ldb

ldb Accessor

Syntax:

ldb bytespec integer ! byte

(setf (ldb bytespec place**)** new-byte**)**

Pronunciation:

[ lidib ] or [ lidεb ] or [ el d—e b—e ]

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^(s1) through 2⁰ are the same as those in integer with weights 2^(p+s1) through 2^p, 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 e↵ect 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.”

Expanded Reference: ldb

tip

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(ldb )