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<title>MMIX-Pseudos - Using as</title>
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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<div class="node">
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<p>
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<a name="MMIX_002dPseudos"></a>Previous: <a rel="previous" accesskey="p" href="MMIX_002dRegs.html#MMIX_002dRegs">MMIX-Regs</a>,
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Up: <a rel="up" accesskey="u" href="MMIX_002dSyntax.html#MMIX_002dSyntax">MMIX-Syntax</a>
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<hr><br>
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</div>
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<h5 class="subsubsection">9.23.3.4 Assembler Directives</h5>
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<p><a name="index-assembler-directives_002c-MMIX-1268"></a><a name="index-pseudo_002dops_002c-MMIX-1269"></a><a name="index-MMIX-assembler-directives-1270"></a><a name="index-MMIX-pseudo_002dops-1271"></a>
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<dl>
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<dt><code>LOC</code><dd><a name="index-assembler-directive-LOC_002c-MMIX-1272"></a><a name="index-pseudo_002dop-LOC_002c-MMIX-1273"></a><a name="index-MMIX-assembler-directive-LOC-1274"></a><a name="index-MMIX-pseudo_002dop-LOC-1275"></a>
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<a name="MMIX_002dloc"></a>
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The <code>LOC</code> directive sets the current location to the value of the
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operand field, which may include changing sections. If the operand is a
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constant, the section is set to either <code>.data</code> if the value is
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<code>0x2000000000000000</code> or larger, else it is set to <code>.text</code>.
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Within a section, the current location may only be changed to
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monotonically higher addresses. A LOC expression must be a previously
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defined symbol or a “pure” constant.
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<p>An example, which sets the label <var>prev</var> to the current location, and
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updates the current location to eight bytes forward:
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<pre class="smallexample"> prev LOC @+8
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</pre>
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<p>When a LOC has a constant as its operand, a symbol
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<code>__.MMIX.start..text</code> or <code>__.MMIX.start..data</code> is defined
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depending on the address as mentioned above. Each such symbol is
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interpreted as special by the linker, locating the section at that
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address. Note that if multiple files are linked, the first object file
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with that section will be mapped to that address (not necessarily the file
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with the LOC definition).
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<br><dt><code>LOCAL</code><dd><a name="index-assembler-directive-LOCAL_002c-MMIX-1276"></a><a name="index-pseudo_002dop-LOCAL_002c-MMIX-1277"></a><a name="index-MMIX-assembler-directive-LOCAL-1278"></a><a name="index-MMIX-pseudo_002dop-LOCAL-1279"></a>
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<a name="MMIX_002dlocal"></a>
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Example:
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<pre class="smallexample"> LOCAL external_symbol
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LOCAL 42
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.local asymbol
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</pre>
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<p>This directive-operation generates a link-time assertion that the operand
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does not correspond to a global register. The operand is an expression
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that at link-time resolves to a register symbol or a number. A number is
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treated as the register having that number. There is one restriction on
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the use of this directive: the pseudo-directive must be placed in a
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section with contents, code or data.
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<br><dt><code>IS</code><dd><a name="index-assembler-directive-IS_002c-MMIX-1280"></a><a name="index-pseudo_002dop-IS_002c-MMIX-1281"></a><a name="index-MMIX-assembler-directive-IS-1282"></a><a name="index-MMIX-pseudo_002dop-IS-1283"></a>
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<a name="MMIX_002dis"></a>
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The <code>IS</code> directive:
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<pre class="smallexample"> asymbol IS an_expression
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</pre>
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<p>sets the symbol <span class="samp">asymbol</span> to <span class="samp">an_expression</span>. A symbol may not
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be set more than once using this directive. Local labels may be set using
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this directive, for example:
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<pre class="smallexample"> 5H IS @+4
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</pre>
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<br><dt><code>GREG</code><dd><a name="index-assembler-directive-GREG_002c-MMIX-1284"></a><a name="index-pseudo_002dop-GREG_002c-MMIX-1285"></a><a name="index-MMIX-assembler-directive-GREG-1286"></a><a name="index-MMIX-pseudo_002dop-GREG-1287"></a>
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<a name="MMIX_002dgreg"></a>
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This directive reserves a global register, gives it an initial value and
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optionally gives it a symbolic name. Some examples:
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<pre class="smallexample"> areg GREG
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breg GREG data_value
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GREG data_buffer
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.greg creg, another_data_value
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</pre>
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<p>The symbolic register name can be used in place of a (non-special)
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register. If a value isn't provided, it defaults to zero. Unless the
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option <span class="samp">--no-merge-gregs</span> is specified, non-zero registers allocated
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with this directive may be eliminated by <code>as</code>; another
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register with the same value used in its place.
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Any of the instructions
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<span class="samp">CSWAP</span>,
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<span class="samp">GO</span>,
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<span class="samp">LDA</span>,
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<span class="samp">LDBU</span>,
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<span class="samp">LDB</span>,
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<span class="samp">LDHT</span>,
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<span class="samp">LDOU</span>,
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<span class="samp">LDO</span>,
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<span class="samp">LDSF</span>,
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<span class="samp">LDTU</span>,
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<span class="samp">LDT</span>,
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<span class="samp">LDUNC</span>,
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<span class="samp">LDVTS</span>,
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<span class="samp">LDWU</span>,
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<span class="samp">LDW</span>,
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<span class="samp">PREGO</span>,
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<span class="samp">PRELD</span>,
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<span class="samp">PREST</span>,
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<span class="samp">PUSHGO</span>,
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<span class="samp">STBU</span>,
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<span class="samp">STB</span>,
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<span class="samp">STCO</span>,
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<span class="samp">STHT</span>,
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<span class="samp">STOU</span>,
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<span class="samp">STSF</span>,
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<span class="samp">STTU</span>,
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<span class="samp">STT</span>,
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<span class="samp">STUNC</span>,
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<span class="samp">SYNCD</span>,
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<span class="samp">SYNCID</span>,
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can have a value nearby <a name="GREG_002dbase"></a>an initial value in place of its
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second and third operands. Here, “nearby” is defined as within the
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range 0<small class="dots">...</small>255 from the initial value of such an allocated register.
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<pre class="smallexample"> buffer1 BYTE 0,0,0,0,0
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buffer2 BYTE 0,0,0,0,0
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...
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GREG buffer1
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LDOU $42,buffer2
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</pre>
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<p>In the example above, the <span class="samp">Y</span> field of the <code>LDOUI</code> instruction
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(LDOU with a constant Z) will be replaced with the global register
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allocated for <span class="samp">buffer1</span>, and the <span class="samp">Z</span> field will have the value
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5, the offset from <span class="samp">buffer1</span> to <span class="samp">buffer2</span>. The result is
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equivalent to this code:
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<pre class="smallexample"> buffer1 BYTE 0,0,0,0,0
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buffer2 BYTE 0,0,0,0,0
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...
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tmpreg GREG buffer1
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LDOU $42,tmpreg,(buffer2-buffer1)
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</pre>
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<p>Global registers allocated with this directive are allocated in order
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higher-to-lower within a file. Other than that, the exact order of
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register allocation and elimination is undefined. For example, the order
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is undefined when more than one file with such directives are linked
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together. With the options <span class="samp">-x</span> and <span class="samp">--linker-allocated-gregs</span>,
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<span class="samp">GREG</span> directives for two-operand cases like the one mentioned above
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can be omitted. Sufficient global registers will then be allocated by the
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linker.
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<br><dt><code>BYTE</code><dd><a name="index-assembler-directive-BYTE_002c-MMIX-1288"></a><a name="index-pseudo_002dop-BYTE_002c-MMIX-1289"></a><a name="index-MMIX-assembler-directive-BYTE-1290"></a><a name="index-MMIX-pseudo_002dop-BYTE-1291"></a>
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<a name="MMIX_002dbyte"></a>
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The <span class="samp">BYTE</span> directive takes a series of operands separated by a comma.
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If an operand is a string (see <a href="Strings.html#Strings">Strings</a>), each character of that string
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is emitted as a byte. Other operands must be constant expressions without
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forward references, in the range 0<small class="dots">...</small>255. If you need operands having
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expressions with forward references, use <span class="samp">.byte</span> (see <a href="Byte.html#Byte">Byte</a>). An
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operand can be omitted, defaulting to a zero value.
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<br><dt><code>WYDE</code><dt><code>TETRA</code><dt><code>OCTA</code><dd><a name="index-assembler-directive-WYDE_002c-MMIX-1292"></a><a name="index-pseudo_002dop-WYDE_002c-MMIX-1293"></a><a name="index-MMIX-assembler-directive-WYDE-1294"></a><a name="index-MMIX-pseudo_002dop-WYDE-1295"></a><a name="index-assembler-directive-TETRA_002c-MMIX-1296"></a><a name="index-pseudo_002dop-TETRA_002c-MMIX-1297"></a><a name="index-MMIX-assembler-directive-TETRA-1298"></a><a name="index-MMIX-pseudo_002dop-TETRA-1299"></a><a name="index-assembler-directive-OCTA_002c-MMIX-1300"></a><a name="index-pseudo_002dop-OCTA_002c-MMIX-1301"></a><a name="index-MMIX-assembler-directive-OCTA-1302"></a><a name="index-MMIX-pseudo_002dop-OCTA-1303"></a>
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<a name="MMIX_002dconstants"></a>
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The directives <span class="samp">WYDE</span>, <span class="samp">TETRA</span> and <span class="samp">OCTA</span> emit constants of
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two, four and eight bytes size respectively. Before anything else happens
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for the directive, the current location is aligned to the respective
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constant-size boundary. If a label is defined at the beginning of the
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line, its value will be that after the alignment. A single operand can be
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omitted, defaulting to a zero value emitted for the directive. Operands
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can be expressed as strings (see <a href="Strings.html#Strings">Strings</a>), in which case each
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character in the string is emitted as a separate constant of the size
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indicated by the directive.
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<br><dt><code>PREFIX</code><dd><a name="index-assembler-directive-PREFIX_002c-MMIX-1304"></a><a name="index-pseudo_002dop-PREFIX_002c-MMIX-1305"></a><a name="index-MMIX-assembler-directive-PREFIX-1306"></a><a name="index-MMIX-pseudo_002dop-PREFIX-1307"></a>
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<a name="MMIX_002dprefix"></a>
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The <span class="samp">PREFIX</span> directive sets a symbol name prefix to be prepended to
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all symbols (except local symbols, see <a href="MMIX_002dSymbols.html#MMIX_002dSymbols">MMIX-Symbols</a>), that are not
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prefixed with <span class="samp">:</span>, until the next <span class="samp">PREFIX</span> directive. Such
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prefixes accumulate. For example,
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<pre class="smallexample"> PREFIX a
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PREFIX b
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c IS 0
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</pre>
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<p>defines a symbol <span class="samp">abc</span> with the value 0.
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<br><dt><code>BSPEC</code><dt><code>ESPEC</code><dd><a name="index-assembler-directive-BSPEC_002c-MMIX-1308"></a><a name="index-pseudo_002dop-BSPEC_002c-MMIX-1309"></a><a name="index-MMIX-assembler-directive-BSPEC-1310"></a><a name="index-MMIX-pseudo_002dop-BSPEC-1311"></a><a name="index-assembler-directive-ESPEC_002c-MMIX-1312"></a><a name="index-pseudo_002dop-ESPEC_002c-MMIX-1313"></a><a name="index-MMIX-assembler-directive-ESPEC-1314"></a><a name="index-MMIX-pseudo_002dop-ESPEC-1315"></a>
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<a name="MMIX_002dspec"></a>
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A pair of <span class="samp">BSPEC</span> and <span class="samp">ESPEC</span> directives delimit a section of
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special contents (without specified semantics). Example:
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<pre class="smallexample"> BSPEC 42
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TETRA 1,2,3
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ESPEC
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</pre>
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<p>The single operand to <span class="samp">BSPEC</span> must be number in the range
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0<small class="dots">...</small>255. The <span class="samp">BSPEC</span> number 80 is used by the GNU binutils
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implementation.
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</dl>
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