Why 255 is a hard limit in Windows

You may have noticed that 255 often comes up as a hard limit in computing again and again.

A short text field in a MS Access database cannot be more than 255 characters.

It is not possible to search for more than 255 characters in the MS Word find tool.

And of course as discussed in the Tip of the Night for December 22, 2019, the number of characters in a single file path that can be accessed by Windows Explorer cannot exceed 255.

Why is 255 the numerical limit? It has to do with 8-bit computing. Compuer systems use binary code - a series of 0s and 1s to represent values. So for example, a lower case 'x' in binary code will be represented as 01111000. A combination of zeros and ones is used to represent letters and numbers. The earliest computers had 8 bits to a byte - so the most basic data blocks could have no more than 255 characters - two to the eight power, 2^8 equals 256. Modern computers can use 64-bit data blocks, which can represent a far greater number of values (2^64 equals 18446744073709552000). However, software devs often design programs so that they don't take up more storage space than necessary - and the smallest possible storage spaces are 8-bit blocks. The limit of 255 is imposed, so larger 32-bit or 64-bit data blocks are not taken up.