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### 7.51 `.if` absolute expression

`.if` marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an absolute expression) is non-zero. The end of the conditional section of code must be marked by `.endif` (see `.endif`); optionally, you may include code for the alternative condition, flagged by `.else` (see `.else`). If you have several conditions to check, `.elseif` may be used to avoid nesting blocks if/else within each subsequent `.else` block.

The following variants of `.if` are also supported:

`.ifdef` symbol
  Assembles the following section of code if the specified symbol has been defined. Note a symbol which has been referenced but not yet defined is considered to be undefined.

`.ifb` text
  Assembles the following section of code if the operand is blank (empty).

`.ifc` string1`,`string2
  Assembles the following section of code if the two strings are the same. The strings may be optionally quoted with single quotes. If they are not quoted, the first string stops at the first comma, and the second string stops at the end of the line. Strings which contain whitespace should be quoted. The string comparison is case sensitive.

`.ifeq` absolute expression
  Assembles the following section of code if the argument is zero.

`.ifeqs` string1`,`string2
  Another form of `.ifc`. The strings must be quoted using double quotes.

`.ifge` absolute expression
  Assembles the following section of code if the argument is greater than or equal to zero.

`.ifgt` absolute expression
  Assembles the following section of code if the argument is greater than zero.

`.ifle` absolute expression
  Assembles the following section of code if the argument is less than or equal to zero.

`.iflt` absolute expression
  Assembles the following section of code if the argument is less than zero.

`.ifnb` text
  Like `.ifb`, but the sense of the test is reversed: this assembles the following section of code if the operand is non-blank (non-empty).

`.ifnc` string1`,`string2`.`
  Like `.ifc`, but the sense of the test is reversed: this assembles the following section of code if the two strings are not the same.

`.ifndef` symbol
`.ifnotdef` symbol
  Assembles the following section of code if the specified symbol has not been defined. Both spelling variants are equivalent. Note a symbol which has been referenced but not yet defined is considered to be undefined.

`.ifne` absolute expression
  Assembles the following section of code if the argument is not equal to zero (in other words, this is equivalent to `.if`).

`.ifnes` string1`,`string2
  Like `.ifeqs`, but the sense of the test is reversed: this assembles the following section of code if the two strings are not the same.