Computers, Program Execution, and Data Representation

Tom Kelliher, CS 102

Jan. 25, 1999

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Introductory exercises.

Assignment

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Outline

1. Program execution.

2. A model computer.

3. Data representation.

Coming Up

MS Word --- bring the workbook.

Program Execution

Consider the following simple program:

 1:   let sum = 0
 2:   print "How many numbers? "
 3:   read count
 4:   let loopCount = count
 5:   if loopCount equals 0 goto 11
 6:   print "Next number: "
 7:   read input
 8:   let sum = sum + input
 9:   let loopCount = loopCount - 1
10:   goto 5
11:   let average = sum / count
12:   print "The average is:", average, "."
13:   end

1. Where do we begin?

2. After completing one step, where do we proceed?

3. Operations: assignment, arithmetic, decision, branch, I/O.

4. Operands: Variables, constants (numeric and string).

The Basic Parts of a Computer

A block diagram:

1. What is memory? How does it differ from storage?

2. What parts of a PC are considered input devices? Output devices? Both?

Data Representation

1. Computers use the binary system. Why?

2. Binary digits.

3. Bits, bytes, words.

4. Memory locations: cells with addresses.

5. Converting between binary and decimal:
   1. Binary to decimal: Consider some examples. 1, 10, 100, 110. What to do in general?

   2. Decimal to binary: We need a little algorithm.

      read decimalNumber
      while decimalNumber does not equal 0
      {
         let remainder be the remainder of decimalNumber / 2
         let decimalNumber be the integer quotient of decimalNumber / 2
         print remainder to the left of any other digits printed
      }
      

6. How do we represent characters? ASCII code:
   1. A: 01000001

   2. 4: 00110100

7. How does the computer know if a memory location contains numbers, characters, variables, or instructions?