CPU architecture for a simple calculator:

Instruction set:

1.     NOP(0000) – no operation.

2.     AND(0001) – checks if the accumulator is the same as data in memory at the current address, then sets the accumulator to 1 if it is.

3.     OR(0010) – if either the accumulator or data in memory at the current address are not 0, then set the accumulator to 1.

4.     ADD(0011) – adds data from memory at the current address to the accumulator.

5.     SUB(0100) – subtracts the accumulator by data in memory at the current address.

6.     LDA(0101) – loads data from memory at the current address into the accumulator.

7.     STA(0111) – transfers the data in the accumulator to the next memory address, setting the accumulator to 0.

8.     IN1(1000) – load data from the first input register into the accumulator.

9.     IN2(1001) – load data from the second input register into the accumulator.

10.  OUT1(1010) – transfers the current data in the accumulator to the first output register but does is not reset the accumulator.

11.  OUT2(1011) – transfers the current data in the accumulator to the second output register but does is not reset the accumulator.

12.  JMP(1100) – adds the data at the current memory address to the program counter.

13.  JZADD(1101) – will ADD the memory at the current address to the program counter if the accumulator is 0.

14.  JZSUB(1110) – will subtract the memory at the current address to the program counter if the accumulator is 0.

15.  HLT(1111) – Halt the execution of data.

Flags:

1.     C – The carry flag will be a 1-bit register that is part of the accumulator that will let the ALU know if it needs to consider a carry in a future cycle or when outputting to the output register; this flag will be put as the first digit of the output-2 registers if the number is within 4 bits but with a carry.

2.     B – The borrow flag will be a 1-bit register next to the accumulator 

The architecture of CPU:

·      8-bit memory address

·      4-bit data bus

·      4-bit instruction set

·      Times two 4-bit register inputs.

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·      Times two 4-bit register outputs.