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diff --git a/binutils-2.20.1/gas/doc/c-i386.texi b/binutils-2.20.1/gas/doc/c-i386.texi new file mode 100644 index 0000000..cf0bfa8 --- /dev/null +++ b/binutils-2.20.1/gas/doc/c-i386.texi @@ -0,0 +1,954 @@ +@c Copyright 1991, 1992, 1993, 1994, 1995, 1997, 1998, 1999, 2000, +@c 2001, 2003, 2004, 2005, 2006, 2007, 2008, 2009 +@c Free Software Foundation, Inc. +@c This is part of the GAS manual. +@c For copying conditions, see the file as.texinfo. +@ifset GENERIC +@page +@node i386-Dependent +@chapter 80386 Dependent Features +@end ifset +@ifclear GENERIC +@node Machine Dependencies +@chapter 80386 Dependent Features +@end ifclear + +@cindex i386 support +@cindex i80386 support +@cindex x86-64 support + +The i386 version @code{@value{AS}} supports both the original Intel 386 +architecture in both 16 and 32-bit mode as well as AMD x86-64 architecture +extending the Intel architecture to 64-bits. + +@menu +* i386-Options:: Options +* i386-Directives:: X86 specific directives +* i386-Syntax:: AT&T Syntax versus Intel Syntax +* i386-Mnemonics:: Instruction Naming +* i386-Regs:: Register Naming +* i386-Prefixes:: Instruction Prefixes +* i386-Memory:: Memory References +* i386-Jumps:: Handling of Jump Instructions +* i386-Float:: Floating Point +* i386-SIMD:: Intel's MMX and AMD's 3DNow! SIMD Operations +* i386-16bit:: Writing 16-bit Code +* i386-Arch:: Specifying an x86 CPU architecture +* i386-Bugs:: AT&T Syntax bugs +* i386-Notes:: Notes +@end menu + +@node i386-Options +@section Options + +@cindex options for i386 +@cindex options for x86-64 +@cindex i386 options +@cindex x86-64 options + +The i386 version of @code{@value{AS}} has a few machine +dependent options: + +@table @code +@cindex @samp{--32} option, i386 +@cindex @samp{--32} option, x86-64 +@cindex @samp{--64} option, i386 +@cindex @samp{--64} option, x86-64 +@item --32 | --64 +Select the word size, either 32 bits or 64 bits. Selecting 32-bit +implies Intel i386 architecture, while 64-bit implies AMD x86-64 +architecture. + +These options are only available with the ELF object file format, and +require that the necessary BFD support has been included (on a 32-bit +platform you have to add --enable-64-bit-bfd to configure enable 64-bit +usage and use x86-64 as target platform). + +@item -n +By default, x86 GAS replaces multiple nop instructions used for +alignment within code sections with multi-byte nop instructions such +as leal 0(%esi,1),%esi. This switch disables the optimization. + +@cindex @samp{--divide} option, i386 +@item --divide +On SVR4-derived platforms, the character @samp{/} is treated as a comment +character, which means that it cannot be used in expressions. The +@samp{--divide} option turns @samp{/} into a normal character. This does +not disable @samp{/} at the beginning of a line starting a comment, or +affect using @samp{#} for starting a comment. + +@cindex @samp{-march=} option, i386 +@cindex @samp{-march=} option, x86-64 +@item -march=@var{CPU}[+@var{EXTENSION}@dots{}] +This option specifies the target processor. The assembler will +issue an error message if an attempt is made to assemble an instruction +which will not execute on the target processor. The following +processor names are recognized: +@code{i8086}, +@code{i186}, +@code{i286}, +@code{i386}, +@code{i486}, +@code{i586}, +@code{i686}, +@code{pentium}, +@code{pentiumpro}, +@code{pentiumii}, +@code{pentiumiii}, +@code{pentium4}, +@code{prescott}, +@code{nocona}, +@code{core}, +@code{core2}, +@code{corei7}, +@code{l1om}, +@code{k6}, +@code{k6_2}, +@code{athlon}, +@code{opteron}, +@code{k8}, +@code{amdfam10}, +@code{generic32} and +@code{generic64}. + +In addition to the basic instruction set, the assembler can be told to +accept various extension mnemonics. For example, +@code{-march=i686+sse4+vmx} extends @var{i686} with @var{sse4} and +@var{vmx}. The following extensions are currently supported: +@code{8087}, +@code{287}, +@code{387}, +@code{no87}, +@code{mmx}, +@code{nommx}, +@code{sse}, +@code{sse2}, +@code{sse3}, +@code{ssse3}, +@code{sse4.1}, +@code{sse4.2}, +@code{sse4}, +@code{nosse}, +@code{avx}, +@code{noavx}, +@code{vmx}, +@code{smx}, +@code{xsave}, +@code{aes}, +@code{pclmul}, +@code{fma}, +@code{movbe}, +@code{ept}, +@code{clflush}, +@code{syscall}, +@code{rdtscp}, +@code{3dnow}, +@code{3dnowa}, +@code{sse4a}, +@code{sse5}, +@code{svme}, +@code{abm} and +@code{padlock}. +Note that rather than extending a basic instruction set, the extension +mnemonics starting with @code{no} revoke the respective functionality. + +When the @code{.arch} directive is used with @option{-march}, the +@code{.arch} directive will take precedent. + +@cindex @samp{-mtune=} option, i386 +@cindex @samp{-mtune=} option, x86-64 +@item -mtune=@var{CPU} +This option specifies a processor to optimize for. When used in +conjunction with the @option{-march} option, only instructions +of the processor specified by the @option{-march} option will be +generated. + +Valid @var{CPU} values are identical to the processor list of +@option{-march=@var{CPU}}. + +@cindex @samp{-msse2avx} option, i386 +@cindex @samp{-msse2avx} option, x86-64 +@item -msse2avx +This option specifies that the assembler should encode SSE instructions +with VEX prefix. + +@cindex @samp{-msse-check=} option, i386 +@cindex @samp{-msse-check=} option, x86-64 +@item -msse-check=@var{none} +@item -msse-check=@var{warning} +@item -msse-check=@var{error} +These options control if the assembler should check SSE intructions. +@option{-msse-check=@var{none}} will make the assembler not to check SSE +instructions, which is the default. @option{-msse-check=@var{warning}} +will make the assembler issue a warning for any SSE intruction. +@option{-msse-check=@var{error}} will make the assembler issue an error +for any SSE intruction. + +@cindex @samp{-mmnemonic=} option, i386 +@cindex @samp{-mmnemonic=} option, x86-64 +@item -mmnemonic=@var{att} +@item -mmnemonic=@var{intel} +This option specifies instruction mnemonic for matching instructions. +The @code{.att_mnemonic} and @code{.intel_mnemonic} directives will +take precedent. + +@cindex @samp{-msyntax=} option, i386 +@cindex @samp{-msyntax=} option, x86-64 +@item -msyntax=@var{att} +@item -msyntax=@var{intel} +This option specifies instruction syntax when processing instructions. +The @code{.att_syntax} and @code{.intel_syntax} directives will +take precedent. + +@cindex @samp{-mnaked-reg} option, i386 +@cindex @samp{-mnaked-reg} option, x86-64 +@item -mnaked-reg +This opetion specifies that registers don't require a @samp{%} prefix. +The @code{.att_syntax} and @code{.intel_syntax} directives will take precedent. + +@end table + +@node i386-Directives +@section x86 specific Directives + +@cindex machine directives, x86 +@cindex x86 machine directives +@table @code + +@cindex @code{lcomm} directive, COFF +@item .lcomm @var{symbol} , @var{length}[, @var{alignment}] +Reserve @var{length} (an absolute expression) bytes for a local common +denoted by @var{symbol}. The section and value of @var{symbol} are +those of the new local common. The addresses are allocated in the bss +section, so that at run-time the bytes start off zeroed. Since +@var{symbol} is not declared global, it is normally not visible to +@code{@value{LD}}. The optional third parameter, @var{alignment}, +specifies the desired alignment of the symbol in the bss section. + +This directive is only available for COFF based x86 targets. + +@c FIXME: Document other x86 specific directives ? Eg: .code16gcc, +@c .largecomm + +@end table + +@node i386-Syntax +@section AT&T Syntax versus Intel Syntax + +@cindex i386 intel_syntax pseudo op +@cindex intel_syntax pseudo op, i386 +@cindex i386 att_syntax pseudo op +@cindex att_syntax pseudo op, i386 +@cindex i386 syntax compatibility +@cindex syntax compatibility, i386 +@cindex x86-64 intel_syntax pseudo op +@cindex intel_syntax pseudo op, x86-64 +@cindex x86-64 att_syntax pseudo op +@cindex att_syntax pseudo op, x86-64 +@cindex x86-64 syntax compatibility +@cindex syntax compatibility, x86-64 + +@code{@value{AS}} now supports assembly using Intel assembler syntax. +@code{.intel_syntax} selects Intel mode, and @code{.att_syntax} switches +back to the usual AT&T mode for compatibility with the output of +@code{@value{GCC}}. Either of these directives may have an optional +argument, @code{prefix}, or @code{noprefix} specifying whether registers +require a @samp{%} prefix. AT&T System V/386 assembler syntax is quite +different from Intel syntax. We mention these differences because +almost all 80386 documents use Intel syntax. Notable differences +between the two syntaxes are: + +@cindex immediate operands, i386 +@cindex i386 immediate operands +@cindex register operands, i386 +@cindex i386 register operands +@cindex jump/call operands, i386 +@cindex i386 jump/call operands +@cindex operand delimiters, i386 + +@cindex immediate operands, x86-64 +@cindex x86-64 immediate operands +@cindex register operands, x86-64 +@cindex x86-64 register operands +@cindex jump/call operands, x86-64 +@cindex x86-64 jump/call operands +@cindex operand delimiters, x86-64 +@itemize @bullet +@item +AT&T immediate operands are preceded by @samp{$}; Intel immediate +operands are undelimited (Intel @samp{push 4} is AT&T @samp{pushl $4}). +AT&T register operands are preceded by @samp{%}; Intel register operands +are undelimited. AT&T absolute (as opposed to PC relative) jump/call +operands are prefixed by @samp{*}; they are undelimited in Intel syntax. + +@cindex i386 source, destination operands +@cindex source, destination operands; i386 +@cindex x86-64 source, destination operands +@cindex source, destination operands; x86-64 +@item +AT&T and Intel syntax use the opposite order for source and destination +operands. Intel @samp{add eax, 4} is @samp{addl $4, %eax}. The +@samp{source, dest} convention is maintained for compatibility with +previous Unix assemblers. Note that @samp{bound}, @samp{invlpga}, and +instructions with 2 immediate operands, such as the @samp{enter} +instruction, do @emph{not} have reversed order. @ref{i386-Bugs}. + +@cindex mnemonic suffixes, i386 +@cindex sizes operands, i386 +@cindex i386 size suffixes +@cindex mnemonic suffixes, x86-64 +@cindex sizes operands, x86-64 +@cindex x86-64 size suffixes +@item +In AT&T syntax the size of memory operands is determined from the last +character of the instruction mnemonic. Mnemonic suffixes of @samp{b}, +@samp{w}, @samp{l} and @samp{q} specify byte (8-bit), word (16-bit), long +(32-bit) and quadruple word (64-bit) memory references. Intel syntax accomplishes +this by prefixing memory operands (@emph{not} the instruction mnemonics) with +@samp{byte ptr}, @samp{word ptr}, @samp{dword ptr} and @samp{qword ptr}. Thus, +Intel @samp{mov al, byte ptr @var{foo}} is @samp{movb @var{foo}, %al} in AT&T +syntax. + +@cindex return instructions, i386 +@cindex i386 jump, call, return +@cindex return instructions, x86-64 +@cindex x86-64 jump, call, return +@item +Immediate form long jumps and calls are +@samp{lcall/ljmp $@var{section}, $@var{offset}} in AT&T syntax; the +Intel syntax is +@samp{call/jmp far @var{section}:@var{offset}}. Also, the far return +instruction +is @samp{lret $@var{stack-adjust}} in AT&T syntax; Intel syntax is +@samp{ret far @var{stack-adjust}}. + +@cindex sections, i386 +@cindex i386 sections +@cindex sections, x86-64 +@cindex x86-64 sections +@item +The AT&T assembler does not provide support for multiple section +programs. Unix style systems expect all programs to be single sections. +@end itemize + +@node i386-Mnemonics +@section Instruction Naming + +@cindex i386 instruction naming +@cindex instruction naming, i386 +@cindex x86-64 instruction naming +@cindex instruction naming, x86-64 + +Instruction mnemonics are suffixed with one character modifiers which +specify the size of operands. The letters @samp{b}, @samp{w}, @samp{l} +and @samp{q} specify byte, word, long and quadruple word operands. If +no suffix is specified by an instruction then @code{@value{AS}} tries to +fill in the missing suffix based on the destination register operand +(the last one by convention). Thus, @samp{mov %ax, %bx} is equivalent +to @samp{movw %ax, %bx}; also, @samp{mov $1, %bx} is equivalent to +@samp{movw $1, bx}. Note that this is incompatible with the AT&T Unix +assembler which assumes that a missing mnemonic suffix implies long +operand size. (This incompatibility does not affect compiler output +since compilers always explicitly specify the mnemonic suffix.) + +Almost all instructions have the same names in AT&T and Intel format. +There are a few exceptions. The sign extend and zero extend +instructions need two sizes to specify them. They need a size to +sign/zero extend @emph{from} and a size to zero extend @emph{to}. This +is accomplished by using two instruction mnemonic suffixes in AT&T +syntax. Base names for sign extend and zero extend are +@samp{movs@dots{}} and @samp{movz@dots{}} in AT&T syntax (@samp{movsx} +and @samp{movzx} in Intel syntax). The instruction mnemonic suffixes +are tacked on to this base name, the @emph{from} suffix before the +@emph{to} suffix. Thus, @samp{movsbl %al, %edx} is AT&T syntax for +``move sign extend @emph{from} %al @emph{to} %edx.'' Possible suffixes, +thus, are @samp{bl} (from byte to long), @samp{bw} (from byte to word), +@samp{wl} (from word to long), @samp{bq} (from byte to quadruple word), +@samp{wq} (from word to quadruple word), and @samp{lq} (from long to +quadruple word). + +@cindex encoding options, i386 +@cindex encoding options, x86-64 + +Different encoding options can be specified via optional mnemonic +suffix. @samp{.s} suffix swaps 2 register operands in encoding when +moving from one register to another. + +@cindex conversion instructions, i386 +@cindex i386 conversion instructions +@cindex conversion instructions, x86-64 +@cindex x86-64 conversion instructions +The Intel-syntax conversion instructions + +@itemize @bullet +@item +@samp{cbw} --- sign-extend byte in @samp{%al} to word in @samp{%ax}, + +@item +@samp{cwde} --- sign-extend word in @samp{%ax} to long in @samp{%eax}, + +@item +@samp{cwd} --- sign-extend word in @samp{%ax} to long in @samp{%dx:%ax}, + +@item +@samp{cdq} --- sign-extend dword in @samp{%eax} to quad in @samp{%edx:%eax}, + +@item +@samp{cdqe} --- sign-extend dword in @samp{%eax} to quad in @samp{%rax} +(x86-64 only), + +@item +@samp{cqo} --- sign-extend quad in @samp{%rax} to octuple in +@samp{%rdx:%rax} (x86-64 only), +@end itemize + +@noindent +are called @samp{cbtw}, @samp{cwtl}, @samp{cwtd}, @samp{cltd}, @samp{cltq}, and +@samp{cqto} in AT&T naming. @code{@value{AS}} accepts either naming for these +instructions. + +@cindex jump instructions, i386 +@cindex call instructions, i386 +@cindex jump instructions, x86-64 +@cindex call instructions, x86-64 +Far call/jump instructions are @samp{lcall} and @samp{ljmp} in +AT&T syntax, but are @samp{call far} and @samp{jump far} in Intel +convention. + +@section AT&T Mnemonic versus Intel Mnemonic + +@cindex i386 mnemonic compatibility +@cindex mnemonic compatibility, i386 + +@code{@value{AS}} supports assembly using Intel mnemonic. +@code{.intel_mnemonic} selects Intel mnemonic with Intel syntax, and +@code{.att_mnemonic} switches back to the usual AT&T mnemonic with AT&T +syntax for compatibility with the output of @code{@value{GCC}}. +Several x87 instructions, @samp{fadd}, @samp{fdiv}, @samp{fdivp}, +@samp{fdivr}, @samp{fdivrp}, @samp{fmul}, @samp{fsub}, @samp{fsubp}, +@samp{fsubr} and @samp{fsubrp}, are implemented in AT&T System V/386 +assembler with different mnemonics from those in Intel IA32 specification. +@code{@value{GCC}} generates those instructions with AT&T mnemonic. + +@node i386-Regs +@section Register Naming + +@cindex i386 registers +@cindex registers, i386 +@cindex x86-64 registers +@cindex registers, x86-64 +Register operands are always prefixed with @samp{%}. The 80386 registers +consist of + +@itemize @bullet +@item +the 8 32-bit registers @samp{%eax} (the accumulator), @samp{%ebx}, +@samp{%ecx}, @samp{%edx}, @samp{%edi}, @samp{%esi}, @samp{%ebp} (the +frame pointer), and @samp{%esp} (the stack pointer). + +@item +the 8 16-bit low-ends of these: @samp{%ax}, @samp{%bx}, @samp{%cx}, +@samp{%dx}, @samp{%di}, @samp{%si}, @samp{%bp}, and @samp{%sp}. + +@item +the 8 8-bit registers: @samp{%ah}, @samp{%al}, @samp{%bh}, +@samp{%bl}, @samp{%ch}, @samp{%cl}, @samp{%dh}, and @samp{%dl} (These +are the high-bytes and low-bytes of @samp{%ax}, @samp{%bx}, +@samp{%cx}, and @samp{%dx}) + +@item +the 6 section registers @samp{%cs} (code section), @samp{%ds} +(data section), @samp{%ss} (stack section), @samp{%es}, @samp{%fs}, +and @samp{%gs}. + +@item +the 3 processor control registers @samp{%cr0}, @samp{%cr2}, and +@samp{%cr3}. + +@item +the 6 debug registers @samp{%db0}, @samp{%db1}, @samp{%db2}, +@samp{%db3}, @samp{%db6}, and @samp{%db7}. + +@item +the 2 test registers @samp{%tr6} and @samp{%tr7}. + +@item +the 8 floating point register stack @samp{%st} or equivalently +@samp{%st(0)}, @samp{%st(1)}, @samp{%st(2)}, @samp{%st(3)}, +@samp{%st(4)}, @samp{%st(5)}, @samp{%st(6)}, and @samp{%st(7)}. +These registers are overloaded by 8 MMX registers @samp{%mm0}, +@samp{%mm1}, @samp{%mm2}, @samp{%mm3}, @samp{%mm4}, @samp{%mm5}, +@samp{%mm6} and @samp{%mm7}. + +@item +the 8 SSE registers registers @samp{%xmm0}, @samp{%xmm1}, @samp{%xmm2}, +@samp{%xmm3}, @samp{%xmm4}, @samp{%xmm5}, @samp{%xmm6} and @samp{%xmm7}. +@end itemize + +The AMD x86-64 architecture extends the register set by: + +@itemize @bullet +@item +enhancing the 8 32-bit registers to 64-bit: @samp{%rax} (the +accumulator), @samp{%rbx}, @samp{%rcx}, @samp{%rdx}, @samp{%rdi}, +@samp{%rsi}, @samp{%rbp} (the frame pointer), @samp{%rsp} (the stack +pointer) + +@item +the 8 extended registers @samp{%r8}--@samp{%r15}. + +@item +the 8 32-bit low ends of the extended registers: @samp{%r8d}--@samp{%r15d} + +@item +the 8 16-bit low ends of the extended registers: @samp{%r8w}--@samp{%r15w} + +@item +the 8 8-bit low ends of the extended registers: @samp{%r8b}--@samp{%r15b} + +@item +the 4 8-bit registers: @samp{%sil}, @samp{%dil}, @samp{%bpl}, @samp{%spl}. + +@item +the 8 debug registers: @samp{%db8}--@samp{%db15}. + +@item +the 8 SSE registers: @samp{%xmm8}--@samp{%xmm15}. +@end itemize + +@node i386-Prefixes +@section Instruction Prefixes + +@cindex i386 instruction prefixes +@cindex instruction prefixes, i386 +@cindex prefixes, i386 +Instruction prefixes are used to modify the following instruction. They +are used to repeat string instructions, to provide section overrides, to +perform bus lock operations, and to change operand and address sizes. +(Most instructions that normally operate on 32-bit operands will use +16-bit operands if the instruction has an ``operand size'' prefix.) +Instruction prefixes are best written on the same line as the instruction +they act upon. For example, the @samp{scas} (scan string) instruction is +repeated with: + +@smallexample + repne scas %es:(%edi),%al +@end smallexample + +You may also place prefixes on the lines immediately preceding the +instruction, but this circumvents checks that @code{@value{AS}} does +with prefixes, and will not work with all prefixes. + +Here is a list of instruction prefixes: + +@cindex section override prefixes, i386 +@itemize @bullet +@item +Section override prefixes @samp{cs}, @samp{ds}, @samp{ss}, @samp{es}, +@samp{fs}, @samp{gs}. These are automatically added by specifying +using the @var{section}:@var{memory-operand} form for memory references. + +@cindex size prefixes, i386 +@item +Operand/Address size prefixes @samp{data16} and @samp{addr16} +change 32-bit operands/addresses into 16-bit operands/addresses, +while @samp{data32} and @samp{addr32} change 16-bit ones (in a +@code{.code16} section) into 32-bit operands/addresses. These prefixes +@emph{must} appear on the same line of code as the instruction they +modify. For example, in a 16-bit @code{.code16} section, you might +write: + +@smallexample + addr32 jmpl *(%ebx) +@end smallexample + +@cindex bus lock prefixes, i386 +@cindex inhibiting interrupts, i386 +@item +The bus lock prefix @samp{lock} inhibits interrupts during execution of +the instruction it precedes. (This is only valid with certain +instructions; see a 80386 manual for details). + +@cindex coprocessor wait, i386 +@item +The wait for coprocessor prefix @samp{wait} waits for the coprocessor to +complete the current instruction. This should never be needed for the +80386/80387 combination. + +@cindex repeat prefixes, i386 +@item +The @samp{rep}, @samp{repe}, and @samp{repne} prefixes are added +to string instructions to make them repeat @samp{%ecx} times (@samp{%cx} +times if the current address size is 16-bits). +@cindex REX prefixes, i386 +@item +The @samp{rex} family of prefixes is used by x86-64 to encode +extensions to i386 instruction set. The @samp{rex} prefix has four +bits --- an operand size overwrite (@code{64}) used to change operand size +from 32-bit to 64-bit and X, Y and Z extensions bits used to extend the +register set. + +You may write the @samp{rex} prefixes directly. The @samp{rex64xyz} +instruction emits @samp{rex} prefix with all the bits set. By omitting +the @code{64}, @code{x}, @code{y} or @code{z} you may write other +prefixes as well. Normally, there is no need to write the prefixes +explicitly, since gas will automatically generate them based on the +instruction operands. +@end itemize + +@node i386-Memory +@section Memory References + +@cindex i386 memory references +@cindex memory references, i386 +@cindex x86-64 memory references +@cindex memory references, x86-64 +An Intel syntax indirect memory reference of the form + +@smallexample +@var{section}:[@var{base} + @var{index}*@var{scale} + @var{disp}] +@end smallexample + +@noindent +is translated into the AT&T syntax + +@smallexample +@var{section}:@var{disp}(@var{base}, @var{index}, @var{scale}) +@end smallexample + +@noindent +where @var{base} and @var{index} are the optional 32-bit base and +index registers, @var{disp} is the optional displacement, and +@var{scale}, taking the values 1, 2, 4, and 8, multiplies @var{index} +to calculate the address of the operand. If no @var{scale} is +specified, @var{scale} is taken to be 1. @var{section} specifies the +optional section register for the memory operand, and may override the +default section register (see a 80386 manual for section register +defaults). Note that section overrides in AT&T syntax @emph{must} +be preceded by a @samp{%}. If you specify a section override which +coincides with the default section register, @code{@value{AS}} does @emph{not} +output any section register override prefixes to assemble the given +instruction. Thus, section overrides can be specified to emphasize which +section register is used for a given memory operand. + +Here are some examples of Intel and AT&T style memory references: + +@table @asis +@item AT&T: @samp{-4(%ebp)}, Intel: @samp{[ebp - 4]} +@var{base} is @samp{%ebp}; @var{disp} is @samp{-4}. @var{section} is +missing, and the default section is used (@samp{%ss} for addressing with +@samp{%ebp} as the base register). @var{index}, @var{scale} are both missing. + +@item AT&T: @samp{foo(,%eax,4)}, Intel: @samp{[foo + eax*4]} +@var{index} is @samp{%eax} (scaled by a @var{scale} 4); @var{disp} is +@samp{foo}. All other fields are missing. The section register here +defaults to @samp{%ds}. + +@item AT&T: @samp{foo(,1)}; Intel @samp{[foo]} +This uses the value pointed to by @samp{foo} as a memory operand. +Note that @var{base} and @var{index} are both missing, but there is only +@emph{one} @samp{,}. This is a syntactic exception. + +@item AT&T: @samp{%gs:foo}; Intel @samp{gs:foo} +This selects the contents of the variable @samp{foo} with section +register @var{section} being @samp{%gs}. +@end table + +Absolute (as opposed to PC relative) call and jump operands must be +prefixed with @samp{*}. If no @samp{*} is specified, @code{@value{AS}} +always chooses PC relative addressing for jump/call labels. + +Any instruction that has a memory operand, but no register operand, +@emph{must} specify its size (byte, word, long, or quadruple) with an +instruction mnemonic suffix (@samp{b}, @samp{w}, @samp{l} or @samp{q}, +respectively). + +The x86-64 architecture adds an RIP (instruction pointer relative) +addressing. This addressing mode is specified by using @samp{rip} as a +base register. Only constant offsets are valid. For example: + +@table @asis +@item AT&T: @samp{1234(%rip)}, Intel: @samp{[rip + 1234]} +Points to the address 1234 bytes past the end of the current +instruction. + +@item AT&T: @samp{symbol(%rip)}, Intel: @samp{[rip + symbol]} +Points to the @code{symbol} in RIP relative way, this is shorter than +the default absolute addressing. +@end table + +Other addressing modes remain unchanged in x86-64 architecture, except +registers used are 64-bit instead of 32-bit. + +@node i386-Jumps +@section Handling of Jump Instructions + +@cindex jump optimization, i386 +@cindex i386 jump optimization +@cindex jump optimization, x86-64 +@cindex x86-64 jump optimization +Jump instructions are always optimized to use the smallest possible +displacements. This is accomplished by using byte (8-bit) displacement +jumps whenever the target is sufficiently close. If a byte displacement +is insufficient a long displacement is used. We do not support +word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump +instruction with the @samp{data16} instruction prefix), since the 80386 +insists upon masking @samp{%eip} to 16 bits after the word displacement +is added. (See also @pxref{i386-Arch}) + +Note that the @samp{jcxz}, @samp{jecxz}, @samp{loop}, @samp{loopz}, +@samp{loope}, @samp{loopnz} and @samp{loopne} instructions only come in byte +displacements, so that if you use these instructions (@code{@value{GCC}} does +not use them) you may get an error message (and incorrect code). The AT&T +80386 assembler tries to get around this problem by expanding @samp{jcxz foo} +to + +@smallexample + jcxz cx_zero + jmp cx_nonzero +cx_zero: jmp foo +cx_nonzero: +@end smallexample + +@node i386-Float +@section Floating Point + +@cindex i386 floating point +@cindex floating point, i386 +@cindex x86-64 floating point +@cindex floating point, x86-64 +All 80387 floating point types except packed BCD are supported. +(BCD support may be added without much difficulty). These data +types are 16-, 32-, and 64- bit integers, and single (32-bit), +double (64-bit), and extended (80-bit) precision floating point. +Each supported type has an instruction mnemonic suffix and a constructor +associated with it. Instruction mnemonic suffixes specify the operand's +data type. Constructors build these data types into memory. + +@cindex @code{float} directive, i386 +@cindex @code{single} directive, i386 +@cindex @code{double} directive, i386 +@cindex @code{tfloat} directive, i386 +@cindex @code{float} directive, x86-64 +@cindex @code{single} directive, x86-64 +@cindex @code{double} directive, x86-64 +@cindex @code{tfloat} directive, x86-64 +@itemize @bullet +@item +Floating point constructors are @samp{.float} or @samp{.single}, +@samp{.double}, and @samp{.tfloat} for 32-, 64-, and 80-bit formats. +These correspond to instruction mnemonic suffixes @samp{s}, @samp{l}, +and @samp{t}. @samp{t} stands for 80-bit (ten byte) real. The 80387 +only supports this format via the @samp{fldt} (load 80-bit real to stack +top) and @samp{fstpt} (store 80-bit real and pop stack) instructions. + +@cindex @code{word} directive, i386 +@cindex @code{long} directive, i386 +@cindex @code{int} directive, i386 +@cindex @code{quad} directive, i386 +@cindex @code{word} directive, x86-64 +@cindex @code{long} directive, x86-64 +@cindex @code{int} directive, x86-64 +@cindex @code{quad} directive, x86-64 +@item +Integer constructors are @samp{.word}, @samp{.long} or @samp{.int}, and +@samp{.quad} for the 16-, 32-, and 64-bit integer formats. The +corresponding instruction mnemonic suffixes are @samp{s} (single), +@samp{l} (long), and @samp{q} (quad). As with the 80-bit real format, +the 64-bit @samp{q} format is only present in the @samp{fildq} (load +quad integer to stack top) and @samp{fistpq} (store quad integer and pop +stack) instructions. +@end itemize + +Register to register operations should not use instruction mnemonic suffixes. +@samp{fstl %st, %st(1)} will give a warning, and be assembled as if you +wrote @samp{fst %st, %st(1)}, since all register to register operations +use 80-bit floating point operands. (Contrast this with @samp{fstl %st, mem}, +which converts @samp{%st} from 80-bit to 64-bit floating point format, +then stores the result in the 4 byte location @samp{mem}) + +@node i386-SIMD +@section Intel's MMX and AMD's 3DNow! SIMD Operations + +@cindex MMX, i386 +@cindex 3DNow!, i386 +@cindex SIMD, i386 +@cindex MMX, x86-64 +@cindex 3DNow!, x86-64 +@cindex SIMD, x86-64 + +@code{@value{AS}} supports Intel's MMX instruction set (SIMD +instructions for integer data), available on Intel's Pentium MMX +processors and Pentium II processors, AMD's K6 and K6-2 processors, +Cyrix' M2 processor, and probably others. It also supports AMD's 3DNow!@: +instruction set (SIMD instructions for 32-bit floating point data) +available on AMD's K6-2 processor and possibly others in the future. + +Currently, @code{@value{AS}} does not support Intel's floating point +SIMD, Katmai (KNI). + +The eight 64-bit MMX operands, also used by 3DNow!, are called @samp{%mm0}, +@samp{%mm1}, ... @samp{%mm7}. They contain eight 8-bit integers, four +16-bit integers, two 32-bit integers, one 64-bit integer, or two 32-bit +floating point values. The MMX registers cannot be used at the same time +as the floating point stack. + +See Intel and AMD documentation, keeping in mind that the operand order in +instructions is reversed from the Intel syntax. + +@node i386-16bit +@section Writing 16-bit Code + +@cindex i386 16-bit code +@cindex 16-bit code, i386 +@cindex real-mode code, i386 +@cindex @code{code16gcc} directive, i386 +@cindex @code{code16} directive, i386 +@cindex @code{code32} directive, i386 +@cindex @code{code64} directive, i386 +@cindex @code{code64} directive, x86-64 +While @code{@value{AS}} normally writes only ``pure'' 32-bit i386 code +or 64-bit x86-64 code depending on the default configuration, +it also supports writing code to run in real mode or in 16-bit protected +mode code segments. To do this, put a @samp{.code16} or +@samp{.code16gcc} directive before the assembly language instructions to +be run in 16-bit mode. You can switch @code{@value{AS}} back to writing +normal 32-bit code with the @samp{.code32} directive. + +@samp{.code16gcc} provides experimental support for generating 16-bit +code from gcc, and differs from @samp{.code16} in that @samp{call}, +@samp{ret}, @samp{enter}, @samp{leave}, @samp{push}, @samp{pop}, +@samp{pusha}, @samp{popa}, @samp{pushf}, and @samp{popf} instructions +default to 32-bit size. This is so that the stack pointer is +manipulated in the same way over function calls, allowing access to +function parameters at the same stack offsets as in 32-bit mode. +@samp{.code16gcc} also automatically adds address size prefixes where +necessary to use the 32-bit addressing modes that gcc generates. + +The code which @code{@value{AS}} generates in 16-bit mode will not +necessarily run on a 16-bit pre-80386 processor. To write code that +runs on such a processor, you must refrain from using @emph{any} 32-bit +constructs which require @code{@value{AS}} to output address or operand +size prefixes. + +Note that writing 16-bit code instructions by explicitly specifying a +prefix or an instruction mnemonic suffix within a 32-bit code section +generates different machine instructions than those generated for a +16-bit code segment. In a 32-bit code section, the following code +generates the machine opcode bytes @samp{66 6a 04}, which pushes the +value @samp{4} onto the stack, decrementing @samp{%esp} by 2. + +@smallexample + pushw $4 +@end smallexample + +The same code in a 16-bit code section would generate the machine +opcode bytes @samp{6a 04} (i.e., without the operand size prefix), which +is correct since the processor default operand size is assumed to be 16 +bits in a 16-bit code section. + +@node i386-Bugs +@section AT&T Syntax bugs + +The UnixWare assembler, and probably other AT&T derived ix86 Unix +assemblers, generate floating point instructions with reversed source +and destination registers in certain cases. Unfortunately, gcc and +possibly many other programs use this reversed syntax, so we're stuck +with it. + +For example + +@smallexample + fsub %st,%st(3) +@end smallexample +@noindent +results in @samp{%st(3)} being updated to @samp{%st - %st(3)} rather +than the expected @samp{%st(3) - %st}. This happens with all the +non-commutative arithmetic floating point operations with two register +operands where the source register is @samp{%st} and the destination +register is @samp{%st(i)}. + +@node i386-Arch +@section Specifying CPU Architecture + +@cindex arch directive, i386 +@cindex i386 arch directive +@cindex arch directive, x86-64 +@cindex x86-64 arch directive + +@code{@value{AS}} may be told to assemble for a particular CPU +(sub-)architecture with the @code{.arch @var{cpu_type}} directive. This +directive enables a warning when gas detects an instruction that is not +supported on the CPU specified. The choices for @var{cpu_type} are: + +@multitable @columnfractions .20 .20 .20 .20 +@item @samp{i8086} @tab @samp{i186} @tab @samp{i286} @tab @samp{i386} +@item @samp{i486} @tab @samp{i586} @tab @samp{i686} @tab @samp{pentium} +@item @samp{pentiumpro} @tab @samp{pentiumii} @tab @samp{pentiumiii} @tab @samp{pentium4} +@item @samp{prescott} @tab @samp{nocona} @tab @samp{core} @tab @samp{core2} +@item @samp{corei7} @tab @samp{l1om} +@item @samp{k6} @tab @samp{k6_2} @tab @samp{athlon} @tab @samp{k8} +@item @samp{amdfam10} +@item @samp{generic32} @tab @samp{generic64} +@item @samp{.mmx} @tab @samp{.sse} @tab @samp{.sse2} @tab @samp{.sse3} +@item @samp{.ssse3} @tab @samp{.sse4.1} @tab @samp{.sse4.2} @tab @samp{.sse4} +@item @samp{.avx} @tab @samp{.vmx} @tab @samp{.smx} @tab @samp{.xsave} +@item @samp{.aes} @tab @samp{.pclmul} @tab @samp{.fma} @tab @samp{.movbe} +@item @samp{.ept} @tab @samp{.clflush} +@item @samp{.3dnow} @tab @samp{.3dnowa} @tab @samp{.sse4a} @tab @samp{.sse5} +@item @samp{.syscall} @tab @samp{.rdtscp} @tab @samp{.svme} @tab @samp{.abm} +@item @samp{.padlock} +@end multitable + +Apart from the warning, there are only two other effects on +@code{@value{AS}} operation; Firstly, if you specify a CPU other than +@samp{i486}, then shift by one instructions such as @samp{sarl $1, %eax} +will automatically use a two byte opcode sequence. The larger three +byte opcode sequence is used on the 486 (and when no architecture is +specified) because it executes faster on the 486. Note that you can +explicitly request the two byte opcode by writing @samp{sarl %eax}. +Secondly, if you specify @samp{i8086}, @samp{i186}, or @samp{i286}, +@emph{and} @samp{.code16} or @samp{.code16gcc} then byte offset +conditional jumps will be promoted when necessary to a two instruction +sequence consisting of a conditional jump of the opposite sense around +an unconditional jump to the target. + +Following the CPU architecture (but not a sub-architecture, which are those +starting with a dot), you may specify @samp{jumps} or @samp{nojumps} to +control automatic promotion of conditional jumps. @samp{jumps} is the +default, and enables jump promotion; All external jumps will be of the long +variety, and file-local jumps will be promoted as necessary. +(@pxref{i386-Jumps}) @samp{nojumps} leaves external conditional jumps as +byte offset jumps, and warns about file-local conditional jumps that +@code{@value{AS}} promotes. +Unconditional jumps are treated as for @samp{jumps}. + +For example + +@smallexample + .arch i8086,nojumps +@end smallexample + +@node i386-Notes +@section Notes + +@cindex i386 @code{mul}, @code{imul} instructions +@cindex @code{mul} instruction, i386 +@cindex @code{imul} instruction, i386 +@cindex @code{mul} instruction, x86-64 +@cindex @code{imul} instruction, x86-64 +There is some trickery concerning the @samp{mul} and @samp{imul} +instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding +multiplies (base opcode @samp{0xf6}; extension 4 for @samp{mul} and 5 +for @samp{imul}) can be output only in the one operand form. Thus, +@samp{imul %ebx, %eax} does @emph{not} select the expanding multiply; +the expanding multiply would clobber the @samp{%edx} register, and this +would confuse @code{@value{GCC}} output. Use @samp{imul %ebx} to get the +64-bit product in @samp{%edx:%eax}. + +We have added a two operand form of @samp{imul} when the first operand +is an immediate mode expression and the second operand is a register. +This is just a shorthand, so that, multiplying @samp{%eax} by 69, for +example, can be done with @samp{imul $69, %eax} rather than @samp{imul +$69, %eax, %eax}. + |