14.3.2. Indirect Addressing Mode

In indirect mode, the memory location specified in the instruction does not contain the operand itself but points to another memory location where the operand is stored.

In indirect addressing mode, the instruction specifies a memory address that points to another address, which contains the actual data (the operand). For example, if an instruction indicates that memory address M contains the value 30, this means the real data is stored at memory location 30. Therefore, the execution must first retrieve the address from location M and then access the data stored at that address.

Think of it like having a library catalog number. The catalog number (indirect address) points to a specific shelf (the memory address), where you can find the actual book (the operand). You first look up the catalog number to know where the book is located, and then you go to that shelf to retrieve the book.

The execution process involves several steps: step 4, load the memory address from the instruction register; step 5, wait for memory readiness; step 6, perform another memory access to get the operand.

1. Step 4: The value of M (the indirect address) is loaded from the instruction register to the memory address register. This tells the system which address to access next.
2. Step 5: The system waits for the memory to signal that it's ready with the data. The memory retrieves the data stored at the address specified by M (in our earlier example, this means looking for the address 30).
3. Step 6: Once the value is ready, the data stored in the memory location pointed to by M (which could be any address like 30 or 72) is accessed and loaded into the memory data register, and subsequently transferred to the desired register for further computations.

Imagine you have a friend who is holding the key to a storage box that contains your favorite toy. Here, your friend represents the memory address (M) that points to the box. First, you ask your friend for the key (load M into the memory address register), wait for them to hand it over (waiting for memory readiness), and then use the key to open the box and get your toy (accessing the final operand).

Indirect addressing requires more steps compared to direct and immediate addressing modes. In direct mode, the operand is accessed directly from the specified address, while in immediate mode, the operand is part of the instruction itself.

Both direct addressing and immediate addressing modes can be executed in fewer steps:

• Direct addressing: In this mode, if the address M points directly to data, the process typically takes six steps, as the operand can be fetched directly from the memory location mentioned in the instruction.
• Immediate addressing: This mode requires just four steps, as the operand is included in the instruction itself, allowing immediate access without fetching from any memory location.

Think of direct addressing as having a friend hand over a book (the operand) directly from a specified shelf in a library (the address). Immediate addressing is like writing down the title of a book (the operand) directly on a note, so you just read it without going to the library. Indirect addressing, on the other hand, involves first asking where the book is located (the address) before you can get it.

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