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| ANSI Common Lisp 12 Numbers 12.2 Dictionary of Numbers
12.2.69 ldb | Accessor
  Syntax: ldb bytespec integer byte (setf (ldb bytespec place) new-byte) Pronunciation: [ 'lidib ] or [ 'lidEb ] or [ 'el'dê'bê ] Arguments and Values: bytespec - a byte specifier. integer - an integer. byte, new-byte - a non-negative integer. Description: ldb extracts and returns the byte of integer specified by bytespec. ldb returns an integer in which the bits with weights 2(s-1) through 20 are the same as those in integer with weights 2(p+s-1) through 2p, and all other bits zero; s is `(byte-size bytespec)` and p is `(byte-position bytespec)`. setf may be used with ldb to modify a byte within the integer that is stored in a given place. The order of evaluation, when an ldb form is supplied to setf, is exactly left-to-right. The effect is to perform a dpb operation and then store the result back into the place. Examples:
       (ldb (byte 2 1) 10)  1
       (setq a (list 8))  (8)
       (setf (ldb (byte 2 1) (car a)) 1)  1
       a  (10)
  See Also: byte, byte-position, byte-size, dpb Notes:
       (logbitp j (ldb (byte s p) n))
          ==(and (< j s) (logbitp (+ j p) n))
  In general,
       (ldb (byte 0 x) y)  0
for all valid values of x and y. Historically, the name "ldb" comes from a DEC PDP-10 assembly language instruction meaning "load byte." Allegro CL Implementation Details: None.

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