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<!DOCTYPE HTML>
<HTML LANG="en-us"
><HEAD
><TITLE
>ldb | Common Lisp Nova Spec</TITLE
><META CHARSET="US-ASCII"
><LINK REL="canonical" HREF="f_ldb.html"
><LINK REL="next" HREF="f_ldb-test.html" TYPE="text/html" TITLE="ldb-test"
><LINK REL="prev" HREF="f_dpb.html" TYPE="text/html" TITLE="dpb"
><LINK REL="up" HREF="12_2_Numbers_Dictionary.html" TYPE="text/html" TITLE="12.2 Numbers Dictionary"
><LINK REL="start" HREF="index.html" TYPE="text/html" TITLE="Common Lisp Nova Spec"
><META NAME="VIEWPORT" CONTENT="width=device-width, initial-scale=1.0"
><LINK REL="STYLESHEET" HREF="dpans.css%3F3909942064.css"
><SCRIPT SRC="dpans.js%3F3909942064"
></SCRIPT
><SCRIPT SRC="apropos.js%3F3909942064"
></SCRIPT
></HEAD
><BODY
><DIV
><DIV CLASS="topnav"
><DIV CLASS="breadcrumb"
><SPAN CLASS="breadcrumb-item"
><A HREF="index.html"
>Common Lisp Nova Spec</A
></SPAN
> <SPAN CLASS="breadcrumb-item"
>&#8594; <A HREF="12_Numbers.html"
>12. Numbers</A
></SPAN
> <SPAN CLASS="breadcrumb-item"
>&#8594; <A HREF="12_2_Numbers_Dictionary.html"
>12.2 Numbers Dictionary</A
></SPAN
> <SPAN CLASS="breadcrumb-item"
>&#8594; <A HREF="f_ldb.html"
>ldb</A
></SPAN
></DIV
><DIV CLASS="apropos"
><DIV CLASS="apropos-io"
><A HREF="f_dpb.html" CLASS="prev"
>&#8592;</A
><SPAN ID="apropos-label"
>Apropos </SPAN
><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
><A HREF="f_ldb-test.html" CLASS="next"
>&#8594;</A
></DIV
><DIV ID="apropos-res"
></DIV
></DIV
></DIV
><DIV CLASS="matter"
><DIV CLASS="com"
><DIV CLASS="begincom"
><HR
><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
><TR
><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="ldb"
></SPAN
><B
>ldb</B
></TD
><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
><I
>Accessor</I
></TD
></TR
></TABLE
><HR
></DIV
><UL CLASS="subtoc"
></UL
><DL
><DT
><B
>Syntax</B
></DT
><DD
><P CLASS="j"
><B
>ldb</B
> <SPAN CLASS="cmssi"
>bytespec</SPAN
> <SPAN CLASS="cmssi"
>integer</SPAN
> <SPAN CLASS="arrow"
>&#8594;</SPAN
> <SPAN CLASS="cmssi"
>byte</SPAN
></P
><P CLASS="j"
><B
>(setf (ldb</B
> <SPAN CLASS="cmssi"
>bytespec</SPAN
> <SPAN CLASS="cmssi"
>place</SPAN
><B
>)</B
> <SPAN CLASS="cmssi"
>new-byte</SPAN
><B
>)</B
></P
></DD
><DT
><B
>Pronunciation</B
></DT
><DD
><P CLASS="j"
>[ <B CLASS="b"
>lidib</B
>&#8201;] or [ <B CLASS="b"
>lid<SPAN CLASS="cmr"
>&#1013;</SPAN
>b</B
>&#8201;] or [ <B CLASS="b"
>el d&#275; b&#275;</B
>&#8201;]</P
></DD
><DT
><B
>Arguments and Values</B
></DT
><DD
><P CLASS="j"
><VAR CLASS="param"
>bytespec</VAR
> &#8212; a <A HREF="26_1_Glossary.html#byte_specifier"
><EM CLASS="term"
>byte specifier</EM
></A
>. </P
><P CLASS="j"
><VAR CLASS="param"
>integer</VAR
> &#8212; an <A HREF="26_1_Glossary.html#integer"
><EM CLASS="term"
>integer</EM
></A
>. </P
><P CLASS="j"
><VAR CLASS="param"
>byte</VAR
>, <VAR CLASS="param"
>new-byte</VAR
> &#8212; a non-negative <A HREF="26_1_Glossary.html#integer"
><EM CLASS="term"
>integer</EM
></A
>.</P
></DD
><DT
><B
>Description</B
></DT
><DD
><P CLASS="j"
><A HREF="f_ldb.html" CLASS="funref"
><B
>ldb</B
></A
> extracts and returns the <A HREF="26_1_Glossary.html#byte"
><EM CLASS="term"
>byte</EM
></A
> of <VAR CLASS="param"
>integer</VAR
> specified by <VAR CLASS="param"
>bytespec</VAR
>. </P
><P CLASS="j"
><A HREF="f_ldb.html" CLASS="funref"
><B
>ldb</B
></A
> returns an <A HREF="26_1_Glossary.html#integer"
><EM CLASS="term"
>integer</EM
></A
> in which the bits with weights 2<SUP CLASS="sup"
><SPAN CLASS="cmr"
>(</SPAN
><I CLASS="i"
><I
>s</I
></I
><SPAN CLASS="cmr"
>-1)</SPAN
></SUP
> through 2<SUP CLASS="sup"
><SPAN CLASS="cmr"
>0</SPAN
></SUP
> are the same as those in <VAR CLASS="param"
>integer</VAR
> with weights 2<SUP CLASS="sup"
><SPAN CLASS="cmr"
>(</SPAN
><I CLASS="i"
><I
>p</I
></I
><SPAN CLASS="cmr"
>+</SPAN
><I CLASS="i"
><I
>s</I
></I
><SPAN CLASS="cmr"
>-1)</SPAN
></SUP
> through 2<SUP CLASS="sup"
><I CLASS="i"
><I
>p</I
></I
></SUP
>, and all other bits zero; <I CLASS="i"
><I
>s</I
></I
> is <CODE CLASS="f"
>(byte-size <VAR CLASS="param"
>bytespec</VAR
>)</CODE
> and <I CLASS="i"
><I
>p</I
></I
> is <CODE CLASS="f"
>(byte-position <VAR CLASS="param"
>bytespec</VAR
>)</CODE
>. </P
><P CLASS="j"
><A HREF="f_setf.html" CLASS="macref"
><B
>setf</B
></A
> may be used with <A HREF="f_ldb.html" CLASS="funref"
><B
>ldb</B
></A
> to modify a byte within the <VAR CLASS="param"
>integer</VAR
> that is stored in a given <VAR CLASS="param"
>place</VAR
>. The order of evaluation, when an <A HREF="f_ldb.html" CLASS="funref"
><B
>ldb</B
></A
> form is supplied to <A HREF="f_setf.html" CLASS="macref"
><B
>setf</B
></A
>, is exactly left-to-right. <SPAN CLASS="idx" DATA-KIND="idxtext" DATA-TERM="order of evaluation"
></SPAN
><SPAN CLASS="idx" DATA-KIND="idxtext" DATA-TERM="evaluation order"
></SPAN
>The effect is to perform a <A HREF="f_dpb.html" CLASS="funref"
><B
>dpb</B
></A
> operation and then store the result back into the <VAR CLASS="param"
>place</VAR
>.</P
></DD
><DT
><B
>Examples</B
></DT
><DD
><PRE CLASS="screen"
>(ldb (byte 2 1) 10) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> 1
(setq a (list 8)) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> (8)
(setf (ldb (byte 2 1) (car a)) 1) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> 1
a <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> (10)</PRE
></DD
><DT
><B
>See Also</B
></DT
><DD
><P CLASS="j"
><A HREF="f_byte.html" CLASS="funref"
><B
>byte</B
></A
>, <A HREF="f_byte.html" CLASS="funref"
><B
>byte-position</B
></A
>, <A HREF="f_byte.html" CLASS="funref"
><B
>byte-size</B
></A
>, <A HREF="f_dpb.html" CLASS="funref"
><B
>dpb</B
></A
></P
></DD
><DT
><B
>Notes</B
></DT
><DD
><PRE CLASS="screen"
>(logbitp <VAR CLASS="param"
>j</VAR
> (ldb (byte <VAR CLASS="param"
>s</VAR
> <VAR CLASS="param"
>p</VAR
>) <VAR CLASS="param"
>n</VAR
>))
<SPAN CLASS="cmsy"
><SPAN STYLE="font-size:16px;vertical-align:-2px"
>&#8801;</SPAN
></SPAN
> (and (&lt; <VAR CLASS="param"
>j</VAR
> <VAR CLASS="param"
>s</VAR
>) (logbitp (+ <VAR CLASS="param"
>j</VAR
> <VAR CLASS="param"
>p</VAR
>) <VAR CLASS="param"
>n</VAR
>))</PRE
><P CLASS="j"
>In general,</P
><PRE CLASS="screen"
>(ldb (byte 0 <VAR CLASS="param"
>x</VAR
>) <VAR CLASS="param"
>y</VAR
>) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> 0</PRE
><P CLASS="j"
>for all valid values of <VAR CLASS="param"
>x</VAR
> and <VAR CLASS="param"
>y</VAR
>. </P
><P CLASS="j"
>Historically, the name &#8220;ldb&#8221; comes from a DEC PDP-10 assembly language instruction meaning &#8220;load byte.&#8221;</P
></DD
></DL
></DIV
></DIV
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><DIV CLASS="btmnav"
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>&#8592;</A
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>&#8594;</A
></DIV
><DIV CLASS="trail"
>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
></DIV
></DIV
><SCRIPT
>domReady();</SCRIPT
></BODY
></HTML
>
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