Computer's Brain - Adders to RAM

It's been a very long time since I have visited this topic.

Adders :

They are the basic functionality of a computer to add 2 binary. There are two types of adders. Half Adders, Full Adders.

Half Adders : They are just XOR gate and AND gate combined. The AND gate is responsible for getting the Carry of Addition. Look at the examples bellow-
A B Output
0 0 0
1 0 1
0 1 1
1 1 10
Full Adder : On full adder we tend to take 3 inputs instead of 2, A, B and Cin which is carry in
Input Output
A B Cin Sum Carry
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
A Full Adder combines two Half Adders: the first takes inputs A and B, and the second takes the output from the first along with the carry-in (Cin) from the previous calculation. Finally, the carries from both Half Adders pass through an OR gate to produce the new carry and sum.

The 3 important flags that an ALU generates are

Bit Overflow : It is a condition when the output of the last logic full / half adder has a carryover. This is the same case what happens to pacman after level 255 in which the ALU overflows which cause a bunch of errors and glitches.
Zero : To check if the output of the ALU is zero the logical unit does a bunch of checks with OR gates to get one answer
If any of the input is 1 the output is going to be 0 else the output is going to be 1.
Negative : There is a special flag given by every ALU to check if the given output is negative or not.

AND OR Latch : It remembers 1 bit of information, there are two inputs going into an AND OR Latch. 1 is to SET and 2 is to Reset it. If the set and reset is both tuned off than the output of this latch is whatever the last value was put in.
Gated Latch : It has two inputs, 1 Data and 2 Write Enable. When the write enable wire is on the latch changes it's value .

A group of latches operating at a single time (8 latches mean 1 byte of memory) is called a register.

The number of bits a register can hold is called a width.

A multiplexer is a combinational circuit that has many data inputs and a single output, depending on control or select inputs.

RAM has bunch of latches that are arranged in a grid like structure which is inturn powered by a multiplexer.

There are a bunch of wires that connect it to the CPU.

Address Bus : Address Bus is Uni directional as RAM never sends the cup an address.
Control Bus : It either writes the data to the RAM or read data from RAM.
Data Bus : Data bus is bi directional, it is the wires from where data is transferred to and from the CPU. If the instruction is to read the data of the address 1001, the data is going to be sent from the data bus by the ram and vice versa.

Date: 2024-10-09