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as: Sub-Sections

 
 4.4 Sub-Sections
 ================
 
 Assembled bytes conventionally fall into two sections: text and data.
 You may have separate groups of data in named sections that you want to
 end up near to each other in the object file, even though they are not
 contiguous in the assembler source.  'as' allows you to use
 "subsections" for this purpose.  Within each section, there can be
 numbered subsections with values from 0 to 8192.  Objects assembled into
 the same subsection go into the object file together with other objects
 in the same subsection.  For example, a compiler might want to store
 constants in the text section, but might not want to have them
 interspersed with the program being assembled.  In this case, the
 compiler could issue a '.text 0' before each section of code being
 output, and a '.text 1' before each group of constants being output.
 
    Subsections are optional.  If you do not use subsections, everything
 goes in subsection number zero.
 
    Each subsection is zero-padded up to a multiple of four bytes.
 (Subsections may be padded a different amount on different flavors of
 'as'.)
 
    Subsections appear in your object file in numeric order, lowest
 numbered to highest.  (All this to be compatible with other people's
 assemblers.)  The object file contains no representation of subsections;
 'ld' and other programs that manipulate object files see no trace of
 them.  They just see all your text subsections as a text section, and
 all your data subsections as a data section.
 
    To specify which subsection you want subsequent statements assembled
 into, use a numeric argument to specify it, in a '.text EXPRESSION' or a
 '.data EXPRESSION' statement.  When generating COFF output, you can also
 use an extra subsection argument with arbitrary named sections:
 '.section NAME, EXPRESSION'.  When generating ELF output, you can also
 use the '.subsection' directive (SubSection) to specify a
 subsection: '.subsection EXPRESSION'.  EXPRESSION should be an absolute
 expression (Expressions).  If you just say '.text' then '.text
 0' is assumed.  Likewise '.data' means '.data 0'.  Assembly begins in
 'text 0'.  For instance:
      .text 0     # The default subsection is text 0 anyway.
      .ascii "This lives in the first text subsection. *"
      .text 1
      .ascii "But this lives in the second text subsection."
      .data 0
      .ascii "This lives in the data section,"
      .ascii "in the first data subsection."
      .text 0
      .ascii "This lives in the first text section,"
      .ascii "immediately following the asterisk (*)."
 
    Each section has a "location counter" incremented by one for every
 byte assembled into that section.  Because subsections are merely a
 convenience restricted to 'as' there is no concept of a subsection
 location counter.  There is no way to directly manipulate a location
 counter--but the '.align' directive changes it, and any label definition
 captures its current value.  The location counter of the section where
 statements are being assembled is said to be the "active" location
 counter.
 

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