Undocumented 8085 Flags And Instructions

Thread Starter


Joined May 28, 2007
Its gr8 to say that 8085 has undocumented instructions (10) out of 256 instructions !

check the link to see the undocumented 8085 instructions and flags

Does anybody knows actually what the Intel is meaning with the undocumented codes ?

Do they really designed by the designer or are they created by their own during the manufacturing process ?

Why Intel is not releasing the undocumented codes of the mps?

What is the reason for undocumenting them?


Joined Apr 20, 2004
I can recall other microprocessors with some undocumented instructions. They were not always particularly useful, as they tended to be byproducts of the manufacturing process. In the case of the 6502, the odd instructions varied depending on the manufacturer.

It is very much not a plot to keep these things secret. You might notice that these microprocessors with the odd codes tend to be early 8 bit wonders, and long past any kind of useful application by now.


Joined Nov 21, 2004
I spent many years programming assembler language projects for the 8085 back in the mid 1980's and I am familiar with the undocumented opcodes. Although some of them are very useful (such as a 16 bit subtract), I was warned by Intel tech support to not use them because their operation, and in fact their very presence in any particular chip, can not be guaranteed. So, if you are a home hobbyist who wants to use undocumented opcodes to your advantage, that's perfectly fine and dandy, but if you are a professional programmer who's software is used in manufactured products, then you definitely should not use undocumented opcodes, no matter how useful or convenient they may be!


Joined Jan 28, 2005
The fact that the opcodes are undocumented makes them only useful in a one-off design where you have control of the ramifications. To use undocumented opcodes would be a risky endeavor if used in a mass produced product for the reason already stated by others within this thread.

In a mass produced design someone is bound to replace a failed microcontroller with one that does not support the undocumented opcodes and then be frustrated when the board does not work.



Joined Apr 27, 2007
I think these "undocumented opcodes" are a byproduct of faulty logic. Of course, for different manufacturers, the results may be different. It reminds me of the old 7447 display driver, that would yield odd digits when a binary value above 9 was present at the input. Perhaps certain opcodes were not predicted in order to simplify the logic.