Overflow and Carry flags

Question

是否可以将两个带符号的8位数字相加并设置进位和溢出位？

Answer 1

Per your comments, your question seems to be "is it possible to have both carry and overflow set for a two's complement add involving signed number?" It is. The typical implementation is to take the exclusive-OR of the carry-in for the last adder with the carry-out at the end of the chain -- hence, an overflowing addition of negative numbers will cause the carry-out bit to be set and the overflow bit to be set.

Here's an example, add -1 to -128:

Carry 10000 0000 
       1000 0000  (-128)
       1111 1111  (-1)
       ---------
       0111 1111 (oops, this is 127!)

Carry will be set, since the last add resulted in a carry -- and overflow will be set based on the rule above (also, note that -128 added to -1 is obviously not 127)

Answer 2

您无法访问C中的标志，即使您可以让编译器生成设置它们的代码，您也无法使用它们。

Answer 3

You can write your own add routine in C that will return carry and overflow flags for signed 8-bit operands. If you're referring to the hardware carry and overflow bits inside the processor, no, that cannot be done portably in C.