= Instantaneous description of a Turing machine
{title2=$C$}
= Turing machine configuration
{c}
{synonym}
An instantaneous description records the current control state, head position, and tape contents. For a tape blank outside a finite window, a word $h\,u\,q\,v\,h$ records the symbols $u$ to the left of the head and $v$ from the scanned cell rightward, with unrecorded cells blank. If $v$ is empty, the head scans an implicit blank. Extra outer blanks can represent the same infinite tape. The <instruction of a Turing machine> determines its successor description.
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