Processor Addressing Modes

Summary

This script explains the instruction register in a CPU, detailing various addressing modes like immediate, direct, indirect, register direct, register indirect, relative, and indexed addressing. It illustrates how each mode affects memory access, with immediate addressing being the fastest and indirect addressing being slower due to multiple memory accesses. The explanation is designed to help users understand how CPUs execute machine code instructions.

Takeaways

• 💡 The instruction register in a CPU holds the program instruction currently being executed.
• 🔢 A modern PC processor uses more bits than the 16 bits shown in the simplistic diagram.
• 🛠️ Each instruction has a fixed number of bits for the operation code and operand, which can be a value, CPU register, or memory address.
• 📈 More bits allocated to the operation code allows for a larger instruction set but limits the operand size.
• 🔄 Addressing modes help overcome memory limitations and provide efficient ways to use registers and memory.
• 📋 Two bits of the operation code can indicate the addressing mode used in an instruction.
• 🏷️ Immediate addressing is when the operand is the actual value to be operated on.
• 📍 Direct addressing (or absolute addressing) is when the operand is a memory address containing the value.
• 🔄 Indirect addressing is when the operand is a memory address that contains another effective memory address.
• 🏎️ Register direct mode is the fastest addressing mode because it doesn't require memory access after instruction fetch.
• 🔄 Relative addressing calculates the effective memory address by adding the operand to the program counter's address.
• 🔢 Indexed addressing adds a value in the index register to a base memory address to access array variables sequentially.

Q & A

What is the function of the instruction register in a CPU?

The instruction register holds the program instruction that is currently being executed.

How does the number of bits in the instruction register affect the machine's capabilities?

More bits allow for a larger instruction set, but fewer bits for operands and memory addressing; fewer bits allow for fewer commands but more memory addressing.

What is meant by the operation code in a machine code instruction?

The operation code represents the operation such as add, subtract, load, or store.

What is an operand in the context of machine code instructions?

An operand could be a value, a CPU register, or a memory address that will be operated upon.

How do addressing modes help overcome limitations on memory addressing?

Addressing modes provide efficient ways to utilize the processor's registers and memory, allowing more memory locations to be accessible.

What is immediate addressing and how is it indicated?

Immediate addressing is when the operand is the actual value to be operated on, indicated by a hash symbol in front of the value.

What is direct addressing and how does it differ from indirect addressing?

Direct addressing is when the operand is a memory address containing the value to be operated on, while indirect addressing is when the operand is a memory address of a location that contains the effective memory address, requiring more memory accesses.

How does register direct mode differ from register indirect mode?

In register direct mode, the operand is a processor register containing a value, which is faster as it does not require memory access. In register indirect mode, the operand is a register containing a memory address, requiring one memory access.

What is relative addressing and why is it advantageous?

Relative addressing calculates the effective memory address by adding the operand to the program counter's address, making jump instructions independent of their position in memory.

How does indexed addressing work and what is it useful for?

Indexed addressing calculates the effective memory address by adding the operand to the value in the index register, useful for accessing contiguous memory locations, such as array variables.

What are some other addressing modes not mentioned in the script?

Other addressing modes include indexed direct, indexed immediate, indexed indirect, displacement mode, auto increment, auto decrement, and some architectures have a base-plus-offset addressing mode.