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80 lines
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Accessor LDB</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="f_dpb.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_number.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_ldb_te.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="ldb"><I>Accessor</I> <B>LDB</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>ldb</B> <I>bytespec integer</I> => <I>byte</I><P>
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<P>
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<TT>(setf (</TT><B>ldb</B> <I>bytespec place</I><TT>) </TT><I>new-byte</I><TT>)</TT><P>
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<P>
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<P><B>Pronunciation:</B><P>
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<P>
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['lidib] or ['liduhb] or ['el'dee'bee] <P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>bytespec</I>---a <A REL=DEFINITION HREF="26_glo_b.htm#byte_specifier"><I>byte specifier</I></A>. <P>
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<I>integer</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>. <P>
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<I>byte</I>, <I>new-byte</I>---a non-negative <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#ldb"><B>ldb</B></A> extracts and returns the <A REL=DEFINITION HREF="26_glo_b.htm#byte"><I>byte</I></A> of <I>integer</I> specified by <I>bytespec</I>. <P>
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<A REL=DEFINITION HREF="#ldb"><B>ldb</B></A> returns an <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A> in which the bits with weights 2^(<I>s</I>-1) through 2^0 are the same as those in <I>integer</I> with weights 2^(<I>p</I>+<I>s</I>-1) through 2^<I>p</I>, and all other bits zero; <I>s</I> is <TT>(byte-size </TT><I>bytespec</I><TT>)</TT> and <I>p</I> is <TT>(byte-position </TT><I>bytespec</I><TT>)</TT>. <P>
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<A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> may be used with <A REL=DEFINITION HREF="#ldb"><B>ldb</B></A> to modify a byte within the <I>integer</I> that is stored in a given <I>place</I>. The order of evaluation, when an <A REL=DEFINITION HREF="#ldb"><B>ldb</B></A> form is supplied to <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A>, is exactly left-to-right. The effect is to perform a <A REL=DEFINITION HREF="f_dpb.htm#dpb"><B>dpb</B></A> operation and then store the result back into the <I>place</I>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(ldb (byte 2 1) 10) => 1
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(setq a (list 8)) => (8)
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(setf (ldb (byte 2 1) (car a)) 1) => 1
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a => (10)
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</PRE>
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</TT> <P>
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<P><B>Side Effects:</B> None.
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<P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_by_by.htm#byte"><B>byte</B></A>, <A REL=DEFINITION HREF="f_by_by.htm#byte-position"><B>byte-position</B></A>, <A REL=DEFINITION HREF="f_by_by.htm#byte-size"><B>byte-size</B></A>, <A REL=DEFINITION HREF="f_dpb.htm#dpb"><B>dpb</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<PRE>
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(logbitp j (ldb (byte s p) n))
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== (and (< j s) (logbitp (+ j p) n))
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</PRE>
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</TT> <P>
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In general, <P>
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<PRE>
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(ldb (byte 0 x) y) => 0
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</PRE>
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</TT> <P>
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for all valid values of <I>x</I> and <I>y</I>. <P>
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Historically, the name ``ldb'' comes from a DEC PDP-10 assembly language instruction meaning ``load byte.'' <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss279.htm">PUSH-EVALUATION-ORDER:FIRST-ITEM</A><P></UL><HR>
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