[Assembly] compile program development in Linux

I. Introduction
The code written in assembly language in Linux has two different forms.

1) complete assembly code

It means that the entire program is all written in assembly language. Despite the complete compilation code, the compilation tools on the Linux platform also absorb the advantages of the C language, so that programmers can use the # include, # ifdef, and other pre-processing commands, and can simplify the Code through macro definition.

2) Embedded Assembly Code

It refers to the assembly code snippets that can be embedded into C language programs. Although ansi c language standards do not provide relevant provisions on Embedded Assembly Code, all the actually used C compilers have been expanded in this regard, of course, this includes GCC on the Linux platform.

Ii. Classification of assembly languages in Linux

The vast majority of Linux programmers have previously only been familiar with DOS/Windows assembly languages. These Assembly codes are intel-style. However, in UNIX and Linux systems, the at&t format is mostly used. The two are quite different in syntax format:

1) In at&t Assembly format, Register names must be prefixed with '%'; pushl % eax

In Intel assembly format, Register names do not need to be prefixed. Push eax

2) In at&t Assembly format, the prefix '$' is used to represent an immediate operand. pushl $1

In Intel assembly format, the representation of the immediate number does not contain any prefix. Push 1

3) the source and target operands in at&t and Intel are in the opposite position.

In Intel assembly format, the destination operand is on the left of the source operand; addl $1, % eax

In at&t Assembly format, the target operand is on the right of the source operand. Add eax, 1

4) In at&t Assembly format, the length of an operand is determined by the last letter of the operator. The suffixes 'B', 'w', and 'l' indicate that the operands are bytes, 8 bits), words (word, 16 bits), and long words (long, 32 bits );

In Intel assembly format, the length of the operand is expressed by prefix such as "Byte PTR" and "word PTR. For example:
At&t format: movb Val, % Al

Intel format: mov Al, byte PTR Val

5) in at&t Assembly format, the prefix '*' must be added before the operands of absolute transfer and call commands (jump/call), but not in Intel format.

6) The operation code of the remote Transfer Instruction and remote sub-call instruction is "ljump" and "lcall" in the at&t Assembly format ", in Intel assembly format, it is "JMP far" and "Call far", that is:
 
At&t format: ljump $ section, $ offset

Lcall $ section, $ offset

Intel format: Call far section: Offset

JMP far section: Offset

The corresponding remote return command is:
At&t format: LRET $ stack_adjust

Intel format: Ret far stack_adjust

7) in at&t Assembly format, the addressing mode of memory operands is Section: disp (base, index, scale)

In Intel assembly format, the addressing mode of memory operands is: Section: [base + Index * scale + disp]

Because Linux uses 32-bit linear addresses in protection mode, the following address calculation method is used instead of considering the segment base address and offset when calculating the address: disp + base + Index * Scale

Hello World

Linux is a 32-bit operating system running in protected mode. It adopts the flat memory mode. Currently, binary code in ELF format is the most commonly used. An executable program in the ELF format is generally divided into the following parts :. text ,. data and. BSS, where. text is a read-only code area ,. data is a readable and writable data area, while. BSS is a readable and writable data zone without initialization. Code and data zones are collectively called sections in elf. You can use other standard sections or add custom sections as needed.
Section, but an elf executable program should have at least one. text section. The following is our first assembler, In the at&t assembly language format:

Example 1. at&t format

# Hello. S. Data # Data Segment declaration MSG:. String "Hello, world! \ N "# the string to be output Len =. -MSG # String Length. text # code snippet declaration. global _ start # specify the entry function _ start: # display a string movl $ Len, % edX # parameter 3: String Length movl $ MSG, % ECx # parameter 2: movl $1, % EBX # parameter 1: file descriptor (stdout) movl $4, % eax # system call number (sys_write) int $0x80 # Call kernel function # exit program movl $0, % EBX # parameter 1: Exit code movl $1, % eax # system call number (sys_exit) int $0x80 # Call the kernel function

When I first came into contact with at&t-formatted assembly code, many programmers thought it was too obscure. It doesn't matter. On the Linux platform, you can also use the Intel format to compile the assembly program:

Example 2. Intel format

; Hello. ASM section. data; data segment declaration msg db "Hello, world! ", 0xa; string to be output Len equ $-MSG; String Length section. text; the Code segment declares global _ start; specifies the entry function _ start:; displays a string mov edX, Len on the screen; parameter 3: String Length mov ECx, MSG; parameter 2: the string mov EBX, 1 to be displayed. Parameter 1: file descriptor (stdout) mov eax, 4; system call number (sys_write) int 0x80; kernel function is called; exit program mov EBX, 0; parameter 1: Exit code mov eax, 1; system call number (sys_exit) int 0x80; call Kernel Function

Although the syntax used by the above two assembler programs is completely different, the function is to call the sys_write provided by the Linux kernel to display a string, and then call sys_exit to exit the program. In the Linux kernel source File Include/asm-i386/unistd. H, you can find the definitions of all system calls.

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Iv. Linux assembly tools

There are many types of assembler tools on Linux, but like DOS/Windows, the most basic tools are assembler, connector, and debugger.

1) Assembler
The assembler is used to convert source programs written in assembly languages into binary-format target codes. The standard Assembler for Linux is
Gas, which is the background Assembly Tool on which GCC depends, is usually included in the binutils software package. Gas uses the standard at&t Assembly syntax and can be used to compile programs written in at&t format:

As-O hello. O hello. s

Another commonly used assembler on Linux is NASM. It provides good macro commands and supports a considerable number of target code formats, including bin and. out, coff, elf, and RDF. NASM uses a manually compiled syntax analyzer, so the execution speed is much faster than that of gas. More importantly, it uses Intel assembly syntax, it can be used to compile assembler programs written in Intel syntax format:

NASM-F elf hello. ASM

2) linker

The target code generated by the assembler cannot run directly on the computer. It must be processed by the linker to generate executable code. The linker is usually used to connect multiple target codes into one executable code. In this way, the entire program can be divided into several modules for separate development before they can be combined into an application.

Linux uses LD as a standard linking program, which is also included in the binutils package. The assembler successfully passes
After NASM is compiled and the target code is generated, you can use LD to link it to an executable program:

LD-S-O hello. o

3) Debugger

In Linux, You can debug assembly code by using a general debugger such as GDB and DDD, or by using an ALD (assembly language debugger) Specially Used to debug assembly code ).

From the perspective of debugging, the advantage of using gas is that you can include the symbol table in the generated target code ), in this way, you can use GDB and DDD for source code-level debugging. To include the symbol table in the generated executable program, you can compile and link the table in the following way:

As -- maid-O hello. O hello. s
LD-O hello. o

When running the as command, the parameter-ststabs can tell the assembler to add a symbol table to the generated target code. Note that the-S parameter is not added when the LD command is used for link, otherwise, the symbol table in the target code will be deleted during the link.
Debugging assembly code in GDB and DDD is the same as debugging C-language code. You can set breakpoints to interrupt program running and view the current values of variables and registers, you can also track the code in one step.

Assembly programmers usually face some harsh software and hardware environments. The short and concise ALD may better meet the actual needs. Therefore, the following describes how to use the ALD to debug the assembly program. First, run the ALD command in the command line mode to start the debugger. The parameter of this command is the executable program to be debugged:

ALD hello
Assembly Language debugger 0.1.3
Copyright (c) 2000-2002 Patrick alken
Hello: Elf intel 80386 (32 bit), LSB, executable, Version 1 (current)
Loading debugging symbols... (15 symbols loaded)
ALD>

When the ALD prompt appears, run the disassemble command to decompile the code segment:

ALD> disassemble-S. Text
Disconfiguring section. Text (0x08048074-0x08048096)
08048074 ba0f000000 mov edX, 0xf
08048079 b998900408 mov ECx, 0x8049098
0804807e bb0000000 mov EBX, 0x1
08048083 b804000000 mov eax, 0x4
08048088 CD80 int 0x80
0804808a bb00000000 mov EBX, 0x0
0804808f b80000000 mov eax, 0x1
08048094 CD80 int 0x80

The first column of the output information is the address code corresponding to the command. It can be used to set the breakpoint during program execution:

ALD> Break 0x08048088
Breakpoint 1 set for 0x08048088

After the breakpoint is set, run the Run Command to run the program. When a breakpoint occurs, the system automatically suspends the program and displays the current values of all registers:

ALD> RUN
Starting program: Hello
Breakpoint 1 encountered at 0x08048088
Eax = 0x00000004 EBX = 0x00000001 ECx = 0x08049098 edX = 0x0000000f
ESP = 0xbffff6c0 EBP = 0x00000000 ESI = 0x00000000 EDI = 0x00000000
DS = 0x0000002b es = 0x0000002b FS = 0x00000000 GS = 0x00000000
Ss = 0x0000002b cs = 0x00000023 EIP = 0x08048088 eflags = 0x00000246
Flags: pf ZF if
08048088 CD80 int 0x80

To debug the assembly code in one step, run the following command:

ALD> next
Hello, world!
Eax = 0x0000000f EBX = 0x00000000 ECx = 0x08049098 edX = 0x0000000f
ESP = 0xbffff6c0 EBP = 0x00000000 ESI = 0x00000000 EDI = 0x00000000
DS = 0x0000002b es = 0x0000002b FS = 0x00000000 GS = 0x00000000
Ss = 0x0000002b cs = 0x00000023 EIP = 0x0804808f eflags = 0x00000346
Flags: pf zf tf if
0804808f b80000000 mov eax, 0x1

If you want to obtain a detailed list of all the Debugging commands supported by ALD, you can use the help command: Ald> help
Commands may be abbreviated.
If a blank command is entered, the last command is repeated.
Type 'help <command> 'for more specific information on <command>.
General commands
Attach clear continue detach disassemble
Enter examine file help load
Next quit register run set
Step unload window write
Breakpoint related commands
Break Delete disable enable ignore
Lbreak tbreak

V. System Call

Even the simplest assembler program will inevitably use operations such as input, output, and exit. To perform these operations, you must call the services provided by the operating system, that is, system calls. Unless your program only performs addition, subtraction, multiplication, division, and other mathematical operations, it will be difficult to avoid using system calls. In fact, except for different system calls, assembly programming of various operating systems is often very similar.

There are two methods to use system calling on Linux:

1> use the encapsulated C library (libc)

2> directly calling through assembly.

The method of using the Linux kernel service is the most efficient way to directly call the system call through the assembly language, because the generated program does not need to be linked to any library, but directly communicates with the kernel.

Like Dos, system calls in Linux are also implemented through interruptions (INT 0x80. When executing the int 80 command, the register eax stores the function number of the system call, and the parameters passed to the system call must be placed in the registers EBX, ECx, EDX, ESI, in EDI, after the system call is completed, the return value can be obtained in the register eax.

All system call function numbers can be found in the file/usr/include/bits/syscall. h. For ease of use, they are defined using macros such as sys _ <Name>, such as sys_write and sys_exit. For example, the frequently used write function is defined as follows: ssize_t write (int fd, const void * Buf, size_t count );

The function is ultimately implemented through the sys_write system call. According to the above conventions, the parameters FB, Buf, and count exist in the registers EBX, ECx, and EDX respectively, while the system call number sys_write is placed in the register eax, after the int 0x80 command is executed, the returned value can be obtained from the register eax.

You may have discovered that at most five registers can be used to save parameters during system calls. Is the number of parameters called by all systems not greater than 5? Of course not. For example, the MMAP function has six parameters. These parameters must be passed to the system to call sys_mmap: void * MMAP (void * Start, size_t length, int Prot, int flags, int FD, off_t offset );

When the number of parameters required for a system call is greater than 5, when the int 0x80 command is executed, the system call function number still needs to be saved in the register eax, the difference is that all parameters should be placed in a contiguous memory area, and the pointer pointing to the memory area should be saved in the register EBX. After the system call is complete, the returned values are still stored in the register eax.

Because we only need a contiguous memory area to store system call parameters, we can use stacks to pass the parameters required for system calls just like common function calls. Note that Linux uses the C-language call mode, which means that all parameters must be pushed to the stack in the reverse order, that is, the last parameter is pushed to the stack first, the first parameter is then last written to the stack. If the stack is used to pass the parameters required by the system call, the current value of the stack pointer should also be copied to the Register EBX when the int 0x80 command is executed.

Vi. Command Line Parameters

In Linux, when an executable program is started through a command line, the required parameters are saved to the stack: argc first, next, the array argv pointing to the parameters of each command line, and finally the environment variable pointer data envp. When compiling an assembly language program, you often need to process these parameters. The following Code demonstrates how to process command line parameters in assembly code:

Example 3. process command line parameters

# Args. s. text. globl _ start: popl % ECx # argcvnext: popl % ECx # argv test % ECx, % ECx # NULL pointer indicates end of jzexit movl % ECx, % EBX xorl % edX, % edxstrlen: movb (% EBX), % Al Inc % edX Inc % EBX test % Al, % Al jnzstrlen movb $10,-1 (% EBX) movl $4, % eax # system call number (sys_write) movl $1, % EBX # file descriptor (stdout) int $0x80 jmpvnextexit: movl $1, % eax # system call number (sys_exit) xorl % EBX, % EBX # exit code int $0x80 RET

VII. GCC inline assembly

Although compiled programs run fast, the development speed is very slow and the efficiency is very low. If you only want to optimize key code segments, it may be better to embed Assembly commands into a C-language program to take full advantage of the respective features of the advanced language and assembly language. But in general, embedding Assembly statements in C code is much more complicated than the "pure" assembly language code, because it is necessary to solve how to allocate registers, and how to combine with the variables in C code.

GCC provides good support for inline assembly. The most basic format is __asm _ ("ASM statements ");

Example: __asm _ ("NOP ");

If you need to execute multiple Assembly statements at the same time, separate the statements with "\ n \ t,

Example: __asm _ ("pushl % eax \ n \ t"
"Movl $0, % eax \ n \ t"
"Popl % eax ");

The Assembly statements embedded in C code are hard to have nothing to do with other parts. Therefore, the complete inline assembly format must be used more often:

_ ASM _ ("ASM statements": outputs: Inputs: registers-modified );

The Assembly statement inserted into the C code is separated by ":". The first part is the assembly code, which is usually called the instruction department, the format is basically the same as that used in assembly languages. The command part is required, while the other part can be omitted based on the actual situation.

When embedding Assembly statements into C code, how to combine operands with variables in C code is a big problem. GCC uses the following method to solve this problem: the programmer provides specific instructions, and the use of registers only requires the "sample" and constraints, GCC and gas are responsible for how to combine registers and variables.

In the command department of the GCC inline assembly statement, the number prefixed with '%' (for example, % 0, % 1) indicates the "sample" operand of the register. When the instruction Department uses several sample operands, it indicates that several variables need to be combined with registers, so that GCC and gas will properly process the compilation and compilation according to the given constraints. Because the sample operand uses '%' as the prefix, two '%' should be added before the register name in case of specific registers to avoid confusion.

The output department is followed by the instruction Department. It is a condition that specifies how the output variable is combined with the sample operand. Each condition is called a "constraint" and can contain multiple constraints when necessary, separate them with commas. Each output constraint starts with the '=' sign, followed by a description of the operand type, and finally the constraint on how to combine with the variable. All registers or operands that combine with the operands described in the output part do not retain the content before execution after the embedded assembly code is executed, this is the basis for GCC in scheduling registers.

The output part is followed by the input part. The format of the input constraint is similar to that of the output constraint, but it does not contain the '=' sign. If a register is required for an input constraint, GCC allocates a register for it during preprocessing and inserts necessary commands to load the operands into the register. Registers or operands that are combined with the operands described in the input part are not reserved after the embedded assembly code is executed.

Sometimes in some operations, in addition to the registers used for data input and output, multiple registers are also used to save the intermediate calculation results, which will inevitably destroy the content of the original register. In the last part of the GCC inline assembly format, you can describe the registers that will produce side effects so that GCC can take appropriate measures.

The following is a simple example of inline assembly:

Example 4: inline assembly

/* Inline. C */INT main () {int A = 10, B = 0; _ ASM _ volatile _ ("movl % 1, % eax; \ n \ r "" movl % eax, % 0; ":" = r "(B)/* output */:" R "() /* input */: "% eax");/* unaffected REGISTERS */printf ("Result: % d, % d \ n",, B );}

The preceding Procedure assigns the value of variable A to variable B, which must be described as follows:
Variable B is the output operand, which is referenced by % 0, and variable A is the input operand, which is referenced by % 1.
Both the input and output operations use R constraints to store variables A and B in registers. The difference between an input constraint and an output constraint is that an output constraint has one constraint modifier '= '.
When using the register eax in an inline assembly statement, add two '%' before the register name, that is, % eax. In inline assembly, variables are identified by % 0, % 1, and so on. Any identifier with only one '%' is regarded as an operand rather than a register.
The last part of the inline assembly statement tells GCC that it will change the value in the register eax. GCC should not use this register to store any other value during processing.
Because variable B is specified as the output operand, after the inline assembly statement is executed, the saved value is updated.

The operands used in inline assembly start from the first constraint in the output part and start from 0. Each constraint is counted once. When the instruction Part references these operands, you only need to add '%' before the serial number as the prefix. Note that When referencing an operand, the instruction department of an inline assembly statement always uses it as a 32-bit long word, but the actual situation may need words or bytes, therefore, the correct qualifier should be specified in the constraints: