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Accessor

mask-field

mask-field bytespec integer ⇒ masked-integer

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

Arguments and Values

bytespeca byte specifier.
integeran integer.
masked-integer, new-masked-integera 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; byte-size; byte-position, 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))

Backlinks

Byte Operations on Integers, deposit-field, Function Call Forms as Places, Numbers Dictionary, Symbols in the COMMON-LISP Package