Transcript assembler
Computer Science 101 Assembly Language Problems with Machine Language Uses binary - No English-like words to make it more readable Only numeric memory addresses - Can not name an instruction or data location Difficult to change - If we leave out one instruction, all addresses from that point on will be incorrect. Difficult to create data - Must use internal binary representation Assembly Language to the Rescue! Symbolic operation codes: Load, Store, Symbolic names (user defined) for memory addresses Jump, Compare, etc. Load Pay Pseudo-operations provide services such as data generation. Assembler An assembler is a program that takes a source code program written in assembly language and converts it to a machine language object file. Essentially a line by line translation. Assembly lang Source file Assembler Machine lang Object file Labels: Giving names to locations. We can attach a symbolic name to any instruction or data location by beginning the line with a label with that name. A label consists of the name followed by a colon. You use labels in all of the places that addresses would occur in machine language; i.e. you no longer use numeric addresses. – Address fields in instructions Operands of computations Targets of branch instructions Pseudo-operations Pseudo-operations are not statements that are converted to machine code. They are used to request services of the assembler. Performed only when program assembled. Hypothetical Machine Pseudo-Ops: All of the pseudo-ops begin with a period. .begin -- required to mark beginning .end -- required to mark end .data 30 -- puts a value at the memory location; value in decimal assembler converts to binary Example: Add X to Y and Store in Z Z=X+Y .begin load X add Y store Z halt X: .data 33 Y: .data 40 Z: .data 0 .end Developing Assembly Programs First write your algorithm in pseudocode. Then convert to assembly. Convert computations using labeled memory locations and the register for variables document these. We’ll look at techniques for converting common control flow constructs to assembly language. Converting Set (Assignment) Statements Set <variable> to <expression> Semantics: Compute value of the expression Give this value to the variable Assembly language: Accumulate value of expression in R Store the value in the variable’s location Example Set X to 2Y + Z – 5 load Y add Y add Z subtract FIVE store X … X: .data 0 Y: .data 20 Z: .data 30 FIVE: .data 5 … Converting If-statements If XY then Statements A Statements B Cond T load X compare Y jumpgt BSpot … Translation of A ... A Code F B Code BSpot: … Translation of B If-statement example .begin User inputs number. If number is greater than 5 we output the number. Pseudocode: Get N If N>5 then Print N Stop DONE: N: FIVE: .end in N load N compare FIVE jumpgt DONE jumpeq DONE out N halt .data 0 .data 5 Converting If-else-statements If XY then Statements A else Statements B Statements C load X compare Y jumpgt Else … Translation of A F BCode Cond T A Code ... jump CSpot Else: … Translation of B … … C Code CSpot: Translation of C Converting While-statements While XY Do Statements A Statements B Cond T WHILE: load X compare Y jumpgt BSpot … Translation of A ... jump WHILE F A Code BSpot: … Translation of B B Code … While-statement example Program outputs sum of 10 numbers entered by the user. Pseudocode: Set Ct to 10 Set Sum to 0 While Ct>0 Get Num Set Sum to Sum+Num Set Ct to Ct-1 Print Sum Stop .begin WHILE: PRNT: load TEN store CT clear SUM load CT compare ZERO jumpeq PRNT in NUM load NUM add SUM store SUM decrement CT jump WHILE out SUM halt While-statement example (cont.) .begin WHILE: PRNT: load TEN store CT clear Sum load CT compare ZERO jumpeq PRNT in NUM load NUM add SUM store SUM decrement CT jump WHILE out SUM halt TEN: CT: SUM: NUM: ZERO: .end .data 10 .data 0 .data 0 .data 0 .data 0