16.6.1. Achievements of Course Objectives

We have designed 3 Instructions: the codes assigned are 1, 2, and 5. The additional code 4 is for a new Instruction called SUB M, which means subtraction. This Instruction means that the Accumulator is equal to the Accumulator minus the contents of the Memory.

In this part, we discuss the initial design of the first three Instructions (codes 1, 2, and 5) and the introduction of a fourth Instruction, SUB M. The purpose of the SUB M Instruction is to facilitate subtraction operations within the processor by assigning a specific code (4) so that the Accumulator can perform a calculation by subtracting the contents of a Memory address.

Think of your Accumulator like a savings account that starts with an initial balance. The SUB M Instruction is like withdrawing some money (subtracting) from that account. If you take out $20 from your initial balance of$100, your new balance will reflect that subtraction.

We follow a consistent Instruction format for all Instructions. This includes defining whether the reference is to Memory or Registers. For example, when the opcode indicates a Memory reference, it will start with 0, while a Register reference will start with 1.

Here, we explain how the Instruction format is structured uniformly across all operations. The distinction between Memory (coded with a leading zero) and Register (coded with a leading one) references allows the processor to understand where to fetch or store data. This structured coding is essential for the proper functioning of the Instructions.

Consider this like having a library catalog where each book has a unique code to specify whether it’s a hardcover (Memory) or paperback (Register). This helps the librarian know exactly where to find or store the books efficiently.

The number of Registers is limited (e.g., R0 to R7), so the Instructions regarding Registers require fewer bits compared to Memory Instructions. For example, an opcode might refer to a Register value and load that value into the Accumulator.

In this chunk, we outline the operation of Registers. Since the processor is designed with a limited number of Registers, it allows for straightforward access and manipulation of their values compared to accessing a broader range of Memory. Each Register's value can be fetched and loaded into the Accumulator swiftly using specific opcodes.

Imagine if your kitchen had limited storage drawers (Registers) compared to your entire pantry (Memory). It’s much easier to grab ingredients from the drawers because you know exactly where they are, rather than having to search through the whole pantry.

We introduce INR for increment and DEC for decrement. These operations allow for increasing or decreasing the value in Memory or Registers without utilizing the ALU for such simple changes.

The focus here is on the instructions used to increment or decrement values in Memory or Registers. Using simple codes like INR and DEC helps in managing these operations efficiently. Instead of more complex computations using the ALU, these codes signal straightforward changes to values.

Think of this like household chores. Instead of putting a whole pot of water to boil (ALU operation), you just turn the faucet to add a little water (increment) or let some out (decrement). This keeps things simple and efficient without needing to engage more complicated processes.