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How can I exchange two bits?

Valentin Tihhomirov

The following does not work (ACC is always 0, despite out[0] is 255):

ACC0 = out[0] & 1 == 1 ? 1 : 0;
ACC1 = out[0] & 2 == 2 ? 1 : 0;
out[0] &= 0xFC;
send_string(" acc1");
send_byte(ACC);
if (ACC0)
out[0] |= 2;
if (ACC1)
out[0] |= 1;

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  • 0

    Well, there's the straightforward way:

    bit temp;

// save bit 0
temp = out[0] & 1;

// bit 0 = bit 1
out[0] &= ~1;
out[0] |= (out[0] & 2) ? 1 : 0;

// bit 1 = temp
out[0] &= ~2;
out[0] |= temp ? 2 : 0;

    You could also use a shift instead of the conditional to get bit 1 to bit 0.

    // bit 0 = bit 1
    out[0] |= (out[0] >> 1) & 1;

    Or you could use the bit-addressable memory if you have a byte to spare in this function.

    bit tempBit;

// temporary swap area
bdata temp;
sbit temp0 = temp^0;
sbit temp1 = temp^1;

temp = out[0];

tempBit = temp0;
temp0 = temp1;
temp1 = tempBit;

out[0] = temp;

    If you don't care about the other bits in out[0], you could save the use of tempBit. Or, you could use the old XOR swap trick on the bit variables:

    temp0 = temp0 ^ temp1;
temp1 = temp0 ^ temp1;
temp0 = temp0 ^ temp1;

    I haven't compiled any of these to see what the generated code might look like.

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  • 0

    Well, there's the straightforward way:

    bit temp;

// save bit 0
temp = out[0] & 1;

// bit 0 = bit 1
out[0] &= ~1;
out[0] |= (out[0] & 2) ? 1 : 0;

// bit 1 = temp
out[0] &= ~2;
out[0] |= temp ? 2 : 0;

    You could also use a shift instead of the conditional to get bit 1 to bit 0.

    // bit 0 = bit 1
    out[0] |= (out[0] >> 1) & 1;

    Or you could use the bit-addressable memory if you have a byte to spare in this function.

    bit tempBit;

// temporary swap area
bdata temp;
sbit temp0 = temp^0;
sbit temp1 = temp^1;

temp = out[0];

tempBit = temp0;
temp0 = temp1;
temp1 = tempBit;

out[0] = temp;

    If you don't care about the other bits in out[0], you could save the use of tempBit. Or, you could use the old XOR swap trick on the bit variables:

    temp0 = temp0 ^ temp1;
temp1 = temp0 ^ temp1;
temp0 = temp0 ^ temp1;

    I haven't compiled any of these to see what the generated code might look like.

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