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Sign flag

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Title: Sign flag  
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Subject: Intel 8080, Intel 80386, Intel 80486, Motorola 6800, Freescale 68HC11, Intel 8008, Intel 8085, Intel 4040, X86-64, Two's complement
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Sign flag

In a computer processor the negative flag or sign flag is a single bit in a system status (flag) register used to indicate whether the result of the last mathematical operation resulted in a value whose most significant bit was set. In a two's complement interpretation of the result, the negative flag is set if the result was negative.

For example, in an 8-bit signed number system, -37 will be represented as 1101 1011 in binary (the most significant bit is 1), while +37 will be represented as 0010 0101 (the most significant bit is 0).

The negative flag is set according to the result in the x86 series processors by the following instructions (referring to the Intel 80386 manual [1]):

  • All arithmetic operations except multiplication and division;
  • compare instructions (equivalent to subtract instructions without storing the result);
  • Logical instructions - XOR, AND, OR;
  • TEST instructions (equivalent to AND instructions without storing the result).

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