//read byte from SPI (mode 3) as a master
BYTE rdSPI(void)
{
   register BYTE BitCnt = 8;   // Do 8 times
   register BYTE Shift;        // Input data register
   do                          // Top of loop
   {
      SPI_SCK_LO();            // SPI clock output low
      Shift <<= 1;             // C left shift one bit
      if(IS_SPI_SDI_LO())      // Check level of data input
         Shift &= ~1;          // Set bit zero to zero
      else
         Shift |= 1;           // Set bit zero to one
      SPI_SCK_HI();            // SPI clock output high
   } while (--BitCnt);         // Do all 8 bits
   return(Shift);              // Return the input value
}

The &= operator is a bitwise and
The |= operator is a bitwise or
The ~ operator is a bitwise negate

The last one will work differently on different variables depending on
the bit-count.  In this example (8 bit), the expression ~1 is equal to
254.
The code in itself looks like it's converted asm-code with the shift
and test performed.  A smoother way would be like this:

//read byte from SPI (mode 3) as a master
BYTE rdSPI(void)
{
   register BYTE BitCnt = 8;   // Do 8 times
   register BYTE Shift;        // Input data register
   do                          // Top of loop
   {
      SPI_SCK_LO();            // SPI clock output low
      Shift <<= 1;             // C left shift one bit
      Shift.0=SPI_SDI_LEVEL(); // Set data
      SPI_SCK_HI();            // SPI clock output high
   } while (--BitCnt);         // Do all 8 bits
   return(Shift);              // Return the input value
}

This is of course dependent on the compiler, if it supports the
bit-addressing withing registers.

Dag S

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