mask-field [Accessor]

mask-field bytespec integer => masked-integer

(setf ( mask-field bytespec place) new-masked-integer)

Arguments and Values::

bytespec---a byte specifier.

integer---an integer.

masked-integer, new-masked-integer---a non-negative integer.

Description::

mask-field performs a "mask" operation on integer. It returns an integer that has the same bits as integer in the byte specified by bytespec, but that has zero-bits everywhere else.

setf may be used with mask-field to modify a byte within the integer that is stored in a given place. The effect is to perform a deposit-field operation and then store the result back into the place.

Examples::

 (mask-field (byte 1 5) -1) =>  32
 (setq a 15) =>  15
 (mask-field (byte 2 0) a) =>  3
 a =>  15
 (setf (mask-field (byte 2 0) a) 1) =>  1
 a =>  13

See Also::

@xref{byte; byte-size; byte-position} , section ldb [Accessor]

Notes::

 (ldb bs (mask-field bs n)) == (ldb bs n)
 (logbitp j (mask-field (byte s p) n))
   == (and (>= j p) (< j s) (logbitp j n))
 (mask-field bs n) == (logand n (dpb -1 bs 0))

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