SUB A, r8/d8 command to set flag C

[hcoderLee]

According to the GBCPUman：C - Set if no borrow. In my opinion, the code should be：

self.reg.set_flag(Flag::C, a >= n);

when the value of register A is greater then the number n, a-n is not borrowed. But in the project, the code is:

self.reg.set_flag(C, u16::from(a) < u16::from(n));

Which I couldn't understand.

[mohanson]

The substract in i8080/z80 is 'unintuitive' behaviour, See https://retrocomputing.stackexchange.com/questions/12558/the-behavior-of-the-auxiliary-carry-flag-in-subtraction-on-intel-8080