Registers and ALU

Registers

Registers are a type of computer memory used to quickly accept, store, and transfer data and instructions that are being used immediately by the CPU. The registers used by the CPU are often termed as Processor registers. 

A processor register may hold an instruction, a storage address, or any data (such as bit sequence or individual characters).

The computer needs processor registers for manipulating data and a register for holding a memory address. The register holding the memory location is used to calculate the address of the next instruction after the execution of the current instruction is completed.

I created 3 different registers:

1. 2 registers for normal data storage; named A and B.

2. 1 register for instructions; instruction register.

In the image above there is a clock module, 1 pseudo bus (yellow LEDs), all 3 registers. The one with red led is instruction register. its work in to provide instruction to the CPU module, which decodes it and perform series of steps that is required to execute the instruction. The other 2 registers in my case are mostly use to temporary store data or to be used with the ALU which does addition and 2s complement subtraction.

2s complement is achieved by inverting all the inputs and adding 1 to the LSB. this was done by using an inverter ICs and to add 1 in LSB a simple XOR gate was used as half adder.

Registers with ALU

The ALU needs no extra clock cycle to add or subtract the data in A & B register, it directly does that even if you not need it.

At this stage my basic computer was ready now I needed a place to transfer data to these registers automatically by not hard coding it every single time so its time to add RAM.

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