相当于 C 语言中的 NOP for Embedded？

Question

I use KEIL to compile a program.

The program uses the code

asm("NOP");

Unfortunately KEIL compiler does not accept the statement.

The idea is to introduce a delay by using NOP (no operation) assembly code.

What is the actual equivalent of this in C ? Does this vary with the embedded controller that I use?

Answer 1

There's an intrinsic nop in most compilers, Keil should have this as well - try __nop()

See - http://www.keil.com/support/man/docs/armccref/armccref_CJABCDAD.htm

Intrinsic functions are usually safer than directly adding assembly code for compatibility reasons.

Answer 2

Does this vary with the embedded controller that I use?

Yes. Inline assembly is not part of the C standard (yet), it varies from compiler to compiler and sometimes even between different target architectures of the same compiler. See Is inline asm part of the ANSI C standard? for more information.

For example, for the C51 Keil compiler, the syntax for inline assembly is

...
#pragma asm
      NOP
#pragma endasm
...

while for ARM , the syntax is something like

...
__asm  {
          NOP
       }
...

You will need to check the manual for the actual compiler you are using.

For some of the more common opcodes, some compilers provide so-called intrinsics - these can be called like a C function but essentially insert assembly code, like _nop_ () .

Answer 3

If you are using Keil for ARM Cortex target (eg stm32), you are most probably also using CMSIS library. It has portable macros and inline functions for all assembly instructions written like this: __NOP() .