35. OCR A Level (H446) SLR7 - 1.2 Addressing memory

Summary

This video explains four key memory addressing modes used in computing: immediate, direct, indirect, and indexed addressing. It demonstrates how each mode works with examples, highlighting their use in executing instructions. Immediate addressing uses the operand directly as the value, direct addressing refers to a memory address, indirect addressing involves finding another memory address, and indexed addressing is used with arrays, adjusting addresses via an index register. These addressing modes are crucial for efficient memory management, especially when dealing with limited address space and large datasets, such as arrays.

Takeaways

• 😀 Immediate addressing mode directly uses the value in the operand, with no memory search required.
• 😀 Direct addressing mode references a memory address where the required value is stored.
• 😀 Indirect addressing mode uses an operand that points to another address containing the actual data address.
• 😀 Indexed addressing mode combines a base address with an index value to access elements in an array.
• 😀 In the immediate mode, the instruction 'add 10' adds the value 10 directly to the accumulator.
• 😀 Direct addressing involves going to a specific memory location (e.g., 'add 10' means accessing memory address 10).
• 😀 Indirect addressing allows for larger address ranges by referencing memory locations that contain other addresses.
• 😀 Indexed addressing is ideal for accessing elements in a contiguous array by incrementing an index register.
• 😀 The operand in immediate addressing is the actual value to be used, making it simpler than other modes.
• 😀 Indirect addressing helps overcome address size limitations by allowing the use of memory locations that store other addresses.
• 😀 In indexed addressing, the index register is incremented to access subsequent elements in an array, optimizing memory access.

Q & A

What are the four main addressing modes discussed in the video?

The four main addressing modes discussed are immediate, direct, indirect, and indexed addressing.

How is an instruction processed once it has been fetched?

Once an instruction has been fetched, it is decoded by splitting the binary into an opcode and an operand. The opcode specifies the operation, and the operand specifies the data or memory address.

What does immediate addressing mean in terms of the operand?

In immediate addressing, the operand is the actual value to be used, not an address. For example, 'add 10' directly means add 10, not the value stored at memory location 10.

How does direct addressing differ from immediate addressing?

In direct addressing, the operand refers to a memory location, and the value at that location is used. For example, 'add 10' means go to memory address 10 and add the value found there.

What is the purpose of indirect addressing?

Indirect addressing allows a memory location to contain the address of another memory location. This mode enables larger address ranges and allows for more flexible referencing of data.

Why is indirect addressing useful for referencing larger memory sizes?

Indirect addressing is useful because it allows smaller address fields in the instruction to reference larger memory spaces by storing addresses within memory locations themselves.

What role does an index register play in indexed addressing?

In indexed addressing, the index register holds a value that is added to a base address, allowing for efficient access to elements in contiguous memory locations, such as array elements.

How would an array of 100 items be processed using indexed addressing?

In indexed addressing, the index register is incremented to access each successive item in the array, rather than writing separate instructions for each item. The register starts at 0 and increases with each instruction.

What value would be loaded into the accumulator using immediate addressing for the instruction 'lda 8'?

In immediate addressing, 'lda 8' would load the value 8 directly into the accumulator.

What would happen if the instruction 'lda 8' were executed using indirect addressing?

Using indirect addressing for 'lda 8' would involve going to memory location 8, finding the address stored there (e.g., 5), then accessing the value at that address, which would be loaded into the accumulator.