Two’s Complement Format for Signed Integers

Virtually all personal computers in use today (including Macs) use two’s complement format for storing signed integers. Two’s complement is a technique for representing negative numbers along with positive numbers. Although the leftmost bit always indicates the sign of the quantity, it is not precisely the same as a sign bit.

With signed formats (for example, int as opposed to unsigned int), only the bottom half of the range is used to represent positive values, along with zero. The top half of the range represents negative values. Consequently, a pattern with leftmost bit set to 1 always indicates a negative value.