16.5.2. Program Counter and Instruction Fetching

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. So now, we are saying that already we have designed...

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In this part, the speaker explains that they have already created three instructions with corresponding codes 1, 2, and 5, which are usable for programming. More instructions can be introduced, and the next designed instruction is called SUB M, which is used for subtraction. It indicates that the accumulator will hold the result of subtracting contents from memory. This introduction sets the stage for designing a structured instruction format to standardize the commands used in programming.

Imagine a recipe that lists three main ingredients (like ‘1’ for sugar, ‘2’ for flour, and ‘5’ for butter). You realize you can add more ingredients such as salt (numbered ‘4’). This is similar to designing new instructions, where old ones are retained and new ones provide extra functionality.

The instruction format is the same whatever we are going to design for all the instruction it is going to follow this particular pattern. Now again I am going to give designing 4 more Instructions. This is a similar Instruction load...

(Refer Slide Time: 37:34)

The speaker discusses the standard instruction format that should be used for all commands, ensuring consistency in the coding system. They mention adding more instructions that involve loading and storing from registers instead of memory. Instructions follow binary encoding rules where specific bits are designated for operations, and the instructions range from direct memory access to register manipulation. It clarifies that the instructions not only work with memory but also interact with limited registers.

Think of a universal remote control that can be programmed to control various devices. Each device can use the same set of buttons (universal instructions) but may perform different functions (like changing channels on a TV vs. adjusting the volume on a sound system). The consistent instruction format ensures that no matter what device you’re controlling, the basic commands remain the same.

If I say that this is your 9000 means it is going to refer to this particular Register R0. The value of the Register R0 will be loaded to the Accumulator. So, similarly if my opcode is your 9001 it is going to say that take the value of the Register R1...

(Refer Slide Time: 39:00)

Here, the speaker explains how specific opcodes represent particular registers. For example, an instruction such as ‘9000’ loads the value from Register 0 into the accumulator, while ‘9001’ does the same for Register 1. This emphasizes how operations can be performed on values stored in registers, which are temporary storage locations within the CPU. By using different codes or opcodes, a program can easily manipulate data during execution.

Consider a small box for each family member where they keep their personal items (registers). When someone wants to retrieve their items (data), they simply call out their name (opcode) to open their box and take what they need (load into accumulator). This organization allows for efficient managing and accessing of individual items without mixing them up.

We are designing some more instructions over here. So, this is INR is basically an increment and DEC is your decrement... If I am going to say 6900. In that particular case what will happen 6 is my decrement.

(Refer Slide Time: 41:41)

This section introduces increment (INR) and decrement (DEC) operations, which can be performed on both registers and memory. For example, an instruction like ‘6900’ indicates to decrement the value at memory location 900. This highlights a straightforward functional concept in programming that helps to easily modify values in a program during execution. The idea behind not using the ALU for these operations is also discussed, favoring a specialized increment/decrement circuit instead.

Imagine you have a savings jar (register) where you can either add coins (increment) or take coins out (decrement). Instead of using a mathematical tool to calculate the amount each time, you simply drop a coin in or take one out. This simple action makes managing your savings straightforward and efficient, similar to how the CPU operates with these instructions.