bitmask, C (gcc) - rextester

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//gcc 7.4.0

#include  <stdio.h>
#include  <stdbool.h>

typedef struct 
{
    bool bit7:1;
    bool bit6:1;
    bool bit5:1;
    bool bit4:1;
    bool bit3:1;
    bool bit2:1;
    bool bit1:1;
    bool bit0:1;

} bitmask;

int main(void)
{
    unsigned int voltage_word = 0;
    unsigned int voltage_word_shifted = 0;
    unsigned char high_byte = 0;
    unsigned char low_byte = 0;
    unsigned char reg13 = 0;
    unsigned char reg14 = 0;
    unsigned char new_high_byte = 0;
    unsigned char new_low_byte  = 0;
    
   
    
 
    // calculate voltage word
       voltage_word = 0x02A5;
       printf("voltage_word:  [0x%04X] \r\n", voltage_word);
    
    // shift left 1 bit
       voltage_word_shifted = (voltage_word << 1);
       printf("voltage_word_shifted: [0x%04X] \r\n", voltage_word_shifted);
    
    // break word into high byte and low byte
       high_byte = (voltage_word_shifted >> 8);
       low_byte  = (voltage_word_shifted << 8) >> 8;
       printf("voltage high/low bytes: [0x%02X] [0x%02X]\r\n", high_byte, low_byte); 
    
   
     // read current high byte & 0x07 to mask off correct bits then | calculated high byte
       reg13 = 0xA0;  // current high byte
       new_high_byte = reg13 | high_byte;
    
     // read current low byte & 0x01 to mask off correct bits then | calulated low byte
       reg14 = 0x0D6;  // current low byte
       new_low_byte = (reg14 & 0x01) | low_byte;
    
     printf("current value: [0x%02X] [0x%02X]\r\n", reg13, reg14); 
     printf("new value:     [0x%02X] [0x%02X]\r\n", new_high_byte, new_low_byte); 
    
   
    return 0;
}

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Compilation time: 0.13 sec, absolute running time: 0.18 sec, cpu time: 0.01 sec, memory peak: 3 Mb, absolute service time: 0,44 sec