Model of a Computer

Tuesday, August 30, 2010

Storage of Data

Data represented in bits

01010001 can be 1,010,001₁₀, 81₂, Q_ASCII

Memory Sequence of locations, one byte ea., 0-indexed

Limits: overflow & roundoff

Types of Storage

• Cache
• Main RAM
• Disk RAM (slower)
• Tape Sequence

Operations

Simplified model of a computer

• CPU
• Arithmetic/logic unit (ALU)
• Registers (like super fast cache)
• Control (between Register and RAM

Example: Add number in addr 3 with addr 6, and put result in addr 10

1. LOAD 3,1
2. LOAD 6,4
3. ADD 1,4,3
4. STORE 3,10

Program Storage

Agreed-upon coding convention:

Interpreted Command	ADD (+)	r1 (210 = 000000102)	r4 (510 = 000001012)	r3 (710 = 000001112)
Binary CPU Command	0010	0001	0100	0011

Program execution

• Instruction register (current execution)
• Program counter (location in program)

JUMP: (if, while)

• unconditional and conditional
• alters Program Counter

Evolution and Abstraction

1. Machine Language (all in binary)
2. Assembly Languages (symbols for operators/addresses)
   • Assembler translates to Machine Language
3. High-level (Fortran, C, Java, C++)
   • Compiler translates to Machine language

Several machine instructions for x = (y+3)/z;

Compilation

Lexical analysis

Break language into Tokens: total = sum + 55.32 → 'total', '=', 'sum', '+', '55.32'

Parsing

Figure out goal of tokens:

• 'sum + 55.32' → Arithmetic expression;
• 'total = ...' → Assignment

(Machine) Code Generation

called object code

Linking

finds references between different files writes new file to disk

Loading

Addresses determined by operating system result is executable