16.1. Instruction Design

These are the 3 Instructions we have designed and the code assigned is your 1, 2 and 5. Other codes are now still available to me. So, now we can design some more Instructions. Now, we are designing one more Instruction called SUB M. This means subtraction, basically the Accumulator is equal to Accumulator minus contents of the Memory.

In this chunk, the speaker refers to three existing instruction codes: 1 for load, 2 for store, and 5 for add. New instruction design is conceptually discussed, specifically focusing on a proposed subtraction instruction named SUB M. This instruction will decrease the value in the Accumulator by the value stored in a specified Memory location, illustrating the basic operation of subtraction within the instruction set of a hypothetical processor.

Think of the Accumulator as a bank account balance. If you have $100 in your account and you subtract$20, your new balance is $80. Similarly, the SUB M instruction takes the current value in the Accumulator (like your balance) and subtracts a value from Memory (like withdrawing money from your account).

The Instruction format is the same whatever we design for all the Instructions. I am going to use one more code this is code 4, which is your subtraction code. We can add more and more Instructions now, referring to Registers instead of Memories.

This part emphasizes the consistency in instruction formatting across all designed instructions, meaning they will follow the same structural rules. Code 4 is designated for the new subtraction instruction (SUB M). The speaker indicates future developments, implying the potential to designate additional codes for future instructions that would also work with Register references, hinting at improved efficiency over memory references.

Consider a standardized form used for filling out a job application; everyone uses the same structure to maintain clarity. Here, the instruction format acts like that job application form. No matter the content, the structure stays the same. The introduction of code 4 for subtraction is like adding a new checklist item to that application; it allows for additional tasks to be documented without altering the fundamental layout.

In this case, the first opcodes return memory references and the latter refer to registers. For instance, if I say the opcode is 9000, it refers to Register R0. If I say the opcode is 9001, it will refer to Register R1.

The explanation distinguishes between memory and register references within instruction coding. Memory and registers serve different purposes in computation, and the speaker outlines how specific opcode values signify whether the operation will affect a memory location or a register. For example, opcode 9000 refers to Register R0, demonstrating how opcode assignment can communicate detailed operational intentions within the computer architecture.

Imagine you’re sending a package (instruction) to a friend (register) or a post office (memory). The package has a specific address that tells you where to send it. Here, the opcode is like the address; it tells the processor whether to directly alter a register (the friend who can instantly receive the package) or access a memory location (the post office holding the package until someone retrieves it).

The instructions for increment (INR) and decrement (DEC) show how we modify values in registers or memory. If I execute a decrement operation on memory at location 900, and it has a value of 15, after execution, it will become 14.

This chunk introduces two specific instruction types: increment and decrement. These instructions add or subtract 1 from the specified memory/register value. The example demonstrates how executing a decrement instruction changes the value at a designated memory location, thereby altering the data state within the computer system. It's noteworthy how this manipulation can occur in a way that doesn't require using the Accumulator for every arithmetic change.

Think of a score tracker in a game. If your score is 15 and you lose a point (decrement), the score changes to 14. The decrement instruction works the same way, simply adjusting the stored value directly—like a game score being modified as you earn or lose points.