How do you use nasm to interface with C or other high-level languages?

Assembly instructions also allow access to hardware specific functions that have no C interfaces. Some examples for x86/x64 architecture are ldgt (load global descriptor table), wrmsr/rdmsr (write/read msr), cli/sti (clear/set interrupt flag), etc.

While it's true that using Assembly allows to have direct access to system calls and hardware features it's important to note that there are potential compiler optimization issues when using C in-line assembly vs using external assembly routines from C. So, it's recommended to understand how in-line ASM may impact your code compilation with the specific C compiler being used.

NASM, the Netwide Assembler, is your low-level beast, while C is the high-level workhorse. Harness their combined power for peak performance! Key Points: 1. Optimize Critical Code: Isolate performance-critical sections, rewrite in NASM, and embed them in your C code. This is where assembly shines! 2. Direct Hardware Access: Need to manipulate registers or memory directly? NASM is your go-to. Create custom assembly functions and call them from C. 3. Cross-Platform Challenges: Beware of architecture-specific instructions. Careful planning is essential when targeting multiple platforms.

This point should provide examples of cdecl, stdcall and fastcall (they work in sligthely different ways). Also the extern "C" is only required when using a C++ compiler (not a C compiler), so this part seems to have been generated by an AI maybe, because, instead, what's important is to declare the ASM routine in C as extern and the extern declaration has to follow one of the standards mentioned above, for example: extern int __stdcall my_func(int a); // for using __stdcall OR extern int my_func(int a); // for using cdecl Unfortunately I don't have enough space here to show some complete examples also for the NASM part.

Calling C from ASM, requires to declare the C function in NASM source as: extern my_func Where my_func is defined in the C sources, and the ASM code has to be aware of the cdecl for example. Before calling the C function the ASM code must prepare the function parameters correctly accordign to the calling convention and finally it can call the C function using "call my_func". The "magic" will be done at linking time, so ensure all the declarations are done correctly otherwise the linker will return errors.

Since this article was created with AI, I think AI could be asked to provide some specific examples of usage of macros Providing test cases to be verified in a development sand box would be also very good thing to have

Follow the platform's system call conventions when NASM functions need to interact with system libraries. For instance, on Linux x86-64, I ensured NASM functions adhered to the syscall convention, passing syscall numbers in the RAX register and parameters in registers like RDI and RSI. I called the mmap syscall directly from NASM to allocate memory efficiently. Develop test harnesses in C to verify that NASM functions are working correctly. I wrote a C test program that checked the results of my NASM optimisation routines for string manipulation, comparing them against the output of standard C functions. This allowed me to validate that the NASM functions were performing as expected and properly interfacing with the C environment.