check if carry flag is set

c gcc x86 intel
hans picture hans · Jun 29, 2010 · Viewed 23k times · Source

Using inline assembler [gcc, intel, c], how to check if the carry flag is set after an operation?

Answer

R.. GitHub STOP HELPING ICE picture R.. GitHub STOP HELPING ICE · Jun 29, 2010

sbb %eax,%eax will store -1 in eax if the carry flag is set, 0 if it is clear. There's no need to pre-clear eax to 0; subtracting eax from itself does that for you. This technique can be very powerful since you can use the result as a bitmask to modify the results of computations in place of using conditional jumps.

You should be aware that it is only valid to test the carry flag if it was set by arithmetic performed INSIDE the inline asm block. You can't test carry of a computation that was performed in C code because there are all sorts of ways the compiler could optimize/reorder things that would clobber the carry flag.

Related questions

Can I use Intel syntax of x86 assembly with GCC?

I want to write a small low level program. For some parts of it I will need to use assembly language, but the rest of the code will be written on C/C++. So, if I will use GCC to …

Read More
How to count clock cycles with RDTSC in GCC x86?

With Visual Studio I can read the clock cycle count from the processor as shown below. How do I do the same thing with GCC? #ifdef _MSC_VER // Compiler: Microsoft Visual Studio #ifdef _M_IX86 // Processor: x86 inline uint64_t …

Read More
Why does GCC generate such radically different assembly for nearly the same C code?

While writing an optimized ftol function I found some very odd behaviour in GCC 4.6.1. Let me show you the code first (for clarity I marked the differences): fast_trunc_one, C: int fast_trunc_one(int i) { int mantissa, exponent, …

Read More