On Apr 29, 2022, at 12:08:22, Bob Raicer wrote: > > For the sake of clarity I am going to paraphrase some of the text > found in the original S/360 Principles of Operation and the > z/Architecture Principles of Operation. > > ... The two's complement of > a number is obtained by forming the one's complement of the number > (i.e., inverting all of the bits), adding a value of one in the > rightmost bit position, allowing a carry into the sign position, and > ignoring any carry out of the sign position. > ... > The two's complement of the maximum negative number cannot be > represented in the same number of bits. ... > Those two paragraphs contradict each other. Surely it is possible to invert all the bits of the maximum negative number, add a value of one in the rightmost bit position, and *ignore*any*carry* out of the sign position; the definition of "two's complement" above. That's exactly what the LCR instruction does.
I believe what it intends to say is that the two's complement of the maximum negative number is not its algebraic negation. -- gil
