Instructions & Programs: Crash Course Computer Science #8

Introduction to CPU and Instructions

In this section, we learn about the Central Processing Unit (CPU) and how it processes instructions.

Components of a CPU

• The CPU is made up of an Arithmetic Logic Unit (ALU), control unit, memory, and a clock.

• The CPU is programmable, meaning it can perform different tasks based on the sequence of instructions given to it.

Understanding Instructions

• Instructions are stored in computer memory as binary numbers.

• Each instruction consists of an opcode (operation code) and an address or register specifier.

• For example, "LOAD_A 14" means loading the value from memory address 14 into Register A.

Program Execution

• Programs are executed by fetching instructions from memory and performing the specified operations.

• Instructions like "ADD" and "SUBTRACT" use the ALU to perform arithmetic operations on registers.

• The "JUMP" instruction allows changing the order of instructions or skipping certain instructions by specifying a new location in memory to jump to.

• The "HALT" instruction stops program execution. It separates instructions from data in memory.

Example Program Execution

• An example program adds two numbers together using LOAD, ADD, STORE, and JUMP instructions.

• The program modifies register values and jumps back to repeat certain instructions.

Conclusion

This section concludes our understanding of CPU components and instruction execution.

Key Takeaways

• The CPU is programmable through sequences of instructions that specify operations on registers and memory.

• Instructions are stored in memory as binary numbers, and the CPU fetches and executes them sequentially.

• The ALU performs arithmetic operations, and the control unit manages instruction execution.

Further Exploration

• Understanding CPU architecture and instruction sets is crucial for computer science and programming.

• Exploring more complex programs and instructions can lead to a deeper understanding of how CPUs work.

New Section

This section discusses conditional jumps in computer programming and introduces different types of conditional jumps.

Conditional Jumps

• A jump that only happens if a certain condition is met.

• Examples of conditional jumps include JUMP_NEGATIVE, JUMP IF EQUAL, and JUMP IF GREATER.

• The program starts by loading values from memory into registers A and B.

New Section

This section explains the execution of conditional jumps using an example code.

Execution of Conditional Jumps

• In the example code, the numbers 11 and 5 are loaded into registers A and B respectively.

• Register B is subtracted from Register A, resulting in 6 being saved into Register A.

• The JUMP_NEGATIVE instruction is encountered but since the last ALU result was positive, the processor does not jump.

• The next instruction is a JUMP 2, which unconditionally jumps to instruction 2.

• The SUBTRACT instruction is executed again, resulting in 1 being saved into Register A.

New Section

This section continues the execution of the example code with conditional jumps.

Continued Execution of Conditional Jumps

• The program reaches the JUMP_NEGATIVE instruction again. Since 1 is also a positive number, the CPU continues to execute the next instruction which is another JUMP 2.

• The SUBTRACT instruction is executed once more. This time, 1 minus 5 results in -4 and sets the negative flag to true for the first time.

• When advancing to the next instruction (JUMP_NEGATIVE), the CPU executes a jump to memory location 5.

• The ADD B to A instruction adds -4 and 5, resulting in 1 being saved into Register A.

• The STORE instruction saves Register A into memory address 13.

• Finally, the HALT instruction is encountered and the computer rests.

New Section

This section discusses the number of instructions executed in a program with conditional jumps.

Number of Executed Instructions

• Although the example code is only 7 instructions long, the CPU ends up executing 13 instructions due to looping twice internally.

• The code calculates the remainder when dividing 5 into 11, which is one.

• By adding a few extra lines of code, it would be possible to keep track of how many loops were performed.

New Section

This section highlights the flexibility of the code and introduces software as a means to extend hardware functionality.

Flexibility and Software Power

• The code can work for any two numbers by changing them in memory.

• Software provides functionality that hardware may not have. In this case, the ALU didn't have division functionality, but it was achieved through programming.

• Other programs can utilize this divide program to perform more complex tasks.

• This demonstrates new levels of abstraction made possible by software.

New Section

This section discusses limitations in CPU design related to instruction length and memory addressing.

Limitations in CPU Design

• Hypothetical CPUs like the one discussed have limited instruction set sizes due to opcode constraints.

• Real CPUs use two strategies: increasing instruction length or using variable-length instructions.

• Bigger instructions with more bits allow for a larger instruction set but come with trade-offs.

• Variable-length instructions involve fetching additional values based on specific instructions' requirements.

New Section

This section explains variable-length instructions and provides an example of a real CPU from history.

Variable-Length Instructions

• CPUs can use variable-length instructions to address more memory and provide flexibility.

• The fetch cycle of the CPU becomes more complicated with variable-length instructions.

• An example is given of the Intel 4004 processor released in 1971, which supported 46 instructions and used 8-bit immediate values for addressing.

New Section

This section highlights the growth in instruction set size in modern processors.

Growth in Instruction Set Size

• Modern computer processors like Intel Core i7 have thousands of different instructions and variants.

• Instructions can range from one to fifteen bytes long, providing a wide range of functionality.

• The increase in instruction set size is due to additional features and capabilities added to processors over time.