most significant bit

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The most significant bit and least significant bit refer to the bits at either end of a byte. For instance, the decimal value 129 will be represented in binary as 10000001, in this case the most significant (left) and least significant (right) bits are set to 1.

Many microprocessor designs include special functionality to easily test these bits. In the 6502, the BIT instruction allows the most significant bit to be tested without having to load a value into the accumulator, and this is widely used in 6502 code. For instance, BASIC sets the most significant bit of its tokens to 1 to allow the interpreter to easily identify them. Many device drivers also use the most significant bit to indicate status, because this can be easily tested.