mask-field bytespec integer → masked-integer

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

bytespec — a byte specifier.

integer — an integer.

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

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.

(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

byte, ldb

(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))