Hi smart guys out there,
I am using µVision with TIs eval-kit LM3S9B92. I use µVisions C-Compiler.
I learned that it is not possible to read the CPU-Registers with C and I have to use asembler for that.
I have no idea of asembler and actually I don't work with it in future and I hope I never have to again :-) ...but right now I need to know the status of the carrybit.
So all I am asking is for a piece of code that gets me the status of bit 29 in the register 17 :) and how to implement that in my project.
I hope this doesn't sound sassy ;-) - at least it isn't meant that way.
Thank you so much.
Point taken - didn't know that C works that way (owning the carry bit). Would it work, if the prior calculation was in assembler, too? 'Cause I thought that the codepart written in assembler will be transcoded to machinecode in a coherent way...won't it?
I wanted to avoid the method Per suggested, because I was just curious if it is possible to avoid it. That was my single goal: Curiosity :-) . Stupid, I guess.
Now I will just do it the standard way without using assembler.
Thanks a lot for your help!
The ARM compiler is "smart" enough that it merges the assembler instructions with the instructions generated from the C code. Then it performs a number of optimization steps.
So inlined assembler need not survive unmodified to the finaly binary.
If you want to implement a large-number library while using the carry flag, then you should implement the evaluations fully in assembler.
Note that a comparison "if (a >= b)" is very fast on the ARM processor, and the optimizer will recognize that it's the same variables in the comparison as in the actual subtraction. So no extra memory accesses. Just a comparison. And the optimizer might even note that the comparison and the subtraction are basically the same thing, and the code is still interested in a subtraction.
When programming in C - do stay with C. Do not try to fool the optimizer by taking short cuts. When you are not happy with C, because of special needs: write full functions in assembler.
That's the whole point of any high-level language (HLL) - you delegate responsibility for the low-level detail to the compiler.
Indeed!
See: http://www.keil.com/forum/17856/
Thanks a lot guys, I will stay with C :-]