14.3. Addressing Modes

As we have already discussed in the last unit that basically the first 3 steps that is step 1, step 2 and step 3, basically consist of instruction fetch.

This chunk introduces the concept of addressing modes as part of instruction execution in a computer. In computing, addressing modes determine how the operand of an instruction can be accessed. The first three steps are universal in fetching the instruction from memory into the CPU which involves reading the program counter, fetching data from memory and passing it into the instruction register.

Think of addressing modes as different ways to reach a destination using a map. You can use direct routes (like direct addressing), you may use hidden paths that lead to the final destination (like indirect addressing), or you might get the destination written down with you (immediate addressing). Each way has its own effect on how quickly and efficiently you can find and use the information.

Now, from 4 onwards depending on different addressing modes or the instruction types they will vary.

In this section, we discuss how the execution of instructions deviates based on the addressing mode used. For example, in direct addressing mode, the instruction directly specifies the memory address from where the value has to be fetched. The steps are numbered sequentially, allowing for a systematic approach to executing instructions. Each addressing mode (direct, indirect, immediate) introduces alterations to the normal procedure followed in fetching data from memory.

Imagine ordering a pizza from different locations. When you call a restaurant and provide the exact address (direct addressing), they know exactly where to deliver. If you give them the address of a nearby landmark instead (indirect addressing), they have to figure out the final destination based on that. In immediate addressing, you already have the pizza with you, so you can just eat it without ordering.

In step 4 from the instruction register, you will get the value of M.

In direct addressing mode, the operand is located at the memory address specified by the instruction. The value of the memory location M is loaded into the memory address register, at which point the CPU reads from this memory location in subsequent steps. This allows for the operand to be retrieved directly as needed.

Consider you have a book with a specific page number containing the information you need. Direct addressing is like a librarian fetching you the exact page as per your request without any detours.

But in case say for example, if it is an indirect instruction... which points to another memory location basically which has the operand.

In indirect addressing mode, the instruction provides a location in memory that contains the actual address of the operand. Therefore, two memory accesses are required: first to get the address and second to retrieve the actual operand. This adds complexity but allows for more dynamic addressing of operands.

Imagine you have a treasure map that points not to the treasure itself but to another map or key that leads you to the treasure. You must first find this secondary location before you can access what you actually need.

So, in this case from up to step 3, everything will be similar in step 4 just you transfer the value basically it will be opcode say R and we have 7 hex which is the immediate value.

Immediate addressing mode suggests that the operand is included directly in the instruction itself. This leads to a simpler and faster execution as no additional memory access is required after fetching the instruction. Instead, the CPU can directly use the value provided.

It's like having a full meal already packed in your bag instead of ordering food. You can begin eating (execution) as soon as you are hungry (instruction fetched), without waiting for any other process.

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