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Counting modified bits in a 32bit data word

I have to count modified bits in a volatile 32 bit data word.

Example:
Initial counter32[32] := 0; olddata := 0x00000010; newdata := 0x0000000001;

So we have moddata := newdata xor olddata => moddata := 00000011.

So bit0 and bit1 have changed.
Now I would have counter[1]++ and counter[0]++;

How this can be done on a Cortex-M0+ fastes way?

Best regards Juergen

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Westonsupermare Pier over 4 years ago +1 verified

int popcount32(uint32_t x) { x -= ((x >> 1) & 0x55555555); x = (x & 0x33333333) + ((x >> 2) & 0x33333333); x = (x + (x >> 4)) & 0x0F0F0F0F; x += (x >> 8); x += (x >> 16); return x & 0x0000003F;...

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0 Juergen Marquardt over 4 years ago in reply to Juergen Marquardt

I think something like this.
But this is tooo slow for our needs and I have no knowledge of ARM Assembler...
MCU is a STM32G0 with Cortex-M0+.

uint32_t counter32[32];
olddata = 0x00000001U;
newdata = 0x00000002U;

void CountBits(uint32_t old, uint32_t new)
{
    uint32_t x;
    uint32_t mask = 0x00000001U;
    int      i;

    x = old ^ new;			// x == 0x00000003
    for (i = 0; i < 32; i++)
    {
        if (x & mask)
            counter[i]++;
        mask <<= 1;
    }        
}


void main (void)
{
    CountBits (olddata, newdata);
}

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0 Juergen Marquardt over 4 years ago in reply to Juergen Marquardt

I think something like this.
But this is tooo slow for our needs and I have no knowledge of ARM Assembler...
MCU is a STM32G0 with Cortex-M0+.

uint32_t counter32[32];
olddata = 0x00000001U;
newdata = 0x00000002U;

void CountBits(uint32_t old, uint32_t new)
{
    uint32_t x;
    uint32_t mask = 0x00000001U;
    int      i;

    x = old ^ new;			// x == 0x00000003
    for (i = 0; i < 32; i++)
    {
        if (x & mask)
            counter[i]++;
        mask <<= 1;
    }        
}


void main (void)
{
    CountBits (olddata, newdata);
}

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