4 Bit Adder/Subtractor Circuits

Hello Lach

A binary add i relaitve simle. 1+0=1 and 1+1=0 + carry = 10. You add bit by bit for the result.

A binary sub is an add with the minus signed value inverted +1. I.e. if you will sub 12 - 7 (= 5). Binary it is 1100 - 0111. Invert 0111 to 1000 and add 1, it will bee 1001. Add 1100 and 1001, result 10101. Drop the carry to 0101.

In hardware you put an exor-gate before the B-inputs on the adder circuit. This can be "programmed" as a non inverter for addition and an inverter for subtraction. Adding the LSB 1 can be done to the adding result.

Anders

Binary subtraction is just two's compliment addition. Take the number you want to subtract, invert it and add one to it - that's the two's compliment. Now add this value to the number you want to subtract from, and you're done.