For any operation affection the Zero flag, number having the highest bit set (bit 7) can be considered negative.
It depends what you do with the number. Doing 255 + 1 will equal doing -1 + 1. It's $FF in both cases and the result is 0. The carry flag is set here. Doing 1 minus 2 will equal 1 but the carry will be cleared. It depends what you do with the carry (BSC, BCC).
Short story, $FF can equal minus 1.
It depends what you do with the number. Doing 255 + 1 will equal doing -1 + 1. It's $FF in both cases and the result is 0. The carry flag is set here. Doing 1 minus 2 will equal 1 but the carry will be cleared. It depends what you do with the carry (BSC, BCC).
Short story, $FF can equal minus 1.
