Assembly Language Ms. V.Anitha AP/CSE SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT Assembly Language Basic Elements of Assembly Language Labels Mnemonics and Operands Comments Reserved words and identifiers Directives Two-Pass Assembler FDP on " Computer Architecture" Prof. V.Anitha, SCT

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. FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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. Assembler Source Program Object Program FDP on " Computer Architecture" Prof. V.Anitha, SCT

Assembly Language Instructions Assembled into machine code by assembler Executed at runtime by the CPU Format LABEL OPCODE OPERANDS ; COMMENTS Label (optional) Mnemonic (required) Operand (depends on the instruction) Comment (optional) Examples MOVE R0,SUM ADD #5,R3 ADDI 5,R3 MOVE #5,(R2) FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

Mnemonics and Operands 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) FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT Comments 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. FDP on " Computer Architecture" Prof. V.Anitha, SCT

Reserved Words and Identifiers Reserved words cannot be used as identifiers Instruction mnemonics (MOV), directives attributes (BYTE, WORD), operators (=) Predefined symbols (@data) Identifiers 1-247 characters, including digits not case sensitive first character must be a letter, _, @, ?, or $ Examples: var1, Count, $first, _main, @myfile FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT Example FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT

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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT 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 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT Symbol Table Example Symbol Address LOOP 112 SUM 200 N 204 NUM1 208 FDP on " Computer Architecture" Prof. V.Anitha, SCT

FDP on " Computer Architecture" Prof. V.Anitha, SCT Thank You… FDP on " Computer Architecture" Prof. V.Anitha, SCT