gas

问题 I have a C file which contain a global variable foo. How I can access foo from another assemby program. I am using i586-elf-as (GNU assembler) and i586-elf-gcc (gnu compiler) for building. 回答1: You can just use the symbol name; as treats all undefined symbols as external. Check compiler output ( gcc -S ) and/or documentation to find out if C variable names get a leading _ prepended or not. ( int myglobal becomes asm _myglobal on many non-ELF platforms, but still myglobal on Linux/ELF.) And of

问题 I have a binary file that I've disassembled using avr-objcopy. The interrupt vector table looks like: 00000000 : ; VECTOR TABLE 0: 13 c0 rjmp .+38 ; 0x28, RESET 2: b8 c1 rjmp .+880 ; 0x374, INT0 4: fd cf rjmp .-6 ; 0x0 6: fc cf rjmp .-8 ; 0x0 8: fb cf rjmp .-10 ; 0x0 a: fa cf rjmp .-12 ; 0x0 c: f9 cf rjmp .-14 ; 0x0 e: f8 cf rjmp .-16 ; 0x0 10: f7 cf rjmp .-18 ; 0x0 12: c7 c1 rjmp .+910 ; 0x3a2, TIMER1 OVF 14: f5 cf rjmp .-22 ; 0x0 16: f4 cf rjmp .-24 ; 0x0 18: f3 cf rjmp .-26 ; 0x0 1a: f2 cf

问题 I've got some assembly functions I've ported to 64-bit ARM, and they work fine on Android, but when I tried to compile the same files in Xcode, I discovered that clang uses a different syntax (different from the official ARM documentation). I've found some scripts which convert a source file from one format to the other, but this is not the ideal solution (and it seems these scripts don't work when the source files contain preprocessor defines). Can I simply use gas in Xcode, or configure

问题 I have a working position independent Linux freestanding x86_64 hello world: main.S .text .global _start _start: asm_main_after_prologue: /* Write */ mov $1, %rax /* syscall number */ mov $1, %rdi /* stdout */ lea msg(%rip), %rsi /* buffer */ mov $len, %rdx /* len */ syscall /* Exit */ mov $60, %rax /* syscall number */ mov $0, %rdi /* exit status */ syscall msg: .ascii "hello\n" len = . - msg which I can assemble and run with: as -o main.o main.S ld -o main.out main.o ./main.out Since it is

问题 So, I have an assembly function, which is called in C. It compiles and gives me no warnings, but when I try to run it, it gives me a segmentation fault. I think it's because I can't move a constant into a register, but to use the mul/div command it requires a value to be in EAX register. How can I multiply or divide two constants in Assembly? Here's the code so far... .section .data .global n .equ A, 50 .equ B, 5 .section .text .global loop_function loop_function: # prologue pushl %ebp # save

问题 Whilst learning the "assembler language" (in linux on a x86 architecture using the GNU as assembler), one of the aha moments was the possibility of using system calls. These system calls come in very handy and are sometimes even necessary as your program runs in user-space. However system calls are rather expensive in terms of performance as they require an interrupt (and of course a system call) which means that a context switch must be made from your current active program in user-space to

问题 Consider the following variable reference in x64 Intel assembly, where the variable a is declared in the .data section: mov eax, dword ptr [rip + _a] I have trouble understanding how this variable reference works. Since a is a symbol corresponding to the runtime address of the variable (with relocation), how can [rip + _a] dereference the correct memory location of a ? Indeed, rip holds the address of the current instruction, which is a large positive integer, so the addition results in an

问题 Consider the following variable reference in x64 Intel assembly, where the variable a is declared in the .data section: mov eax, dword ptr [rip + _a] I have trouble understanding how this variable reference works. Since a is a symbol corresponding to the runtime address of the variable (with relocation), how can [rip + _a] dereference the correct memory location of a ? Indeed, rip holds the address of the current instruction, which is a large positive integer, so the addition results in an

问题 When compiling the following instruction: movl 4(%ebp), 8(%ebp) I got: too many memory reference . What's wrong with it? 回答1: The number before the parenthesis is a byte offset (which causes a memory reference to occur), and you cannot have two of them with movl . You need to move the value temporarily to a register first. movl 4(%ebp), %ecx movl %ecx, 8(%ebp) 回答2: It is not a legal instruction. For most instructions that reference memory you must move it to/from a register. 回答3: movl doesn't

问题 There seem to be a .CFI directive after every line and also there are wide varities of these ex., .cfi_startproc , .cfi_endproc etc.. more here. .file "temp.c" .text .globl main .type main, @function main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 movl $0, %eax leave ret .cfi_endproc .LFE0: .size main, .-main .globl func .type func, @function func: .LFB1: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp