Computer Organization Basics

Basic Computer Organization

• Includes the Central Processing Unit (CPU), registers, memory (RAM), Input/Output devices, the system bus, and storage devices.

Registers

• These are small, high-speed storage locations within the CPU
• Data is lost from registers when power is turned off
• Registers are either for general or special purposes
• The CPU reads the next instruction from memory and places it in the Instruction Register (IR)
• The Program Counter (PC) register holds the address of the current instruction

Register Types

• General-purpose registers are for temporary data storage/operations, such as EAX (x86) and R0-R12 (ARM)
• Special-purpose registers are for dedicated CPU functions, such as PC, SP, and Status Registers
• Floating-Point registers are foe high-precision math, such as XMM (x86) and F0-F31 (MIPS)

Memory Hierarchy

• From fastest to slowest, this includes registers, cache (L1, L2, L3), main memory (RAM), and secondary storage

32-Bit Versus 64-Bit CPU

• A 32-bit processor can perform operations on 32-bit data
• The size of registers is 32 bits, and the ALU performs 32-bit operations on a 32 bit processor
• A 64-bit CPU perform operations on 64-bit data
• A 64 bit CPU contains 64-bit registers and a 64-bit ALU
• Most desktop and server computers use AMD and Intel processors (32 or 64 bits)
• Intel and AMD processors use the same architecture, meaning a program in computer with an Intel processor can run on a computer with an AMD processor

CPU Buses

• A bus is a communication system that transfers data between components inside a computer
• The address bus carries memory addresses (locations) for reading/writing data
• The data bus transfers actual data between the CPU, memory, and I/O devices
• The control bus carries control signals to coordinate operations

USB

• A serial communication bus connects peripherals to a host (PC/motherboard)
• External USB has peripheral connectivity, is slower, uses daisy-chained devices, and does not have direct CPU access
• Internal system buses connect CPU to memory, are faster, use point-to-point connections, and have direct CPU access
• Several types of USB exist with different speeds, from USB 1.x (12 Mbps) to USB4 (40 Gbps)

Instruction Set Architecture (ISA)

• ISA is the abstract interface between hardware and software

CPU Technology

• CISC (Complex Instruction Set Computer) is an ISA where single instructions execute multiple low-level operations
• CISC processors use variable-length instructions, handle complexity in hardware, and use compact code
• Intel x86 and Motorola 68000 are examples of CISC
• RISC (Reduced Instruction Set Computer) is an ISA with simple, fixed-length instructions optimized for single-cycle execution
• RISC processors use load/store-only memory access and manage pipelining in software
• ARM, RISC-V, and MIPS are examples of RISC

CISC vs RISC

• CISC has complex, multi-cycle instructions and RISC has simple, single-cycle instructions
• CISC uses dense code with less RAM, while RISC requires more RAM
• CISC focuses on hardware complexity and RISC on software optimization
• CISC is found in desktops and legacy systems
• RISC is used in mobile and embedded systems

CPU Architecture

• The Von Neumann architecture stores instructions and data in the same memory with a shared bus
• The Harvard architecture uses separate memories and buses for instructions and data

Von Neumann vs Harvard

• Von Neumann architecture has unified memory, is slower, simpler, and is cheaper
• Harvard architecture splits memory, is faster, requires more hardware, and is used in real-time systems