14.3.1. Direct Addressing Mode

In direct addressing mode, the operand is specified explicitly in the instruction. For a direct instruction, you have to read or write something from a specified memory location, denoted as M.

Direct addressing mode is a way for the CPU to access memory directly using a given address. For example, if an instruction contains the operand address (let's say, M = 32), the CPU interprets that as needing to read data from memory location 32. This system allows the CPU to directly work with data without needing additional steps to find out where the data is stored.

Think of direct addressing like using an address to find a friend’s house. If you have the exact address (M = 32) written down, you can directly go to that location without needing to ask anyone for directions. In contrast, if you need to ask someone for help to find their house based on a vague description, that's akin to an indirect addressing mode.

The operation of direct addressing occurs in six stages, outlined as follows: (Steps 1 to 3) Consist of instruction fetch; (Step 4) Take value M from instruction register to memory address register; (Steps 5 and 6) Read from the memory location specified by M, loading the value into the target register.

1. The first three steps fetch the instruction from memory, which is standard regardless of the addressing mode. In these steps, the program counter (PC) is used to reference the memory location of the instruction. 2. In step 4, the instruction specifies a memory location M, which is then loaded into the memory address register (MAR) for reading. 3. Steps 5 and 6 involve the memory accessing that location, retrieving the value stored there (the operand), and transferring that value into the designated register for further processing.

Imagine you go to the library (computer system), look at the catalog (instruction fetch), find the exact row of books (step 4), and then directly take the book (value) off the shelf (steps 5 and 6) without needing to check any other sections. That is how direct addressing works; it eliminates confusion and gets to the point quickly.

In contrast, other instruction types such as indirect and immediate addressing require different mechanisms. Indirect addressing involves accessing another memory location to find the actual operand, while immediate addressing allows the operand to be embedded directly within the instruction itself.

Indirect addressing works by first reading the address of where the operand is located from an instruction, which points to a second address where the actual data is stored. This means an additional step is involved, as an extra memory access is needed to retrieve the operand. In immediate addressing, the operand is part of the instruction itself (like a small note), allowing for immediate use without needing to fetch it from memory.

Think of indirect addressing like getting a letter that tells you where to pick up a package (first memory direct access to find the second address). You have to first go to the address on the letter and then look for another note that tells you where the actual package is located. Conversely, immediate addressing is like having a package delivered to you at home—everything you need is right there without any extra steps.

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