The 286 reveals one of its last secrets

Although largely forgotten today, the Intel 80286 was for a time in the 1980s the processor of choice and the designated successor to the 8086 in the PC world. It brought a new mode that could address up to 16MB of memory and a welcome speed boost over machines using an 8086 or 8088. As with many microprocessors, it has a few undocumented features, and these are a few- some of them who [rep lodsb ] are watching. Along the way, we learn a bit more about the 286 and why Intel had some of these undocumented instructions in the first place.

If you used a 286, it was probably as an end user sitting in front of a PC-AT or clone. During manufacturing and testing, the processor needed some additional functions, both to test the chip itself and to debug designs that used it. It's in these fields that the undocumented instructions are, and they relate to an in-circuit emulator, a 286 with a debug port on some of its unused pins, which would have sat on a plug-in daughterboard for the systems under test. The 286 was famous for its fancy extended mode which took quite a long time to switch, and these instructions are for loading and saving states before and after switching.

The days of the 286, as the new trend was quickly swept away by the 386 with its support for virtual memory, so for most of us it's still just a faster way we've used code 8086 for a few years. They occasionally appear here, even while connected to the internet.

Picture 286: Thomas Nguyen (PttNguyen.net), CC BY-SA 4.0.