Class 04: The Structure of a Computer

Held: Friday, 24 January 2003

Summary: In today's class we will consider a simple model of computation that can ground our further studies.

Assignments:

• Read K&R 1 (due Mon).
• Read Gries 2 (due Tue).

Notes:

• Quiz on associativity.
• I'll try an electronic whiteboard today in class. Please let me know what you think.
• Are there any questions on homework 2? (1) I described a simple Java hierarchy for describing propositions after class. Let me know if you need more details. (2) Yes, a depth-first, left-to-right traversal of the proposition is appropriate.

Overview:

• What is a computer?
• Key operations
• Some simple programs

computer

• Simple model:
  • Input
  • Output
  • Memory
  • Processing
• Both program and data are stored in memory
  • Everything needs an encoding that can be used to generate and interpret the bits.
• Basic Operation:
  • Fetch instruction
  • Decode instruction
  • Execute instruction (potentially fetching values from memory and storing them in memory).
• Common extensions:
  • Registers store values, operations generally require registers as one or more of their parameters.
  • A bus holds various devices (including input, output, and memory).
  • Special flags can be raised and checked.
  • Separate areas for program and data.

assembly

• Preliminary notes:
  • Most instructions are typed.
  • Instructions can be preceded by labels.
• Input instructions. In our discussions, we'll limit ourselves to a typed read.
• Output instructions. In our discussions, we'll limit ourselves to to a typed write.
• Arithmetic instructions (e.g., add, subtract, multiply, divide). All of these are typed.
• Control operations. Primarily jump and various conditional jumps.
  • Conditional jumps can compare values or check flags.

program

• I'll challenge you to help write these (in approximate language).
  • Since I'm challenging you, the code isn't here.
• Read two integers and print out their sum.
• Read integers until you read 0. Print out their sum.
• Read integers until you read 0. Print them out in reverse.