Download Assembly Language

Assembly Language
Ms. V.Anitha
AP/CSE
SCT
Assembly Language
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Basic Elements of Assembly Language
Labels
Mnemonics and Operands
Comments
Reserved words and identifiers
Directives
Two-Pass Assembler
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It’s hard to write code in 1’s &
0’s!
• Machine Instructions are patterns of 0s and 1s.
• Symbolic names are used to represent patterns.
( eg., MOV,ADD,INC)
• A Complete set of symbolic names and rules for their use is
called
Assembly Language.
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Assembler
• Each line of assembly language (Source Program) is translated
into a single Machine Language(Object Program) instruction.
• A program called the Assembler does the translation and
provides useful tools.
Object Program
Source Program
Assembler
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Assembly Language Instructions
• Assembled into machine code by assembler
• Executed at runtime by the CPU
Format
LABEL
OPCODE
OPERANDS
; COMMENTS
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Label (optional)
Mnemonic (required)
Operand (depends on the instruction)
Comment (optional)
Examples
MOVE R0,SUM
ADD #5,R3
ADDI 5,R3
MOVE #5,(R2)
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Labels
• Is an optional name associated with the memory
address.
• Act as place markers.
• marks the address (offset) of code and data
– Code label
– Data label
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Mnemonics and Operands
Mnemonics
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Instruction Mnemonics
memory aid
examples: MOV, ADD, SUB, MUL, INC
Operands
• constant value (96)
• immediate value (#5)
• constant expression (2+4)
• Register ( R1)
• memory (data label) (count)
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Comments
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Explain the program's purpose
When it was written, and by whom
Revision information
Tricky coding techniques
Application-specific explanations
Single-line / multiline comments
Begin and end with the same programmer-chosen
character
• Is ignored by the assembler program.
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Reserved Words and Identifiers
• Reserved Words
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Reserved words cannot be used as identifiers
Instruction mnemonics (MOV), directives
attributes (BYTE, WORD), operators (=)
Predefined symbols (@data)
• Identifiers
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1-247 characters, including digits
not case sensitive
first character must be a letter, _, @, ?, or $
Examples: var1, Count, $first, _main, @myfile
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Directives
• Commands that are recognized and acted upon
by the assembler.
• Used to declare code, data areas.
• Not case sensitive.
• Different assemblers have different directives.
• Examples:DATAWORD,EQU
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Directives cont…
• EQU directive
name EQU expression
name EQU symbol
name EQU <text>
• Define a symbol as either an integer or
text expression.
• Can be useful for non-integer constant
• Cannot be redefined
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Example
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Suggested Coding Standards
• Some approaches to capitalization
– capitalize nothing
– capitalize everything
– capitalize all reserved words, including
instruction mnemonics and register names
– capitalize only directives and operators
• Other suggestions
– descriptive identifier names
– spaces surrounding arithmetic operators
– blank lines between procedures
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Suggested Coding Standards
Cont….
• Indentation and spacing
– code and data labels – no indentation
– executable instructions – indent 4-5 spaces
– comments: begin at column 40-45, aligned
– 1-3 spaces between instruction and its
operands
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•
The
Assembly
Process
Objective
– Translate the AL (Assembly Language) program
into ML (Machine Language).
– Each AL instruction yields one ML instruction word.
• Problem
– An instruction may reference a label.
– If the label hasn’t been encountered yet, the assembler
can't form the instruction word
• Solution
– Two-pass assembly
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Two-Pass Assembly - 1
• First Pass - generating the symbol table
– Scan each line
– Keep track of current address
• Increment by 1 for each instruction
– For each label
• Enter it into the symbol table
• Allocate to it the current address
– Stop when .END is encountered
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Two-Pass Assembly - 2
• Second Pass - generating the ML program
– Scan each line again
– Translate each AL instruction into ML
• Look up symbols in the symbol table instruction
• Ensure that labels are no more than +256 / -255 lines
from instruction
• Determine operand field for the instruction
– Fill memory locations as directed by pseudo-ops
– Stop when .END is encountered
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Symbol Table Example
Symbol
Address
LOOP
112
SUM
200
N
204
NUM1
208
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Thank You…
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