Examples of Addressing Modes

Today, we’re going to learn about how registers work in addressing modes. Let’s start with the values of R1 and R5. Can anyone tell me what initial values we might assign to these registers?

I think R1 could start at 1, and R5 might be 0?

That's correct! Now, when we need to calculate an effective memory address, what do we do?

We add the content of R1 to a displacement value, like 1.

Exactly. So, if R1 is 1 and we add 1 to it, we get 2. What does this mean for our memory access?

It means we’re looking at memory location 2.

Correct! Let’s remember this with the phrase 'Registers guide addresses.' Always think of how the register values lead us to the data we need!

Now, let’s discuss indirect addressing. Who can explain how this differs from direct addressing?

In indirect addressing, we use a memory location to determine where the actual data is stored, while in direct addressing, we go straight to the data location.

Right! Can you provide an example of using indirect addressing with R1?

If R1 holds the address of a memory location, say 8000, then we first check what’s in 8000 to get our required data.

Exactly! And what about displacement addressing?

In displacement, we add a constant value to the address in our index register to find the data.

Very good! A mnemonic to help remember this is 'Direct goes, indirect knows!' It reflects how direct goes right to data, but indirect knows to point elsewhere.

Let’s tackle the complexities of multiple word instructions. Why are they necessary in certain scenarios?

Because some opcodes and operands require more bits than available in a single memory word!

Exactly! For instance, if our instruction requires both opcode and a large address, how would we represent that in an 8-bit architecture?

We’d need to split it into parts, using the first byte for the opcode and the rest for operands spread over two or more memory locations.

Correct! This can lead to non-linear program counter movement. Here's a memory aid: 'Split to fit, don’t forget!' Always ensure you can fit your instruction within the memory limits!