assembly language

1. ASSEMBLY LANGUAGE
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2.  Machine instructions are represented by patterns of 0’s and
1’s
 Use of symbolic words – Move, Add, Increment, Branch etc
 When writing programs such words are replaced by
acronyms called mnemonics like MOV, ADD, INC, BR.
 R3 to represent register 3 and LOC to refer to a memory
location
 Assembly language – A complete set of such symbolic names
and rules for their use constitute a programming language
 Syntax – set of rules
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3. 3
 Assembler – A program that automatically translates the programs
written in an assembly language into a sequence of machine
instructions
 It is one of a collection of utility programs.
 User program in original alphanumeric text format is source
program
 Assembled language program is called object program
 Assembly language program may or may not be case sensitive.

4. 4
 Example
MOVE R0,SUM
 Mnemonic MOVE represents the binary pattern or op code for the
operation
 Assembler translates this mnemonic into binary op code that
computer understands
 Op code followed by operands information (source, destination)
 Here, Source operand …………??????
Destination operand …………??????

5. 5
 Since there are several possible addressing modes for specifying operand
locations, assemble language should indicate the mode
 For example,
ADD #5,R3
In some assemble language this can be denoted by indented op code
mnemonic
ADDI 5,R3
 Indirect addressing modes is specified by putting in ( ),
 For example,
MOVE #5,(R2)
or

6. Assembler Directives
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 In addition to providing a mechanism for representing
instructions in a program, the assemble language allows the
programmer to specify other information needed for
translation
 Suppose SUM is used to represent the value 200
SUM EQU 200
 It simply informs the assembler that the name SUM should
replace the value 200 whenever it appears in the program
 Assembler Directives (Or commands)

7. 7
It shows the memory addresses
where the machine instructions
and the required data items are
to be found after the program
loaded for execution.

8. 8
 Assembler has to know
 How to interpret the names
 Where to place the instructions in the memory
 Where to place the data operands in the memory

9. 9
 Program begins with Assembler Directives
 EQU
 ORIGIN
 DATAWORD
 RESERVE
 RETURN
 END
RETURN – idenfies the point at which the execution of the program should be
terminated.
It causes the assembler to insert an appropriate machine instruction that
returns control to the OS

10. 10
 Most assemble languages requires statements in a source program to
be written as
Label Operation Operand(s) Comment
 Separated by an appropriate delimiter, one or more blank characters.
 Label – optional name associated with the memory address where the
machine language instruction produced from the statement will be
loaded. Also associated with address of the data items
 Operation – op code muemonic or assembler directives
 Operand – addressing information of the operands
 Comment – ignored by the assembler program (only for documentation
purpose)

11. Assembly and Execution of
Programs
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 Key part of the assembly process is determining the values to
replace the names.
 In some cases the value is specified by EQU directive
 Other case, a name is defined in the Label field of a given
instruction
 Hence assembler must keep track of address it generates the
machine code for successive instructions
 In some cases assembler does not directly replace a name
representing an address with the actual value
 Example in branch instructions – Branch target using Relative
addressing mode (branch offset)

12. 12
 Symbol table – it keeps all names and numerical values that
correspond to them while scanning the source program.
 Problem arises when a name appears as an operand before it is given
a value.
 This happens if a forward branch is required.
 A simple solution is to have the assembler scan through the source
program twice.
 During the first pass, it creates a complete symbol table and assigns
numerical value for all names at the end of the pass
 The assembler then goes through the source program a second time
and substitutes values for all names from the symbol table
 Two-pass assembler

13. 13
 Assembler stores the program in the magnetic disk
 Loader – another utility program that loads the object
program into the memory of a computer before it is
executed
 Loader must know the length of the program and the
address in the memory where it will be stored
 Debugger – helps the user to find programming errors

14. Number Notation
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 Numerical values are stored as binary values.
 Most assembler allows numerical values to be specified in
different ways defined by assembly language syntax
 For example,
ADD #93,R1
Can be represented as
ADD #%01011101,R1
In Hexadecimal
ADD #$5D,R1