Q123685: Characteristics of MASM Labels

Article: Q123685
Product(s): Microsoft Macro Assembler
Version(s): 5.1,6.0,6.1,6.11
Operating System(s): 
Keyword(s): 
Last Modified: 04-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.1, 6.0, 6.1, 6.11 
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SUMMARY
=======

Most high-level programming languages use variables for data and functions or
procedures for code. MASM uses labels for both data and code.

It is helpful to think of a MASM label as an address because once the program is
loaded into memory, each label corresponds to a particular address. It is also
helpful to think of a data label as a variable and a code label as a function or
procedure.

You can think of a MASM label as a C pointer; however, this might lead to
confusion because a label is more like a C array name than a C pointer. A MASM
label has no space allocated to store its value, as does a C pointer.

This article discusses the segment and offset of a MASM label, label type, and
label distance.

MORE INFORMATION
================

A MASM label is made up of a segment and an offset. In your MASM program, you
can use the SEG and OFFSET operators to access the corresponding portion of the
label. The the SEG and OFFSET directives work in a way similar to the C FP_SEG
and FP_OFF macros. However, FP_SEG and FP_OFF work on C pointers, not MASM
labels or memory addresses.

The size of the segment of a MASM label is always 16 bits. MASM labels have
either a 16-bit or a 32-bit offset. In general, the offset component of a label
is 16 bits. However, in flat model programs, the offset component of a label is
32 bits.

In addition to a segment and an offset, every label has a type or distance
associated with it. Data labels typically have a type, while code labels have an
associated distance.

The type or distance associated with a label is used to resolve ambiguities. For
example, consider the following instruction:

     mov MyLabel, 0

Should the assembler generate code to move a byte or a word to MyLabel? The
answer depends on how MyLabel was defined. If MyLabel was defined as a byte, a 0
byte is moved to MyLabel, but if MyLabel was defined as a word, a 0 word is
moved to MyLabel.

If you want to move a value to a label that has a size different from the size
for the type associated with the label, cast the label. You can cast a label by
placing the type that you want to associate with the label, followed by PTR, in
front of the data label. For example, the following instruction casts MyLabel to
a label for a WORD:

     mov WORD PTR MyLabel, 0

The WORD PTR does not mean MyLabel contains a pointer to a word; it means the
data at MyLabel should be considered to be a word.

Similarly, you can tell the assembler whether to make a call or a jump NEAR or
FAR by specifying the distance for the code label. For example, the following
instruction tells the assembler to generate a 32-bit NEAR jump to the label
CodeLabel:

     jmp NEAR32 CodeLabel

A jump such as this one is helpful when writing Windows NT or OS/2 flat model
programs.

Additional query words: kbinf 5.10 6.00 6.10

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Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM610 kbMASM611
Version           : :5.1,6.0,6.1,6.11

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