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(nasm.info)Section 4.8.5

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4.8.5. `STRUC' and `ENDSTRUC': Declaring Structure Data Types
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   The core of NASM contains no intrinsic means of defining data
structures; instead, the preprocessor is sufficiently powerful that
data structures can be implemented as a set of macros. The macros
`STRUC' and `ENDSTRUC' are used to define a structure data type.

   `STRUC' takes one parameter, which is the name of the data type. This
name is defined as a symbol with the value zero, and also has the suffix
`_size' appended to it and is then defined as an `EQU' giving the size
of the structure. Once `STRUC' has been issued, you are defining the
structure, and should define fields using the `RESB' family of
pseudo-instructions, and then invoke `ENDSTRUC' to finish the
definition.

   For example, to define a structure called `mytype' containing a
longword, a word, a byte and a string of bytes, you might code

     struc   mytype
     
       mt_long:      resd    1
       mt_word:      resw    1
       mt_byte:      resb    1
       mt_str:       resb    32
     
     endstruc

   The above code defines six symbols: `mt_long' as 0 (the offset from
the beginning of a `mytype' structure to the longword field), `mt_word'
as 4, `mt_byte' as 6, `mt_str' as 7, `mytype_size' as 39, and `mytype'
itself as zero.

   The reason why the structure type name is defined at zero is a side
effect of allowing structures to work with the local label mechanism:
if your structure members tend to have the same names in more than one
structure, you can define the above structure like this:

     struc mytype
     
       .long:        resd    1
       .word:        resw    1
       .byte:        resb    1
       .str:         resb    32
     
     endstruc

   This defines the offsets to the structure fields as `mytype.long',
`mytype.word', `mytype.byte' and `mytype.str'.

   NASM, since it has no _intrinsic_ structure support, does not support
any form of period notation to refer to the elements of a structure once
you have one (except the above local-label notation), so code such as
`mov ax,[mystruc.mt_word]' is not valid. `mt_word' is a constant just
like any other constant, so the correct syntax is `mov
ax,[mystruc+mt_word]' or `mov ax,[mystruc+mytype.word]'.