Re: Arm Cortex M4 floating point

[Mircea Cociuba]

hello, 

This email will be non qemu related, but close to the original problem.

I managed to get the FPU activation sequence to compile, found it on the ARM comunity site:

    // ARM 100166_0001_00_en S7.3.2

    __asm("ldr.w r0, =0xE000ED88"); // CPACR

    __asm("ldr r1, [r0]");

    __asm("orr r1, r1, 0xF << 20"); // [20:23] enable CP10 and CP11

    __asm("str r1, [r0]"); 

The documentation also specifies that this code must be executed in priviledged mode.

There is little to no reference to priviledged mode in the Arm documentation.

It briefly specifies that a certain CONTROL register must have a specific bit(nPriv) set to be in privileged mode.

Where can I find the description of the is register and memory location? and are there specific instructions that work on it?

Mircea Cociuba

"To be or not to be, that is not a question, it's FF".

On Tuesday, August 18, 2020, 06:35:33 PM GMT+3, Peter Maydell <peter.maydell@linaro.org> wrote:

On Tue, 18 Aug 2020 at 15:02, Mircea Cociuba <cociuba_mircea@yahoo.com> wrote:
> gcc params: -Wall -Werror -O2 -nostdlib -nostartfiles -ffreestanding  -mcpu=cortex-m4 -mhard-float -mfpu=vfpv4-d16 (to make sure I have FPU instructions)
>
> the source code:
>
> void PUT32 ( unsigned int, unsigned int );
> #define UART0BASE 0x4000C000
>
> int notmain ( void )
> {
>    unsigned int rx;
>
>
>    unsigned char s[]="QEMU is not that interesting";
>
>    //dummy FPU command
>    asm("vmov s15, r3"); // If you are to remove this, the qemu would print after I type the command continue in gdb, with this line, it shows nothing

This looks like you're trying to use the FPU, but you haven't
enabled it. If you're writing bare metal code then it is
the job of your guest code to make sure it enables the FPU
first, exactly as it has to do on real hardware. Check
the Arm architecture reference manual for the details of
how to do this (I think the main thing is writing to the
CPACR to enable the FPU).

Otherwise QEMU will correctly emulate the behaviour you
get on real h/w if you try to use the FPU when it is disabled:
the CPU takes an exception.

thanks
-- PMM