14.5. Examples of 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. So, what happens in step 1? The program counter value is loaded into the bus. That bus basically or the program counter is read into memory address register. Memory is given in read mode and then actually in these 3 steps, ALU actually adds program counter value which is now in the bus with a constant value, which is the next memory location and then it is again waited to be loaded into the program counter in the next iteration.

In the first three steps of instruction execution, we focus on fetching the instruction. Step 1 involves loading the value of the program counter (PC) onto a bus which carries data. This value is then read into the memory address register (MAR), indicating which memory location is to be accessed. The memory is set to 'read' mode to allow fetching of instructions. During these steps, the Arithmetic Logic Unit (ALU) adds the PC value to a constant that indicates the next memory address to access. After the instruction is fetched, the PC is updated for the next instruction. Essentially, these steps ensure that the processor retrieves the correct instruction to execute next.

Imagine you're cooking a recipe. First, you check your recipe book (the program counter) to see what dish you must prepare next. You open the book to the right page (loading into the memory address register), and then you look for the next ingredient you need (the instruction), ensuring everything is ready for your next step in cooking.

For example, if the instruction is a direct instruction that is you have to read something from or write something from the memory location. Then in the fourth stage, what is going to happen basically, you are going to take from the instruction register the value of M which corresponds to the memory location for an instruction because it’s a direct instruction. So, the value of the operand will be present in memory location M which will be specified in the instruction itself.

In direct addressing mode, the instruction specifies the exact memory location (M) where the relevant operand is stored. In the fourth step of instruction execution, the processor retrieves this memory location value from the instruction register (IR) and loads it into the memory address register (MAR). The next step is to set the memory to read mode, which allows for retrieval of the operand stored at this specified memory address. After the reading process is completed, the operand is transferred to the appropriate registers for execution.

Consider having a written shopping list (the instruction) where it specifically states to get apples from aisle 5 in the supermarket (the memory location). You head directly to aisle 5 (the processor fetching the value from memory) to collect your apples (the operand) without having to search elsewhere.

But in case say for example, if it is an indirect instruction where the memory location M actually points to another memory location which has the operand. So, basically it’s an address to another location maybe say for example.

In indirect addressing mode, the instruction does not directly specify the operand; instead, it provides a memory location (M) that contains the address of the actual operand. This means that the first operation involves reading the address from M into the MAR, and then retrieving the operand stored at that secondary address. This process adds an extra step compared to direct addressing, where the operand is immediately accessible at the address specified in the instruction.

Think of it like getting directions to a friend's house (the memory location) via your navigation app. The directions may lead you to a meeting point (the first address) that gives you additional directions to your friend's actual house (the operand) - therefore, you need to take an extra step to reach your destination.

So, first instruction mode the simplest one is the immediate mode. So, we are just saying LOAD R, 32; that means, the value of 32 constant will be loaded to R.

In immediate addressing mode, the operand is directly specified in the instruction itself, making it the simplest addressing mode. For example, an instruction like 'LOAD R, 32' indicates that the constant value of 32 should be loaded into register R directly. There are fewer steps involved as there is no need to access memory to get the operand, just a straightforward transfer from the instruction register to the targeted register.

Imagine you are buying a ready-made meal (the instruction) that is specified on the packaging to heat for a few minutes (the operand itself). There’s no need to measure or cook anything; you simply place it in the microwave as stated – just like directly loading the value into the register.

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