Computer and Communications Fundamentals. How Computers Work. The Art of (low level) Programming. How Computers Work: Lecture 2 1

Transcription

1 Computer and Communications Fundamentals Shimon Schocken How Computers Work The Art of (low level) Programming How Computers Work: Lecture 2 1

2 Lecture Objectives Acquire a taste for low level programming Relate to high-level language programming Understand the software development cycle How Computers Work: Lecture 2 2

3 First program: add two numbers (cont.) x 2 x ADD x 1 + x 2 21 algorithm 0. read a number 1. store it somewhere (let s say in cell 90) 2. read a number 3. add the contents of cell 90 (to the register) 4. write (the contents of the register) 5. stop program addr instruc read symbolic str read add write stop How Computers Work: Lecture 2 3

4 Program 2: find the maximum of two numbers x 2 x MAX max(x 1, x 2 ) 901 Strategy: read the first number, call it x read the second number, call it y subtract x from y (result=y-x) if the result is positive => y is the larger number else => x is the larger number How Computers Work: Lecture 2 4

5 The machine language revisited (control) symbolic numeric pseudo-code arithmetic commands ADD xx SUB xx 1 xx 2 xx D=D+M[xx] D=D-M[xx] memory commands LD xx STR xx 3 xx 4 xx D=M[xx] M[XX]=D I/O commands READ WRITE D=input output=d control commands GT xx GTZ xx GTP xx 5 xx 6 xx 7 xx goto xx if D=0 goto xx if D>0 goto xx STOP 0 stop How Computers Work: Lecture 2 5

6 Finding the maximum of two numbers (cont.) algorithm 0. read a number (into D) 1. store it somewhere (say in cell 90) 2. read a number (into D) 3. store it somewhere (say in cell 91) 4. subtract M[90] from D 5. if D>0 goto load the contents of cell write 8. stop 9. load the contents of cell write 11. stop program How Computers Work: Lecture 2 6

7 Program 3: sum up a series of numbers 0 x n... x 2 x SUM x 1 + x x n 13 Strategy: Use cell 90 to store the sum of the inputted numbers. Call this cell sum. Assumption: before the program starts running, "sum" should be reset to 0 As long as the inputted number is not 0, add the inputted number to sum When you read 0, write the contents of sum and stop the program. How Computers Work: Lecture 2 7

8 Program 3: sum up a series of numbers (cont.) algorithm 0. read a number (into D) 1. If the number is 0, goto add M[90] to D 3. store the result (D) in M[90] 4. goto 0 5. load M[90] into D 6. write 7. stop program Observation 1: what if cell 90 does not contain 0 when the program starts? Fix the program to take care of this bug. Observation 2: there is one unnecessary instruction in this program. Which one? How Computers Work: Lecture 2 8

9 Diversion: Pascal on brevity "This letter is a bit long because I didn't have the time to make it shorter." Blaise Pascal, He also built the first digital calculator; it took him 3 years The prog. language Pascal was named in his honor He also developed the theory of hydrostatics, invented the binomial coefficients (Pascal's triangle), laid the foundations of probability, and wrote a fundamental philosophical treatise, called Pensées. He died at the age of 39. How Computers Work: Lecture 2 9

10 Program 4: find the maximum in a series of numbers 0 x n... x 2 x MAXn max(x 1, x 2,..., x n ) 5 Algorithm (strategy): Assume that the first number is the maximum of the series, and store it in a cell called max Read the next number (let s call it x ) and subtract max from it. If the result is positive, x is the new maximum, so store it in max Read the next number and do exactly the same thing, until you hit 0. How Computers Work: Lecture 2 10

11 Before you run an assembly program, it must be translated to machine language How Computers Work: Lecture 2 11

12 Let s make life even easier: High Level Languages C code scanf("%d", &max); while (1) { scanf("%d", &x); if (x == 0) break; if (x > max) max = x; } printf("%d", max); C compiler Pascal, Java, Visual Basic,... very similar translations Assembly code label1: read store max read if reg=0 goto label2 store x subtract max if reg > 0 goto label3 goto label1 label3: load x store max goto label1 label2: load max write stop How Computers Work: Lecture 2 12

13 From an abstract algorithm to a working program algorithm HLL language high-level program (source) compiler machine language program (object) loader How Computers Work: Lecture 2 13

14 The program development cycle compile How Computers Work: Lecture 2 14

15 The argument for high level languages (HLL) HLL code is easy to read, maintain and extend; Object code is easy to transmit and execute The user never sees the source code -- s/he gets only the object code (*.exe, *.com, *.dll files). This way, software companies can protect their intellectual property The same HLL program can be compiled to run on any computer platform (using an appropriate compiler). Hence, software companies can extend their market share easily Computer scientists can invent all sorts of HLL s, designed for different purposes. As long as we can translate the HLL to machine language, the HLL can be as abstract as we want The existence of HLL creates a Chinese wall separation between software and hardware. This is a very, very, very powerful and important concept. How Computers Work: Lecture 2 15

16 Summary The logical principles underlying hardware are simple The logical principles underlying software are simple What is good science? Explain maximum phenomena with minimum rules What is good engineering? Build complex artifacts with simple components Computer architectures are a prime example of good engineering Based on these architectures, Alan Turing and John Von Neumann created some of the most beautiful science of the 20 th century Next lecture: back to C How Computers Work: Lecture 2 16