An unsigned number is a plain binary count. Every bit has its usual positive weight, so the value can never be negative.
With n bits, the smallest pattern (all 0s) is 0 and the largest (all 1s) is 1 + 2 + 4 + … + 2ⁿ⁻¹ = 2ⁿ − 1:
- 4 bits: 0 to 15
- 8 bits: 0 to 255
- 16 bits: 0 to 65,535
Unsigned numbers are used wherever negative values make no sense: memory addresses, counters, pixel brightness, array sizes.
The same bit pattern means something else as a signed number. 1111 is 15 unsigned but −1 in twos complement. An adder doesn't care: it produces the same bits either way. What differs is how you check the result. For unsigned addition, a carry out of the MSB means unsigned overflow: the true sum needed one more bit. See signed vs unsigned.
1
128
1
64
1
32
1
16
1
8
1
4
1
2
1
1