What Does It Mean When Mov Moves A Register In Parentheses()
General Information [edit | edit source]
Examples in this article are created using the AT&T assembly syntax used in GNU Every bit. The main advantage of using this syntax is its compatibility with the GCC inline associates syntax. However, this is non the merely syntax that is used to represent x86 operations. For example, NASM uses a different syntax to represent assembly mnemonics, operands and addressing modes, as do some High-Level Assemblers. The AT&T syntax is the standard on Unix-similar systems but some assemblers apply the Intel syntax, or can, similar GAS itself, have both. Encounter X86 assembly linguistic communication Syntax for a comparative table.
GAS instructions generally have the form mnemonic source, destination. For instance, the post-obit mov instruction:
This will move the hexadecimal value five into the register al.
Functioning Suffixes [edit | edit source]
GAS associates instructions are mostly suffixed with the letters "b", "s", "w", "l", "q" or "t" to determine what size operand is being manipulated.
-
b= byte (8 bit). -
s= unmarried (32-fleck floating point). -
west= word (sixteen bit). -
l= long (32 bit integer or 64-bit floating point). -
q= quad (64 flake). -
t= x bytes (80-bit floating indicate).
If the suffix is non specified, and there are no retentivity operands for the instruction, GAS infers the operand size from the size of the destination register operand (the last operand).
Prefixes [edit | edit source]
When referencing a register, the register needs to exist prefixed with a "%". Abiding numbers need to be prefixed with a "$".
Accost operand syntax [edit | edit source]
There are up to 4 parameters of an address operand that are presented in the syntax segment:displacement(base of operations annals, index register, scale cistron). This is equivalent to segment:[base register + deportation + index annals * scale factor] in Intel syntax.
The base, index and displacement components tin can be used in any combination, and every component can exist omitted; omitted components are excluded from the calculation to a higher place[i] [2].
movl -8 ( %ebp , %edx , 4 ), %eax # Full example: load *(ebp + (edx * 4) - 8) into eax movl -4 ( %ebp ), %eax # Typical example: load a stack variable into eax movl ( %ecx ), %edx # No alphabetize: copy the target of a pointer into a register leal eight (, %eax , four ), %eax # Arithmetic: multiply eax past 4 and add 8 leal ( %edx , %eax , 2 ), %eax # Arithmetics: multiply eax by 2 and add edx Introduction [edit | edit source]
This section is written as a short introduction to GAS. GAS is office of the GNU Project, which gives it the following dainty properties:
- It is bachelor on many operating systems.
- It interfaces nicely with the other GNU programming tools, including the GNU C compiler (gcc) and GNU linker (ld).
If you lot are using a computer with the Linux operating system, chances are you already take GAS installed on your system. If you are using a computer with the Windows operating system, yous can install GAS and other useful programming utilities by installing Cygwin or Mingw. The residuum of this introduction assumes you take installed GAS and know how to open a command-line interface and edit files.
Generating assembly [edit | edit source]
Since assembly linguistic communication corresponds directly to the operations a CPU performs, a advisedly written assembly routine may be able to run much faster than the same routine written in a higher-level language, such as C. On the other hand, associates routines typically accept more effort to write than the equivalent routine in C. Thus, a typical method for apace writing a program that performs well is to get-go write the program in a high-level language (which is easier to write and debug), and then rewrite selected routines in associates linguistic communication (which performs ameliorate). A practiced first step to rewriting a C routine in associates language is to utilize the C compiler to automatically generate the associates language. Non simply does this requite you an associates file that compiles correctly, merely it also ensures that the assembly routine does exactly what you intended it to.[iii]
Nosotros will now utilise the GNU C compiler to generate assembly code, for the purposes of examining the GAS assembly linguistic communication syntax.
Here is the classic "Howdy, world" programme, written in C:
#include <stdio.h> int main ( void ) { printf ( "Hello, earth! \n " ); render 0 ; } Save that in a file chosen "hello.c", then blazon at the prompt:
gcc -o hello_c hello.c
This should compile the C file and create an executable file called "hello_c". If yous go an error, brand sure that the contents of "how-do-you-do.c" are correct.
Now you should be able to type at the prompt:
./hello_c
and the program should print "Hello, world!" to the console.
Now that nosotros know that "howdy.c" is typed in correctly and does what we want, allow'south generate the equivalent 32-chip x86 associates language. Type the following at the prompt:
gcc -Due south -m32 hello.c
This should create a file called "hullo.south" (".southward" is the file extension that the GNU system gives to assembly files). On more recent 64-bit systems, the 32-fleck source tree may not exist included, which volition cause a "bits/predefs.h fatal error"; you may supplant the -m32 gcc directive with an -m64 directive to generate 64-bit assembly instead. To compile the assembly file into an executable, type:
gcc -o hello_asm -m32 hello.s
(Notation that gcc calls the assembler (every bit) and the linker (ld) for united states.) Now, if you type the following at the prompt:
./hello_asm
this program should also impress "Hello, world!" to the panel. Not surprisingly, it does the aforementioned thing as the compiled C file.
Let's accept a wait at what is within "hello.due south":
.file "hello.c" .def ___main ; .scl 2; .type 32; .endef .text LC0: .ascii "Hullo, world!\12\0" .globl _main .def _main ; .scl two; .type 32; .endef _main: pushl %ebp movl %esp , %ebp subl $eight , %esp andl $-sixteen , %esp movl $0 , %eax movl %eax , -four ( %ebp ) movl -4 ( %ebp ), %eax call __alloca call ___main movl $LC0 , ( %esp ) call _printf movl $0 , %eax leave ret .def _printf ; .scl 2; .type 32; .endef The contents of "hi.s" may vary depending on the version of the GNU tools that are installed; this version was generated with Cygwin, using gcc version 3.3.1.
The lines outset with periods, like .file, .def, or .ascii are assembler directives — commands that tell the assembler how to assemble the file. The lines beginning with some text followed by a colon, like _main:, are labels, or named locations in the code. The other lines are associates instructions.
The .file and .def directives are for debugging. We tin leave them out:
.text LC0: .ascii "Howdy, world!\12\0" .globl _main _main: pushl %ebp movl %esp , %ebp subl $viii , %esp andl $-xvi , %esp movl $0 , %eax movl %eax , -4 ( %ebp ) movl -4 ( %ebp ), %eax call __alloca call ___main movl $LC0 , ( %esp ) call _printf movl $0 , %eax leave ret "hullo.s" line-by-line [edit | edit source]
This line declares the offset of a section of code. You can name sections using this directive, which gives y'all fine-grained control over where in the executable the resulting machine code goes, which is useful in some cases, similar for programming embedded systems. Using. .text past itself tells the assembler that the following code goes in the default department, which is sufficient for near purposes.
LC0: .ascii "Hi, world!\12\0" This code declares a label, then places some raw ASCII text into the programme, starting at the label'south location. The \12 specifies a line-feed character, while the \0 specifies a zip character at the terminate of the string; C routines mark the end of strings with null characters, and since we are going to call a C string routine, we need this character hither. (Note! String in C is an array of datatype char (char[]) and does non exist in whatsoever other form, merely because i would understand strings as a single entity from the majority of programming languages, it is clearer to express it this way.)
This line tells the assembler that the label _main is a global label, which allows other parts of the program to see information technology. In this case, the linker needs to be able to encounter the _main characterization, since the startup code with which the program is linked calls _main every bit a subroutine.
This line declares the _main characterization, mark the place that is called from the startup lawmaking.
pushl %ebp movl %esp , %ebp subl $8 , %esp These lines save the value of EBP on the stack, then motion the value of ESP into EBP, so decrease 8 from ESP. Annotation that pushl automatically decremented ESP past the advisable length. The l on the stop of each opcode indicates that we want to utilize the version of the opcode that works with long (32-fleck) operands; unremarkably the assembler is able to work out the correct opcode version from the operands, but only to exist condom, information technology's a good idea to include the l, west, b, or other suffix. The per centum signs designate register names, and the dollar sign designates a literal value. This sequence of instructions is typical at the first of a subroutine to salve space on the stack for local variables; EBP is used as the base annals to reference the local variables, and a value is subtracted from ESP to reserve infinite on the stack (since the Intel stack grows from higher memory locations to lower ones). In this case, eight bytes have been reserved on the stack. We shall see why this infinite is needed later.
This lawmaking ands ESP with 0xFFFFFFF0, aligning the stack with the next lowest 16-byte purlieus. An examination of Mingw'due south source code reveals that this may be for SIMD instructions appearing in the _main routine, which operate only on aligned addresses. Since our routine doesn't comprise SIMD instructions, this line is unnecessary.
movl $0 , %eax movl %eax , -4 ( %ebp ) movl -4 ( %ebp ), %eax This code moves zero into EAX, and then moves EAX into the memory location EBP - iv, which is in the temporary space we reserved on the stack at the beginning of the procedure. Then it moves the retentiveness location EBP - iv back into EAX; clearly, this is not optimized code. Note that the parentheses indicate a retention location, while the number in front of the parentheses indicates an outset from that memory location.
telephone call __alloca call ___main These functions are part of the C library setup. Since we are calling functions in the C library, we probably need these. The exact operations they perform vary depending on the platform and the version of the GNU tools that are installed.
movl $LC0 , ( %esp ) call _printf This code (finally!) prints our message. First, it moves the location of the ASCII string to the top of the stack. Information technology seems that the C compiler has optimized a sequence of popl %eax; pushl $LC0 into a unmarried move to the elevation of the stack. Then, it calls the _printf subroutine in the C library to print the message to the console.
This line stores zero, our render value, in EAX. The C calling convention is to store return values in EAX when exiting a routine.
This line, typically establish at the end of subroutines, frees the space saved on the stack by copying EBP into ESP, then popping the saved value of EBP dorsum to EBP.
This line returns command to the calling procedure past popping the saved teaching arrow from the stack.
Communicating straight with the operating system [edit | edit source]
Annotation that we merely have to call the C library setup routines if we need to call functions in the C library, like printf(). Nosotros could avoid calling these routines if we instead communicate directly with the operating system. The disadvantage of communicating directly with the operating system is that we lose portability; our lawmaking will be locked to a specific operating organisation. For instructional purposes, though, let'south expect at how ane might do this under Windows. Here is the C source code, compilable under Mingw or Cygwin:
#include <windows.h> int main ( void ) { LPSTR text = "Hello, world! \n " ; DWORD charsWritten ; HANDLE hStdout ; hStdout = GetStdHandle ( STD_OUTPUT_HANDLE ); WriteFile ( hStdout , text , 14 , & charsWritten , Zippo ); render 0 ; } Ideally, you'd want check the render codes of "GetStdHandle" and "WriteFile" to make sure they are working correctly, just this is sufficient for our purposes. Here is what the generated assembly looks like:
.file "hello2.c" .def ___main ; .scl 2; .blazon 32; .endef .text LC0: .ascii "Hello, world!\12\0" .globl _main .def _main ; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp , %ebp subl $4 , %esp andl $-16 , %esp movl $0 , %eax movl %eax , -16 ( %ebp ) movl -16 ( %ebp ), %eax call __alloca call ___main movl $LC0 , -4 ( %ebp ) movl $-eleven , ( %esp ) telephone call _GetStdHandle@4 subl $four , %esp movl %eax , -12 ( %ebp ) movl $0 , 16 ( %esp ) leal -8 ( %ebp ), %eax movl %eax , 12 ( %esp ) movl $14 , 8 ( %esp ) movl -iv ( %ebp ), %eax movl %eax , 4 ( %esp ) movl -12 ( %ebp ), %eax movl %eax , ( %esp ) phone call _WriteFile@20 subl $20 , %esp movl $0 , %eax leave ret Fifty-fifty though we never apply the C standard library, the generated code initializes information technology for united states of america. Likewise, there is a lot of unnecessary stack manipulation. We tin can simplify:
.text LC0: .ascii "Hi, globe!\12\0" .globl _main _main: pushl %ebp movl %esp , %ebp subl $four , %esp pushl $-11 phone call _GetStdHandle@4 pushl $0 leal -4 ( %ebp ), %ebx pushl %ebx pushl $14 pushl $LC0 pushl %eax call _WriteFile@20 movl $0 , %eax leave ret Analyzing line-by-line:
pushl %ebp movl %esp , %ebp subl $4 , %esp We relieve the old EBP and reserve four bytes on the stack, since the call to WriteFile needs somewhere to store the number of characters written, which is a 4-byte value.
pushl $-eleven call _GetStdHandle@4 Nosotros push the constant value STD_OUTPUT_HANDLE (-xi) to the stack and call GetStdHandle. The returned handle value is in EAX.
pushl $0 leal -four ( %ebp ), %ebx pushl %ebx pushl $14 pushl $LC0 pushl %eax call _WriteFile@xx We button the parameters to WriteFile and phone call it. Notation that the Windows calling convention is to push button the parameters from right-to-left. The load-effective-accost (lea) didactics adds -4 to the value of EBP, giving the location we saved on the stack for the number of characters printed, which we store in EBX and so push onto the stack. Also annotation that EAX all the same holds the return value from the GetStdHandle call, and then we just button it directly.
Hither nosotros prepare our plan'due south render value and restore the values of EBP and ESP using the get out didactics.
Caveats [edit | edit source]
From The GAS manual'due south AT&T Syntax Bugs section:
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 peradventure many other programs use this reversed syntax, and then we're stuck with information technology.
For instance
results in %st(3) being updated to %st - %st(3) rather than the expected %st(iii) - %st. This happens with all the not-commutative arithmetic floating point operations with 2 register operands where the source register is %st and the destination register is %st(i).
Note that even objdump -d -Thousand intel still uses reversed opcodes, and then utilise a different disassembler to bank check this. Run across http://bugs.debian.org/372528 for more than info.
Boosted GAS reading [edit | edit source]
Y'all tin read more about GAS at the GNU GAS documentation page:
https://sourceware.org/binutils/docs/equally/
- X86 Disassembly/Calling Conventions
Quick reference [edit | edit source]
| Instruction | Meaning |
|---|---|
movq %rax , %rbx | rbx ≔rax |
movq $123 , %rax | rax ≔ |
movq %rsi , -16 ( %rbp ) | mem [ rbp-xvi ] ≔rsi |
subq $10 , %rbp | rbp ≔rbp − ten |
cmpl %eax %ebx | Compare ebx with eax and set flags appropriately. If eax =ebx, the cipher flag is set. |
jmp location | unconditional leap |
je location | jump to location if equal flag is set |
jg , jge , jl , jle , jne , … | >, ≥, <, ≤, ≠, … |
Notes [edit | edit source]
- ↑ If
segmentis non specified, as nigh always, it is assumed to beds, unlessbase registerisesporebp; in this case, the address is causeless to be relative toss - ↑ If
index registeris missing, the pointlesscalibration factormust be omitted as well. - ↑ This assumes that the compiler has no bugs and, more chiefly, that the lawmaking you wrote correctly implements your intent. Note besides that compilers can sometimes rearrange the sequence of depression-level operations in order to optimize the lawmaking; this preserves the overall semantics of your code but means the assembly educational activity flow may non friction match up exactly with your algorithm steps.
What Does It Mean When Mov Moves A Register In Parentheses(),
Source: https://en.wikibooks.org/wiki/X86_Assembly/GAS_Syntax
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