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91 lines
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<a name="Explicit-Reg-Vars"></a>
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Next: <a rel="next" accesskey="n" href="Size-of-an-asm.html#Size-of-an-asm">Size of an asm</a>,
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Previous: <a rel="previous" accesskey="p" href="Asm-Labels.html#Asm-Labels">Asm Labels</a>,
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Up: <a rel="up" accesskey="u" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>
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<h4 class="subsection">6.43.5 Variables in Specified Registers</h4>
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<p><a name="index-explicit-register-variables-3492"></a><a name="index-variables-in-specified-registers-3493"></a><a name="index-specified-registers-3494"></a><a name="index-registers_002c-global-allocation-3495"></a>
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GNU C allows you to put a few global variables into specified hardware
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registers. You can also specify the register in which an ordinary
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register variable should be allocated.
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<ul>
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<li>Global register variables reserve registers throughout the program.
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This may be useful in programs such as programming language
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interpreters that have a couple of global variables that are accessed
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very often.
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<li>Local register variables in specific registers do not reserve the
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registers, except at the point where they are used as input or output
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operands in an <code>asm</code> statement and the <code>asm</code> statement itself is
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not deleted. The compiler's data flow analysis is capable of determining
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where the specified registers contain live values, and where they are
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available for other uses. Stores into local register variables may be deleted
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when they appear to be dead according to dataflow analysis. References
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to local register variables may be deleted or moved or simplified.
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<p>These local variables are sometimes convenient for use with the extended
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<code>asm</code> feature (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>), if you want to write one
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output of the assembler instruction directly into a particular register.
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(This works provided the register you specify fits the constraints
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specified for that operand in the <code>asm</code>.)
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</ul>
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<ul class="menu">
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<li><a accesskey="1" href="Global-Reg-Vars.html#Global-Reg-Vars">Global Reg Vars</a>
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<li><a accesskey="2" href="Local-Reg-Vars.html#Local-Reg-Vars">Local Reg Vars</a>
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</ul>
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