Computer Science 101

1. Computer Science 101 Assembly Language

2. 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

3. Assembly Language to the Rescue! • Symbolic operation codes: Load, Store, Jump, Compare, etc. • Symbolic names (user defined) for memory addresses Load Pay • Pseudo-operations provide services such as data generation.

4. Assembler Assembly lang Source file Machine lang Object file 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.

5. 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

6. 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

7. Example: Add X to Y and Store in ZZ = X + Y .begin load X add Y store Z halt X: .data 33 Y: .data 40 Z: .data 0 .end

8. 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.

9. 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

10. Example • Set X to 2Y + Z – 5 • load Y add Y add Z subtract FIVE store X …X: .data 0Y: .data 20Z: .data 30FIVE: .data 5…

11. If XY then Statements AStatements B load X compare Y jumpgtBSpot …Translation of A ... BSpot: …Translation of B Converting If-statements T Cond A Code F B Code

12. User inputs number. If number is greater than 5 we output the number. Pseudocode: Get N If N>5 then Print N Stop .begin in N load N compare FIVE jumpgt DONE jumpeq DONE out N DONE: halt N: .data 0 FIVE: .data 5 .end If-statement example

13. If XY then Statements A else Statements B Statements C load X compare Y jumpgt Else …Translation of A ... Else: …Translation of B … …Translation of C jump CSpot CSpot: Converting If-else-statements F T Cond BCode A Code C Code

14. While XY Do Statements A Statements B load X compare Y jumpgt BSpot …Translation of A ... BSpot: …Translation of B … WHILE: jump WHILE Converting While-statements T Cond A Code F B Code

15. 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 load TEN store CT clear SUM WHILE: load CT compare ZERO jumpeq PRNT in NUM load NUM add SUM store SUM decrement CT jump WHILE PRNT: out SUM halt While-statement example

16. .begin load TEN store CT clear Sum WHILE: load CT compare ZERO jumpeq PRNT in NUM load NUM add SUM store SUM decrement CT jump WHILE PRNT: out SUM halt TEN: .data 10 CT: .data 0 SUM: .data 0 NUM: .data 0 ZERO: .data 0 .end While-statement example (cont.)

17. Cows, horses, supplies,... Man, I can't keep track of everything!