to the first symbol, ‘??0002’ to the second symbol and so on.

REFERENCES

1.      R. S. Gaonkar, Microprocessor Architecture, Programming, and Applications with the 8085, Fifth Edition, Penram International Publishing (India) Private Limited.

2.      S Ghoshal, Microprocessor Based System Design, Macmillan India Limited, 1996

3.      M. Mano, Digital Logic and Computer Design, Prentice – Hall India

4.      B. Ram - Fundamentals of Microprocessor and Microcontrollers

5.      “Microprocessors: Principles and Applications” by A Pal

6.      “Microprocessors and Microcontrollers : Architecture, Programming and Interfacing Using 8085, 8086 and 8051” by Soumitra Kumar Mandal

7.      “Introduction to Microprocessors and Microcontrollers” by Crisp John Crisp

8.      “Microprocessors And Microcontrollers” by A Nagoor Kani

9.      “Microprocessors And Microcontrollers : Architecture, Programming and System Design 8085, 8086, 8051, 8096” by KRISHNA KANT

10.  “8 - Bit Microprocessor” by Vibhute