complement

One's complement(反码)¶

https://en.wikipedia.org/wiki/Ones%27_complement

but in the computer ¬ is same as the not (RA). because the (RA) is stored two's complement in the register.

Two's complement（补码）¶

https://en.wikipedia.org/wiki/Two%27s_complement Conveniently, another way of finding the two's complement of a number is to take its ones' complement and add one: the sum of a number and its ones' complement is all '1' bits, or 2N − 1; and by definition, the sum of a number and its two's complement is 2N.

Two's complement = One's complement + 1
X的Two's complement + X的原码 = 2^N

Property¶

¬(RA) + 1 is the Two's complement, RA is store the two's complement. ¬(Y补码) + 1 = ([-Y]补码)

SUB X, Y

If if Y < 0, ¬([Y补码]) == ¬(RA) != [-Y反码], because -Y > 0 and [-Y原码] =[-Y反码] = [-Y补码]

subf RT, RA, RB
RT <-- ¬(RA) + (RB) + 1

NOR RA, RS, RB
RA <--  ¬ ((RA) | (RB))

not Rx,Ry  <==> nor Rx,Ry,Ry