Offmask

Offmask is a term used in computing to describe a bitmask that is employed to unset or clear specific bits within a binary value. It functions as the complement of an on-mask (set-mask) and is typically formed by inverting the on-mask and applying it with a bitwise operation to a target value.

In common usage, an offmask is applied using a bitwise AND with the inverted on-mask: result = value AND (NOT onmask). This clears the bits indicated by the on-mask while leaving the remaining bits unchanged. For example, with an 8-bit value of 0b11010110 and an on-mask of 0b00001101, the offmask is 0b11110010, and the result of value AND offmask is 0b11010010. This technique is a standard pattern in low-level programming for flag clearing and state reset.

Relations and alternatives: the offmask concept is closely related to the idea of a clear mask or zero mask. In many programming languages, the same operation is implemented as value &= ~onmask, which is equivalent to applying an offmask. The terminology can vary by author or domain, but the underlying idea is the same: use a mask to turn off specified bits.

Applications: offmasks are common in device drivers, embedded systems, and performance-critical code where precise control of individual bits is required. They enable efficient clearing of status flags, control bits, or feature flags without altering other parts of a value.

Notes: the term offmask is not universally standardized; some sources prefer terms like clear mask. Regardless of terminology, the concept relies on bitwise inversion and selective bit clearing.