Question

When I do, for example, -1 % 3, I expect the result to be 2.

When I execute the following TASM instructions I get an unexpected result:

mov ax, -1
xor bh, bh
mov bl,  3
idiv bl ; ah = FF (I expected 02), al = 00

Answer 1

This web page states that:

the sign of the remainder is always the same as the sign of the
dividend.

Wikipedia has some information regarding pitfalls with a signed remainder.

A possible workaround could be to use div instead of idiv, and/or keeping track of signs manually.