A positive integer has an infinite number of leading zeroes, and a negative integer in two's complement has an infinite number of leading ones. On the machine

A positive integer has an infinite number of leading zeroes, and a negative integer in two's complement has an infinite number of leading ones. On the machine level, you have registers of limited width, and there are usually instructions that sign-extend a shorter integer by setting all the high bits to '1' if it is negative. By convention, the highest bit indicates the sign, but mathematically it's "arithmetic modulo 2^n", and any value can be interpreted as both positive and negative. For example, a 4 bit register containing '0111' would normally be interpreted as decimal 7, but could also represent -9. Lisp - like many other interpreted languages - uses arbitrary precision integers, so while not infinite they can be very large.