Define assembler

Prerequisite – Linker is a program in a system which helps to link object modules of a program into a single object file. It performs the process of linking. Linkers are also called as link editors. Linking is a process of collecting and maintaining piece of code and data into a single file. Linker also links a particular module into system library. It takes object modules from assembler as input and forms an executable file as output for the loader. Linking is performed at both compile time, when the source code is translated into machine code and load time, when the program is loaded into memory by the loader. Linking is performed at the last step in compiling a program. Source code -> compiler -> Assembler -> Object code -> Linker -> Executable file -> Loader Linking is of two types: 1. Static Linking – It is performed during the compilation of source program. Linking is performed before execution in static linking. It takes collection of relocatable object file and command-line arguments and generates a fully linked object file that can be loaded and run. Static linker performs two major tasks: • Symbol resolution – It associates each symbol reference with exactly one symbol definition .Every symbol has a predefined task. • Relocation – It relocates code and data section and modifies the symbol references to the relocated memory locations. The linker copies all library routines used in the program into executable image. As a result, it requires more memory space. As it...

While going through some C code having inline assembly I came across the .byte (with a Dot at the beginning) directive. On checking the assembly reference on web I found that it is used to reserve a byte in memory. But in the code there was no label before the statement. So I was wondering what is use of an unlabeled .byte directive or any other data storage directive for that matter. For e.g. if i code .byte 0x0a, how can i use it ? There are a few possibilities... here are a couple I can think of off the top of my head: • You could access it relative to a label that comes after the .byte directive. Example: .byte 0x0a label: mov (label - 1), %eax • Based on the final linked layout of the program, maybe the .byte directives will get executed as code. Normally you'd have a label in this case too, though... • Some assemblers don't support generating x86 instruction prefixes for operand size, etc. In code written for those assemblers, you'll often see something like: .byte 0x66 mov $12, %eax To make the assembler emit the code you want to have. .byte 0x66 is length-changing for mov $12, %eax, so your example will decode as mov $12, %ax / add %al,(%rax) or (%eax) in 32-bit mode. The 2nd instruction is the left-over 00 00 bytes of the 32-bit immediate that mov eax, imm16 doesn't consume. This is tricky for the hardware, too, and causes LCP pre-decode stalls on Intel CPUs (LCP = length-changing prefix). Minimal runnable example .byte spits out bytes wherever you are. Whether th...

asm asm-qualifiers ( AssemblerInstructions ) For the C language, the asm keyword is a GNU extension. When writing C code that can be compiled with -ansi and the -std options that select C dialects without GNU extensions, use __asm__ instead of asm (see asm is a standard keyword, but __asm__ can be used for code compiled with -fno-asm. Qualifiers volatile The optional volatile qualifier has no effect. All basic asm blocks are implicitly volatile. inline If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see asm). Parameters AssemblerInstructions This is a literal string that specifies the assembler code. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. You may place multiple assembler instructions together in a single asm string, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character (written as ‘ \n\t’). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment. Remarks Using extended asm (see asm. However, there are two situations where only basic asm can be used: • Extended asm statements have to be inside a C function, so to write in...

\( \newcommand\) No headers Assembler directives are directions to the assembler to take some action or change a setting. Assembler directives do not represent instructions, and are not translated into machine code. For this assembler, all directives begin with a “.” or “#” (the comment is a #), and the directive must exist on a separate line from any other assembler directive or assembler instruction. There are 4 assembler directives and the comment tag. • .text – The .text directive tells the assembler that the information that follows is program text (assembly instructions), and the translated machine code is to be written to the text segment of memory. • .data – The .data directive tells the assembler that information that follows is program data. The information following a .data instruction will be data values, and will be stored in the data segment. • .label – A label is an address in memory corresponding to either an instruction or data value. It is just a convenience so the programmer can reference an address by a name. It will be used as follow: .label name The label is a tag that can be referenced in place of an address in any assembly instruction that can take a label/adress. Labels and addresses can be used interchangeably. • .number – The number directive tells the assembler to set aside 2 bytes of memory for a data value, and to initialize the memory to the given value. It will often be used with the .label directive to set a label to a 2-byte memory value, ...

asm asm-qualifiers ( AssemblerInstructions ) For the C language, the asm keyword is a GNU extension. When writing C code that can be compiled with -ansi and the -std options that select C dialects without GNU extensions, use __asm__ instead of asm (see asm is a standard keyword, but __asm__ can be used for code compiled with -fno-asm. Qualifiers volatile The optional volatile qualifier has no effect. All basic asm blocks are implicitly volatile. inline If you use the inline qualifier, then for inlining purposes the size of the asm statement is taken as the smallest size possible (see asm). Parameters AssemblerInstructions This is a literal string that specifies the assembler code. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. You may place multiple assembler instructions together in a single asm string, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character (written as ‘ \n\t’). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment. Remarks Using extended asm (see asm. However, there are two situations where only basic asm can be used: • Extended asm statements have to be inside a C function, so to write in...

Prerequisite – Linker is a program in a system which helps to link object modules of a program into a single object file. It performs the process of linking. Linkers are also called as link editors. Linking is a process of collecting and maintaining piece of code and data into a single file. Linker also links a particular module into system library. It takes object modules from assembler as input and forms an executable file as output for the loader. Linking is performed at both compile time, when the source code is translated into machine code and load time, when the program is loaded into memory by the loader. Linking is performed at the last step in compiling a program. Source code -> compiler -> Assembler -> Object code -> Linker -> Executable file -> Loader Linking is of two types: 1. Static Linking – It is performed during the compilation of source program. Linking is performed before execution in static linking. It takes collection of relocatable object file and command-line arguments and generates a fully linked object file that can be loaded and run. Static linker performs two major tasks: • Symbol resolution – It associates each symbol reference with exactly one symbol definition .Every symbol has a predefined task. • Relocation – It relocates code and data section and modifies the symbol references to the relocated memory locations. The linker copies all library routines used in the program into executable image. As a result, it requires more memory space. As it...

\( \newcommand\) No headers Assembler directives are directions to the assembler to take some action or change a setting. Assembler directives do not represent instructions, and are not translated into machine code. For this assembler, all directives begin with a “.” or “#” (the comment is a #), and the directive must exist on a separate line from any other assembler directive or assembler instruction. There are 4 assembler directives and the comment tag. • .text – The .text directive tells the assembler that the information that follows is program text (assembly instructions), and the translated machine code is to be written to the text segment of memory. • .data – The .data directive tells the assembler that information that follows is program data. The information following a .data instruction will be data values, and will be stored in the data segment. • .label – A label is an address in memory corresponding to either an instruction or data value. It is just a convenience so the programmer can reference an address by a name. It will be used as follow: .label name The label is a tag that can be referenced in place of an address in any assembly instruction that can take a label/adress. Labels and addresses can be used interchangeably. • .number – The number directive tells the assembler to set aside 2 bytes of memory for a data value, and to initialize the memory to the given value. It will often be used with the .label directive to set a label to a 2-byte memory value, ...

While going through some C code having inline assembly I came across the .byte (with a Dot at the beginning) directive. On checking the assembly reference on web I found that it is used to reserve a byte in memory. But in the code there was no label before the statement. So I was wondering what is use of an unlabeled .byte directive or any other data storage directive for that matter. For e.g. if i code .byte 0x0a, how can i use it ? There are a few possibilities... here are a couple I can think of off the top of my head: • You could access it relative to a label that comes after the .byte directive. Example: .byte 0x0a label: mov (label - 1), %eax • Based on the final linked layout of the program, maybe the .byte directives will get executed as code. Normally you'd have a label in this case too, though... • Some assemblers don't support generating x86 instruction prefixes for operand size, etc. In code written for those assemblers, you'll often see something like: .byte 0x66 mov $12, %eax To make the assembler emit the code you want to have. .byte 0x66 is length-changing for mov $12, %eax, so your example will decode as mov $12, %ax / add %al,(%rax) or (%eax) in 32-bit mode. The 2nd instruction is the left-over 00 00 bytes of the 32-bit immediate that mov eax, imm16 doesn't consume. This is tricky for the hardware, too, and causes LCP pre-decode stalls on Intel CPUs (LCP = length-changing prefix). Minimal runnable example .byte spits out bytes wherever you are. Whether th...

While going through some C code having inline assembly I came across the .byte (with a Dot at the beginning) directive. On checking the assembly reference on web I found that it is used to reserve a byte in memory. But in the code there was no label before the statement. So I was wondering what is use of an unlabeled .byte directive or any other data storage directive for that matter. For e.g. if i code .byte 0x0a, how can i use it ? There are a few possibilities... here are a couple I can think of off the top of my head: • You could access it relative to a label that comes after the .byte directive. Example: .byte 0x0a label: mov (label - 1), %eax • Based on the final linked layout of the program, maybe the .byte directives will get executed as code. Normally you'd have a label in this case too, though... • Some assemblers don't support generating x86 instruction prefixes for operand size, etc. In code written for those assemblers, you'll often see something like: .byte 0x66 mov $12, %eax To make the assembler emit the code you want to have. .byte 0x66 is length-changing for mov $12, %eax, so your example will decode as mov $12, %ax / add %al,(%rax) or (%eax) in 32-bit mode. The 2nd instruction is the left-over 00 00 bytes of the 32-bit immediate that mov eax, imm16 doesn't consume. This is tricky for the hardware, too, and causes LCP pre-decode stalls on Intel CPUs (LCP = length-changing prefix). Minimal runnable example .byte spits out bytes wherever you are. Whether th...

\( \newcommand\) No headers Assembler directives are directions to the assembler to take some action or change a setting. Assembler directives do not represent instructions, and are not translated into machine code. For this assembler, all directives begin with a “.” or “#” (the comment is a #), and the directive must exist on a separate line from any other assembler directive or assembler instruction. There are 4 assembler directives and the comment tag. • .text – The .text directive tells the assembler that the information that follows is program text (assembly instructions), and the translated machine code is to be written to the text segment of memory. • .data – The .data directive tells the assembler that information that follows is program data. The information following a .data instruction will be data values, and will be stored in the data segment. • .label – A label is an address in memory corresponding to either an instruction or data value. It is just a convenience so the programmer can reference an address by a name. It will be used as follow: .label name The label is a tag that can be referenced in place of an address in any assembly instruction that can take a label/adress. Labels and addresses can be used interchangeably. • .number – The number directive tells the assembler to set aside 2 bytes of memory for a data value, and to initialize the memory to the given value. It will often be used with the .label directive to set a label to a 2-byte memory value, ...