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MASM Reserved Words

This appendix lists the reserved words recognized by MASM. They are divided primarily by their use in the language. The primary categories are:

Reserved words in MASM 6.1 are reserved under all CPU modes. Words enabled in .8086 mode, the default, can be used in all higher CPU modes. To use words from subcategories such as "Special Operands for the 80386" (later in this appendix) requires .386 mode or higher.

You can disable the recognition of any reserved word specified in this appendix by setting the NOKEYWORD option for the OPTION directive. Once disabled, the word can be used in any way as a user-defined symbol (provided the word is a valid identifier). If you want to remove the STR instruction, the MASK operator, and the NAME directive, for instance, from the set of words MASM recognizes as reserved, add this statement to your program:


Words in this appendix identified with an asterisk (*) are new since MASM 5.1.

Operands and Symbols

The words on the two lists in this section are the operands to certain directives. They have special meaning to the assembler. The words on the first list are not reserved words. They can be used in every way as normal identifiers, without affecting their use as operands to directives. The assembler interprets their use from context.

Even though the words on the first list are not reserved, they should not be defined to be text macros or text macro functions. If they are, they will not be recognized in their special contexts. The assembler does not give a warning if such a redefinition occurs.

These operands are reserved words. Reserved words are not case sensitive.

Special Operands for the 80386/486

Predefined Symbols

Unlike most MASM reserved words, predefined symbols are case sensitive.



Operators and Directives

Processor Instructions

Processor instructions are not case sensitive.

8086/8088 Processor Instructions

80186 Processor Instructions

80286 Processor Instructions

80286 and 80386 Privileged-Mode Instructions

80386 Processor Instructions

80486 Processor Instructions







Instruction Prefixes







Coprocessor Instructions

Coprocessor instructions are not case sensitive.

8087 Coprocessor Instructions

80287 Privileged-Mode Instruction

80387 Instructions

See also:

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