Syntax:

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

Side Effects: None.

Affected By: None.

Exceptional Situations: None.

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))