72 lines
4.3 KiB
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72 lines
4.3 KiB
HTML
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<TITLE>CLHS: Accessor MASK-FIELD</TITLE>
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<A NAME="mask-field"><I>Accessor</I> <B>MASK-FIELD</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>mask-field</B> <I>bytespec integer</I> => <I>masked-integer</I><P>
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<TT>(setf (</TT><B>mask-field</B> <I>bytespec place</I><TT>) </TT><I>new-masked-integer</I><TT>)</TT><P>
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<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>masked-integer</I>, <I>new-masked-integer</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="#mask-field"><B>mask-field</B></A> performs a ``mask'' operation on <I>integer</I>. It returns an <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A> that has the same bits as <I>integer</I> in the <A REL=DEFINITION HREF="26_glo_b.htm#byte"><I>byte</I></A> specified by <I>bytespec</I>, but that has zero-bits everywhere else. <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="#mask-field"><B>mask-field</B></A> to modify a byte within the <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A> that is stored in a given <I>place</I>. The effect is to perform a <A REL=DEFINITION HREF="f_deposi.htm#deposit-field"><B>deposit-field</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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(mask-field (byte 1 5) -1) => 32
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(setq a 15) => 15
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(mask-field (byte 2 0) a) => 3
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a => 15
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(setf (mask-field (byte 2 0) a) 1) => 1
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a => 13
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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_ldb.htm#ldb"><B>ldb</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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(ldb bs (mask-field bs n)) == (ldb bs n)
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(logbitp j (mask-field (byte s p) n))
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== (and (>= j p) (< j s) (logbitp j n))
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(mask-field bs n) == (logand n (dpb -1 bs 0))
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</PRE>
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</TT> <P>
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