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Silicon reverse engineering: The 8085's undocumented flags (2013)
The 8085 microprocessor has two undocumented status flags: V and K. These flags can be reverse-engineered by looking at the silicon of the ...
[1] Bit 1 is a signed-number overflow flag, called V, indicating that the result of a signed add or subtract won't fit in a byte. The exclusive-or circuit (which outputs a 1 if exactly one input is 1) is a key component of the flag circuitry, and illustrates how more complex logic can be formed out of simpler gates. [1] The undocumented instructions and flags of the 8085 were discovered by Wolfgang Sehnhardt and Villy M. Sorensen in the process of writing an 8085 assembler, and were written up in the article Unspecified 8085 op codes enhance programming, Engineer's Notebook, "Electronics" magazine, Jan 18, 1979 p 144-145.
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