My machine in a 5 bit calculator i made and it can add and subtract. Now this may not seem like much but it converts the numbers 1-9 to binary then adds A and B then Decodes to BCD and then is displayed. Thats how it goes and you save your inputs for A and B with cells and you can switch between Sub and Add. It includes Binary to BCD and an ALU.
1.(to set machine to zero)Set the machine to 0 by Flicking the lever named by the sign up for A and down for B. Flick to A(up) and make sure no numbers selected and click the button that says write. Do the same for B (A and B lever flicked to B(down))
2.Once machine(calculator) is set to zero start calculating! Flick the Lever thats is to set to A or B and set to A(up). Select the number you want to add then click write! Do the Same for B(Flick A or B lever to B(down)) Once done There is a lever to set subtraction and addition and the sign there will guide you. Up for Add down for Sub. And on the display it should show your result. To set back to zero follow step 1.
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I think this is a bit less for rank Regular, even challenge room says: 4bit calc with sub/add/mul. And yeah i somehow miss multiplication.
I also like if its more than 4 bit, but numbers from 0 to 15 or even negative numbers should be possible on a 4 bit calc.