Fundamental Concepts
Understanding computer memory and system architecture begins with grasping the basics of how data is stored, accessed, and processed. Below are the key concepts to build a strong foundation.
Data Representation
Data is represented in computers using binary digits (bits). Key concepts include:
-
Binary System: Computers use base-2 (binary) numbering, where each digit is either 0 or 1.
-
Hexadecimal System: Often used as a shorthand for binary, hexadecimal (base-16) simplifies representation of large binary numbers.
-
ASCII/Unicode: Standards for encoding characters (e.g., letters, symbols) into binary for storage and processing.
Memory Units
| Unit |
Size |
Typical Use |
| Bit |
1 bit |
Basic unit of information (0 or 1) |
| Byte |
8 bits = [1 byte] |
Characters, small integers |
| Word |
32/64 bits |
Native CPU operations |
| Cache Line |
64/128 bytes |
Memory transfer unit |
| Kilobyte (KB) |
1024 bytes |
Small files, program instructions |
| Megabyte (MB) |
1024 KB |
Medium-sized files, images |
| Gigabyte (GB) |
1024 MB |
Large files, software applications |
| Terabyte (TB) |
1024 GB |
Storage for large datasets, backups |
Bit-Level Operations
Bit manipulation is fundamental to low-level programming and memory management.
// Binary Operations Examples
byte = 0b10101010 // 170 in decimal
// Common Operations:
AND: 10101010 & 11110000 = 10100000
OR: 10101010 | 00001111 = 10101111
XOR: 10101010 ^ 11110000 = 01011010
NOT: ~10101010 = 01010101
LEFT SHIFT: 10101010 << 2 = 10101000
RIGHT SHIFT: 10101010 >> 2 = 00101010
| Operation |
Symbol |
Use Case |
| AND |
& |
Masking bits, testing bit values |
| OR |
| |
Setting bits, combining flags |
| XOR |
^ |
Toggling bits, simple encryption |
| NOT |
~ |
Inverting bits, creating masks |
Memory Hierarchy
Memory hierarchy organizes storage systems based on speed, cost, and capacity. The levels include:
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Registers: Fastest and smallest memory, located inside the CPU.
-
Cache Memory: Divided into L1, L2, and L3 caches, it bridges the gap between CPU registers and main memory.
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Main Memory (RAM): Volatile memory used for active processes and data.
-
Storage (SSD/HDD): Non-volatile memory for long-term data storage.
Addressing and Access
Memory access is governed by addressing mechanisms and access patterns:
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Memory Address: A unique identifier for a location in memory where data is stored.
-
Direct Access: Random Access Memory (RAM) allows any memory location to be accessed directly.
-
Sequential Access: Storage devices like tapes require data to be accessed in a specific order.
Data Storage Types
-
Volatile Memory: Data is lost when power is turned off (e.g., RAM).
-
Non-Volatile Memory: Data persists even without power (e.g., SSD, HDD, ROM).
CPU Registers
~1KB, <1ns access
L1 Cache
32-64KB, ~1ns access
L2 Cache
256KB-1MB, ~4ns access
L3 Cache
Several MB, ~10ns access
Main Memory (RAM)
GB range, ~100ns access
Storage (SSD/HDD)
TB range, μs-ms access
Basic Memory Operations
Memory operations are fundamental to how data is managed:
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Read: Retrieving data from a memory location.
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Write: Storing data into a memory location.
-
Erase: Clearing data from memory (used in flash memory).
Endianness
Endianness defines the order in which bytes are stored in memory:
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Big Endian: The most significant byte is stored at the smallest memory address.
-
Little Endian: The least significant byte is stored at the smallest memory address.
Memory Alignment
Memory alignment ensures data is stored efficiently:
- Data is stored in memory at addresses that are multiples of their size (e.g., 4-byte integers aligned to 4-byte boundaries).
- Improper alignment can lead to performance penalties or crashes.
Memory Bandwidth and Latency
Performance metrics for memory systems:
-
Bandwidth: The amount of data that can be transferred to/from memory per unit time.
-
Latency: The time it takes to access a specific memory location.
Virtual Memory
Virtual memory provides an abstraction layer between programs and physical memory, enabling efficient memory management and protection.
Address Translation Process
Virtual Address Space
|--------------------|
| Page Table |
|--------------------|
| Translation | →→→ TLB Cache →→→ Physical Memory
|--------------------|
| Page Mapping |
|--------------------|
Memory Mapping
Memory mapping bridges physical and virtual memory:
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Physical Memory: Actual hardware memory (RAM).
-
Virtual Memory: An abstraction layer that allows programs to use more memory than physically available.
Page Replacement Algorithms
| Algorithm |
Description |
Advantage |
| LRU |
Least Recently Used |
Optimal for temporal locality |
| Clock |
Second-chance algorithm |
Efficient approximation of LRU |
| Working Set |
Track active pages |
Prevents thrashing |