Undocumented 8086 Instructions, Explained by the Microcode
owl writes:
https://www.righto.com/2023/07/undocumented-8086-instructions.html
What happens if you give the Intel 8086 processor an instruction that doesn't exist? A modern microprocessor (80186 and later) will generate an exception, indicating that an illegal instruction was executed. However, early microprocessors didn't include the circuitry to detect illegal instructions, since the chips didn't have transistors to spare. Instead these processors would do something, but the results weren't specified.1
The 8086 has a number of undocumented instructions. Most of them are simply duplicates of regular instructions, but a few have unexpected behavior, such as revealing the values of internal, hidden registers. In the 8086, most instructions are implemented in microcode, so examining the 8086's microcode can explain why these instructions behave the way they do.
1 The 6502 processor, for instance, has illegal instructions with various effects, including causing the processor to hang. The article How MOS 6502 illegal opcodes really work describes in detail how the instruction decoding results in various illegal opcodes. Some of these opcodes put the internal bus into a floating state, so the behavior is electrically unpredictable.
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