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diff --git a/contrib/binutils/gas/doc/c-i386.texi b/contrib/binutils/gas/doc/c-i386.texi deleted file mode 100644 index f0047f93825a..000000000000 --- a/contrib/binutils/gas/doc/c-i386.texi +++ /dev/null @@ -1,755 +0,0 @@ -@c Copyright 1991, 1992, 1993, 1994, 1995, 1997, 1998, 1999, 2000, 2001 -@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 i80306 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-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. -@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 instructions with more than one -source operand, 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 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{cdo} --- 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. - -@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} (ie. 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 -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{pentium4} @tab @samp{k6} @tab @samp{athlon} -@item @samp{sledgehammer} -@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, 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}. - |