assembly

问题 Why does this code work? http://www.int80h.org/strlen/ says that the string address has to be in EDI register for scasb to work, but this assembly function doesn't seem to do this. Assembly code for mystrlen : global mystrlen mystrlen: sub ecx, ecx not ecx sub al, al cld repne scasb neg ecx dec ecx dec ecx mov eax, ecx ret C main: int mystrlen(const char *); int main() { return (mystrlen("1234")); } Compilation: nasm -f elf64 test.asm gcc -c main.c gcc main.o test.o Output: ./a.out echo $? 4

问题 As the question suggests, I have to write a MASM program to convert an integer to binary. I have tried many different approaches, but none of them helped me at all. The final code I'm working on is as follows. I get an access memory violation error when I debug my code in Visual Studio . Any help on how to solve the error and if I'm on the right track or not will be greatly appreciated. The first code is my C++ code which passes a char array to an .asm file to be converted to binary. #include

问题 So every single resource online tells me that something like this: cmp %eax, %ebx jg < something > would jump to < something > if eax was greater than ebx. But I have another piece of code that seems to contradict this: cmp $0x2, %eax jg < something> as it jumps to < something > when eax has the value 3. Am I missing something, or does cmp a, b - jg execute if b > a and not a>b? And does this apply to other jump statements as well? 回答1: When we read something like cmp $0x2, %eax jg <

问题 I am trying to compile dummy function in gcc with flag -fno-pie and without. void dummy_test_entrypoint() { } When i compile without the flag. gcc -m32 -ffreestanding -c test.c -o test.o I get the following disassembled code. 00000000 <dummy_test_entrypoint>: 0: 55 push ebp 1: 89 e5 mov ebp,esp 3: e8 fc ff ff ff call 4 <dummy_test_entrypoint+0x4> 8: 05 01 00 00 00 add eax,0x1 d: 90 nop e: 5d pop ebp f: c3 ret When i compile with the flag. 00000000 <dummy_test_entrypoint>: 0: 55 push ebp 1: 89

问题 I am trying to compile dummy function in gcc with flag -fno-pie and without. void dummy_test_entrypoint() { } When i compile without the flag. gcc -m32 -ffreestanding -c test.c -o test.o I get the following disassembled code. 00000000 <dummy_test_entrypoint>: 0: 55 push ebp 1: 89 e5 mov ebp,esp 3: e8 fc ff ff ff call 4 <dummy_test_entrypoint+0x4> 8: 05 01 00 00 00 add eax,0x1 d: 90 nop e: 5d pop ebp f: c3 ret When i compile with the flag. 00000000 <dummy_test_entrypoint>: 0: 55 push ebp 1: 89

问题 To multiply a number by any any multiple of 2, I'll shift it those many times. Is there any such technique to multiply a number by 10 in less cycles? 回答1: The 80286 did not have a barrel shifter, that was introduced with the 80386. According to the timing tables in the Microsoft Macro Assembler 5.0 documentation (1987), SHL reg, immed8 takes 5+n cycles, whereas SHL reg, 1 takes 2 cycles. ADD reg, reg takes 2 cycles, as does MOV reg, reg . IMUL reg16, immed takes 21 cycles. Therefore, the

问题 To multiply a number by any any multiple of 2, I'll shift it those many times. Is there any such technique to multiply a number by 10 in less cycles? 回答1: The 80286 did not have a barrel shifter, that was introduced with the 80386. According to the timing tables in the Microsoft Macro Assembler 5.0 documentation (1987), SHL reg, immed8 takes 5+n cycles, whereas SHL reg, 1 takes 2 cycles. ADD reg, reg takes 2 cycles, as does MOV reg, reg . IMUL reg16, immed takes 21 cycles. Therefore, the

问题 I want to know how to properly do it, because the way I'm doing it isn't working. When setting the BP register with 7C00h, then setting the SP register with BP , then pushing some ASCII, then getting the data from the memory to print it with INT 10h , it works just fine. mov ax, 7C00h mov bp, ax mov sp, bp push 'A' mov ah, 0Eh mov al, [7BFEh] int 10h The actual output is A But when I do this: mov ax, 7C00h mov ss, ax mov bp, ax mov sp, bp ... It stops working. The interrupt is called, the

问题 Writing a ZMM register can leave a Skylake-X (or similar) CPU in a state of reduced max-turbo indefinitely. (SIMD instructions lowering CPU frequency and Dynamically determining where a rogue AVX-512 instruction is executing) Presumably Ice Lake is similar. ( Workaround: not a problem for zmm16..31 , according to @BeeOnRope's comments which I quoted in Is it useful to use VZEROUPPER if your program+libraries contain no SSE instructions? So this strlen could just use vpxord xmm16,xmm16,xmm16

问题 What is the simplest way to work with big-endian values in RISC-V at the assembly language level? That is, how to load a big-endian value from memory into a register, work with the register value in native-endian (little-endian), then store it back into memory in big-endian. 16, 32 and 64 bit values are used in many network protocols and file formats. I couldn't find a byte-swap instruction (equivalent to BSWAP on x86 or REV on ARM) in the manual, nor anything about big-endian loads and