as: i386-Variations

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1 9.15.3.1 AT&T Syntax versus Intel Syntax
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1 'as' now supports assembly using Intel assembler syntax.
1 '.intel_syntax' selects Intel mode, and '.att_syntax' switches back to
1 the usual AT&T mode for compatibility with the output of 'gcc'.  Either
1 of these directives may have an optional argument, 'prefix', or
1 'noprefix' specifying whether registers require a '%' prefix.  AT&T
1 System V/386 assembler syntax is quite different from Intel syntax.  We
1 mention these differences because almost all 80386 documents use Intel
1 syntax.  Notable differences between the two syntaxes are:
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1    * AT&T immediate operands are preceded by '$'; Intel immediate
1      operands are undelimited (Intel 'push 4' is AT&T 'pushl $4').  AT&T
1      register operands are preceded by '%'; Intel register operands are
1      undelimited.  AT&T absolute (as opposed to PC relative) jump/call
1      operands are prefixed by '*'; they are undelimited in Intel syntax.
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1    * AT&T and Intel syntax use the opposite order for source and
1      destination operands.  Intel 'add eax, 4' is 'addl $4, %eax'.  The
1      'source, dest' convention is maintained for compatibility with
1      previous Unix assemblers.  Note that 'bound', 'invlpga', and
1      instructions with 2 immediate operands, such as the 'enter'
1      instruction, do _not_ have reversed order.  ⇒i386-Bugs.
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1    * In AT&T syntax the size of memory operands is determined from the
1      last character of the instruction mnemonic.  Mnemonic suffixes of
1      'b', 'w', 'l' and 'q' specify byte (8-bit), word (16-bit), long
1      (32-bit) and quadruple word (64-bit) memory references.  Intel
1      syntax accomplishes this by prefixing memory operands (_not_ the
1      instruction mnemonics) with 'byte ptr', 'word ptr', 'dword ptr' and
1      'qword ptr'.  Thus, Intel 'mov al, byte ptr FOO' is 'movb FOO, %al'
1      in AT&T syntax.
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1      In 64-bit code, 'movabs' can be used to encode the 'mov'
1      instruction with the 64-bit displacement or immediate operand.
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1    * Immediate form long jumps and calls are 'lcall/ljmp $SECTION,
1      $OFFSET' in AT&T syntax; the Intel syntax is 'call/jmp far
1      SECTION:OFFSET'.  Also, the far return instruction is 'lret
1      $STACK-ADJUST' in AT&T syntax; Intel syntax is 'ret far
1      STACK-ADJUST'.
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1    * The AT&T assembler does not provide support for multiple section
1      programs.  Unix style systems expect all programs to be single
1      sections.
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