The Central Processing Unit (CPU): Crash Course Computer Science #7

Understanding the CPU: The Heart of a Computer

Introduction to Processors

• Carrie Anne introduces the episode, warning that it covers complex material about processors.

• Recaps previous episodes on Arithmetic and Logic Units (ALUs) and memory types, including Registers and RAM.

What is a CPU?

• Defines the Central Processing Unit (CPU) as the core component responsible for executing programs made up of instructions.

• Explains that CPUs handle both mathematical operations via ALUs and memory instructions by interacting with RAM.

Building Blocks of a CPU

• Introduces components like RAM with 16 locations and four 8-bit registers (A, B, C, D).

• Discusses how data is stored in binary values and how each instruction has an operation code (opcode).

Instruction Handling

• Describes two additional registers: Instruction Address Register (IAR), which tracks current instruction address, and Instruction Register (IR), which stores the current instruction.

• Outlines the initialization process where all registers start at zero.

Fetch Phase

• Details the fetch phase where the first instruction is retrieved from RAM based on IAR's value.

• Explains decoding of fetched instructions; for example, opcode "0010" corresponds to "LOAD A".

Decode Phase

• Highlights how instructions are decoded by a Control Unit using logic gates to recognize specific opcodes.

Execute Phase

• Describes executing an instruction by enabling appropriate circuits; for "LOAD A", it retrieves data from RAM into Register A.

Completing an Instruction Cycle

• After execution, IAR increments to prepare for fetching the next instruction.

• Notes that different instructions require different decode circuits but can be managed through a single Control Unit abstraction.

Continuing Execution Cycles

Understanding CPU Instructions and Operations

Fetching and Executing an ADD Instruction

• The instruction "1000 01 00" represents an ADD operation, utilizing two sets of 2 bits to select registers. Here, "01" corresponds to Register B and "00" to Register A.

• The Control Unit selects the appropriate registers (A and B) as inputs for the ALU, configuring it to perform the ADD operation by passing in the corresponding opcode.

• To prevent feedback into the ALU, the Control Unit uses an internal register to temporarily store the output before writing it back into Register A.

• After executing with inputs 3 and 14, Register A holds the result (17 or 00010001 in binary), followed by incrementing the instruction address.

Storing Results in Memory

• The next instruction "01001101" is decoded as a STORE_A command with a RAM address of 13. Instead of reading from memory, it writes data from Register A into RAM.

• This marks a successful execution of a simple program that loads values from memory, adds them together, and stores the result back.

Role of the Clock in CPU Operations

• The clock component triggers electrical signals at regular intervals to synchronize CPU operations like fetch-decode-execute cycles.

• Clock speed determines how quickly these cycles occur; it's measured in Hertz (Hz), where one Hz equals one cycle per second.

Historical Context of CPU Speed

• The first single-chip CPU was Intel's 4004 released in 1971 with a clock speed of 740 Kilohertz—significantly faster than manual calculations but slow compared to modern standards.

• Modern CPUs operate at speeds measured in gigahertz (GHz), equating to billions of cycles per second.

Overclocking and Dynamic Frequency Scaling

• Overclocking involves modifying clock settings for increased performance but risks overheating or signal errors if pushed too far.

• Dynamic frequency scaling allows CPUs to adjust their clock speed based on demand, optimizing power usage especially for battery-operated devices like laptops and smartphones.

Conclusion: Completing Our Simplified CPU Design