Read certain bits from one register and write into certain bits of another

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I am looking to implement the prototype for read_and_write, but couldn't get it right. Any help is appreciated. I am able to extract bits.

In this below code for write_val1, my intent is to write only the first 9 bits of final_val into read_data. The rest 7 bits of read_data should remain untouched. Expected output for write_val1 = 0x0080

Similarly for write_val2, my intent is to write only next 8 bits from final_val into read_data. The rest 8 bits of read_data should remain untouched. Expected output for write_val2 = 0x2700

Similarly for write_val3, my intent is to write only next 3 bits(010) from final_val into a position[11:13] of read_data leaving rest of read_data untouched.

Expected output = 0x5448 ; For example: pick 010 extracted bit from write_val3; 0x3048 = 0111 0000 0100 1000; 0x5048 = 0101 0000 0100 1000

#include <stdio.h>
#include <stdint.h>

 // Extracts n bits from a given position in LSB. 
int bitExtracted(uint16_t read_data, uint8_t n_bits, uint8_t pos) 
{ 
    return (((1 << n_bits) - 1) & (read_data >> (pos - 1))); 
} 

void read_and_write(uint32_t* final_val, uint16_t* write_val, uint8_t start_pos, uint8_t end_pos)
{
    uint32_t temp = *final_val;
    *write_val = (uint16_t) ((temp >> start_pos) & ((1 << end_pos) - 1)); // store the desired number of bits in write_val
    *final_val = (temp >> end_pos); //shift final_val by end_pos since those bits are already written
    printf("\n temp %x, write_val %x, final_val %x ", temp, *write_val, *final_val);
    
}

void main() 
{
    uint16_t read_data = 0x0; //assume some read value
    uint16_t ext_val1 = bitExtracted(read_data, 9, 1);  //Read BITS [8:0] from read_data
    uint8_t ext_val2 = bitExtracted(read_data, 8, 1);   //Read BITS [7:0] from read_data
    uint8_t ext_val3 = bitExtracted(read_data, 3, 5);   //Read BITS [7:4] from read_data
    uint32_t final_val = 0x0; //Stores 20 extracted bits from val1, val2 and val3 into final_val (LSB to MSB in order)
    uint16_t write_val1, write_val2, write_val3;
    uint8_t start_pos = 0, end_pos =8;
    ext_val1 = 0x80, ext_val2 = 0x0, ext_val3 = 0x2;
    final_val = (ext_val1 | (ext_val2 << 9) | (ext_val3 << 17));
    printf ("\n final_val %x", final_val);
    
    //Read first 9 bits of final_val and write only into [8:0] position of existing read_data
    read_and_write(&final_val, &write_val1, 0, 9); 
    read_data = 0x80;
    write_val1 = write_val1 | read_data;
    
    //Read next 8 bits of final_val and write only into [7:0] position of existing read_data
    start_pos = 0;
    end_pos = 7;
    read_data = 0x27b7;
    read_and_write(&final_val, &write_val2, start_pos, end_pos);
    write_val2 = write_val2 | read_data;

    //Read next 3 bits of final_val and write only into[13:11] position of existing read_data
    start_pos = 11;
    end_pos = 13;
    read_data = 0x3048;
    read_and_write(&final_val, &write_val3, start_pos, end_pos);
    write_val3 = write_val3 | read_data;
    printf ("\n val1 0x%x val2 0x%x val3 0x%x final_val 0x%x", write_val1, write_val2, ext_val3, final_val);
}
bit bit-shift bit-manipulation c operators
1 Answers

If you want your read_and_write() function to be able to change the values of the first two inputs to the function, (e.g. final_val and write_val1), then they should be passed as pointers to the function. Your function prototype should thus look like this:

void read_and_write(uint32_t* final_val, uint16* write_val1, uint8_t start_pos, uint8_t end_pos);

When calling the function in your main code, you need to use & to take the address of the variables. So you call them using:

read_and_write(&final_val, &write_val1, start_pos, end_pos);

For the reading and writing portion, it is pretty much similar to what you did in the bitExtracted function.

uint32_t temp = *final_val;
*write_val = (uint16_t) ((temp >> start_pos) & ((1 << end_pos) - 1)); // store the desired number of bits in write_val

*final_val = (temp >> end_pos); //shift final_val by end_pos since those bits are already written

The * is to deference the pointer and access the data stored in a particular location. Note that you also need to have a (uint16_t) cast for write_val since the type needs to match.

user_109282800
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