Intel XENIX 286 Programmers Guide (86) - Tenox.tc

XENIX Programming as: Assembler
Assembly Segments
as assembles instruction and data statements in three segments, text, data, and bss.
Segments allow division of instructions and data into separate physical segments in
memory. A location counter keeps the current address within each segment during
assembly and provides reference to the current instruction and data.
Text, Data, and Bss Segments
Every program is divided into at most three distinct segments of assembled code and
data: the text segment, the data segment, and the bss segment. Each segment is
reserved for a specific type of storage and receives different treat ment from the
assembler and from the XENIX linker when the final program is created.
The text segment is normally reserved for instructions but may also be used for data.
Instructions in this segment are assembled, and the code is copied to the output file.
Data definitions in this segment are also assembled and copied; the code is the value of
the data item. The assembler does not separate the instruction and data code. If the
instructions and data definitions are mixed within the source file, the resulting code is
mixed within the output file.
The data segment is reserved for data. The code is copied to a different part of the
output file and receives different treat ment from the XENIX linker.
The bss segment is reserved for uninitialized data only. Instructions or data definitions
with initial values must not be given in this segment. The assembler counts the nu mber
of bytes allocated for this segment and copies this count to the output file. It does not
generate code.
The text segment is implicitly defined at the start of every assembly. Thus, any
instructions or data definitions given when no other segment is explicitly defined are
copied to the text segment. To start a data or bss segment, you must use a .data or .bss
directive. You can explicitly start the text segment with the .text directive (see the
section "Segment Directives," later in this chapter).
Unless otherwise specified, the first statement in the text segment is considered the
program's entry point. In shared-text programs, the instructions and data in the text
segment are write-protected; in nonshared-text programs, they are not. Instructions
and data in the data segment are never write-protected. The bss segment is actually an
extension of the data segment. It begins immediately after the data segment and is
initialized to 0 at the start of program execution.
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