And at one level beneath that at which there are only ADDERs, there are only GATES. There are OR gates, NOR gates, AND gates, NAND gates, and possibly others with more exotic names, but I don't believe that there are any ADD gates. An ADDER circuit functions by using only some combination of logical operations on each pair of bits to be added. The rightmost pair of bits in the two addends has two inputs, and all other bit locations have 3 input bits, the same 2 inputs that the rightmost pair has plus a carry input from the next pair to the right. These primitive logical operations are called BOOLEAN operations, none of which involves anything as "high-level" as a mathematical addition or subtraction.
Bill Fairchild Rocket Software -----Original Message----- From: IBM Mainframe Assembler List [mailto:[email protected]] On Behalf Of William H. Blair Sent: Thursday, September 09, 2010 11:37 AM To: [email protected] Subject: Re: Instruction Set Architecture ... But at the lowest hardware level, one could assert (as several already have) that ONLY _addition_ is being done. That's how the logic circuits that "do" (all of) these things work, period. They _are_ ADDERs. There are _no_ SUBTRACTERs. So how does the machine do subtraction? By complementing one operand and then ADDING the two together.
