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69<a name="g_t_002eif-absolute-expression"></a>
70<h3 class="section">7.45 <code>.if <var>absolute expression</var></code></h3>
71
72<a name="index-conditional-assembly"></a>
73<a name="index-if-directive"></a>
74<p><code>.if</code> marks the beginning of a section of code which is only
75considered part of the source program being assembled if the argument
76(which must be an <var>absolute expression</var>) is non-zero.  The end of
77the conditional section of code must be marked by <code>.endif</code>
78(see <a href="Endif.html#Endif"><code>.endif</code></a>); optionally, you may include code for the
79alternative condition, flagged by <code>.else</code> (see <a href="Else.html#Else"><code>.else</code></a>).
80If you have several conditions to check, <code>.elseif</code> may be used to avoid
81nesting blocks if/else within each subsequent <code>.else</code> block.
82</p>
83<p>The following variants of <code>.if</code> are also supported:
84</p><dl compact="compact">
85<dd><a name="index-ifdef-directive"></a>
86</dd>
87<dt><code>.ifdef <var>symbol</var></code></dt>
88<dd><p>Assembles the following section of code if the specified <var>symbol</var>
89has been defined.  Note a symbol which has been referenced but not yet defined
90is considered to be undefined.
91</p>
92<a name="index-ifb-directive"></a>
93</dd>
94<dt><code>.ifb <var>text</var></code></dt>
95<dd><p>Assembles the following section of code if the operand is blank (empty).
96</p>
97<a name="index-ifc-directive"></a>
98</dd>
99<dt><code>.ifc <var>string1</var>,<var>string2</var></code></dt>
100<dd><p>Assembles the following section of code if the two strings are the same.  The
101strings may be optionally quoted with single quotes.  If they are not quoted,
102the first string stops at the first comma, and the second string stops at the
103end of the line.  Strings which contain whitespace should be quoted.  The
104string comparison is case sensitive.
105</p>
106<a name="index-ifeq-directive"></a>
107</dd>
108<dt><code>.ifeq <var>absolute expression</var></code></dt>
109<dd><p>Assembles the following section of code if the argument is zero.
110</p>
111<a name="index-ifeqs-directive"></a>
112</dd>
113<dt><code>.ifeqs <var>string1</var>,<var>string2</var></code></dt>
114<dd><p>Another form of <code>.ifc</code>.  The strings must be quoted using double quotes.
115</p>
116<a name="index-ifge-directive"></a>
117</dd>
118<dt><code>.ifge <var>absolute expression</var></code></dt>
119<dd><p>Assembles the following section of code if the argument is greater than or
120equal to zero.
121</p>
122<a name="index-ifgt-directive"></a>
123</dd>
124<dt><code>.ifgt <var>absolute expression</var></code></dt>
125<dd><p>Assembles the following section of code if the argument is greater than zero.
126</p>
127<a name="index-ifle-directive"></a>
128</dd>
129<dt><code>.ifle <var>absolute expression</var></code></dt>
130<dd><p>Assembles the following section of code if the argument is less than or equal
131to zero.
132</p>
133<a name="index-iflt-directive"></a>
134</dd>
135<dt><code>.iflt <var>absolute expression</var></code></dt>
136<dd><p>Assembles the following section of code if the argument is less than zero.
137</p>
138<a name="index-ifnb-directive"></a>
139</dd>
140<dt><code>.ifnb <var>text</var></code></dt>
141<dd><p>Like <code>.ifb</code>, but the sense of the test is reversed: this assembles the
142following section of code if the operand is non-blank (non-empty).
143</p>
144<a name="index-ifnc-directive"></a>
145</dd>
146<dt><code>.ifnc <var>string1</var>,<var>string2</var>.</code></dt>
147<dd><p>Like <code>.ifc</code>, but the sense of the test is reversed: this assembles the
148following section of code if the two strings are not the same.
149</p>
150<a name="index-ifndef-directive"></a>
151<a name="index-ifnotdef-directive"></a>
152</dd>
153<dt><code>.ifndef <var>symbol</var></code></dt>
154<dt><code>.ifnotdef <var>symbol</var></code></dt>
155<dd><p>Assembles the following section of code if the specified <var>symbol</var>
156has not been defined.  Both spelling variants are equivalent.  Note a symbol
157which has been referenced but not yet defined is considered to be undefined.
158</p>
159<a name="index-ifne-directive"></a>
160</dd>
161<dt><code>.ifne <var>absolute expression</var></code></dt>
162<dd><p>Assembles the following section of code if the argument is not equal to zero
163(in other words, this is equivalent to <code>.if</code>).
164</p>
165<a name="index-ifnes-directive"></a>
166</dd>
167<dt><code>.ifnes <var>string1</var>,<var>string2</var></code></dt>
168<dd><p>Like <code>.ifeqs</code>, but the sense of the test is reversed: this assembles the
169following section of code if the two strings are not the same.
170</p></dd>
171</dl>
172
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