Computer Storage Fundamentals

The smallest storage unit is a bit, holding either 0 or 1. The basic addressable unit is a byte (8 bits). Storage scales as:

• 1 KB = 1024 bytes
• 1 MB = 1024 KB
• 1 GB = 1024 MB
• 1 TB = 1024 GB

Numeric Representation

Computers store numbers using two's complement:

• Signed types use the highest bit as sign (0=positive, 1=negative)
• Positive numbers: Original, inverse, and complement codes are identical
• Negative numbers:
  • Inverse code = original with bits flipped (except sign)
  • Complement code = inverse + 1

Example (16-bit system):

1:   Original: 0000000000000001
     Inverse:  0000000000000001
     Complement: 0000000000000001

-1:  Original: 1000000000000001
     Inverse:  1111111111111110
     Complement: 1111111111111111

Variable Scope and Storage Classes

extern

Declares globally defined variables from other files:

1. Prevents duplicate definitions
2. Improves code organization

static

Restricts variable scope to file/function while preserving value between calls.

const

Creates immutable values with multiple applications:

1. Consatnt declaration: const int MAX=100;
2. Protecting function parameters
3. Immutable return values
4. Pointer control:
   • const int*: Pointer to constant
   • int* const: Constant pointer
   • const int* const: Fully immutable

Memory Segments

1. Code Segment: Read-only executable instructions
2. Data Segment: Initialized global/static variables
3. BSS: Zero-initialized global/static variables
4. Stack: Automatic storage for function calls (LIFO)
5. Heap: Dynamic memory allocation (malloc/free)

Data Types

Size varies by architecture:

Operators

Bitwise Operations

• XOR (^): Bits differ → 1, same → 0
• NOT (~): Inverts all bits
• Shifts:
  • Left (<<): Multiply by 2 (no overflow)
  • Right (>>): Divide by 2 (sign-preserving)

Division/Modulo

• Division sign: Single negative → negative result
• Modulo sign: Matches dividend

String Functions

Key functions from <string.h>:

1. strlen(): Length excluding null terminator
2. strcpy(): Copies including null terminator
3. strcmp(): Lexical comparison (returns difference)
4. strcat(): Concatenates strings

Arrays

2D Array Initialization

int valid[][3] = {1,2,3,4,5,6}; // OK
int invalid[][] = {1,2,3};      // Error

Partial initialization fills remaining elements with 0.