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12.2.71 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::

byte , ldb

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))