On old Windows versions, you could get even more complicated backwards compatibility fun.
The 8086/8088 only had a 1MB address space that wrapped around: if you used a high enough segment value with a high enough offset, you would actually refer to the beginning of memory.
The 80286 had a much larger address space: a whole 16MB! But when running in compatibility mode (where you could only refer to 1MB), the addresses no longer wrapped around. The result was that you actually had an address space of 1MB+64KB (minus 16 bytes). IBM wanted better compatibility than that so they found an unused AND gate in a TTL chip lying around somewhere and put the A20 signal from the CPU through that before it reached the bus. You could then switch A20 off at will, thus getting better compatibility. So how do you control that gate? Well, the microcontroller on the motherboard that talks to the keyboard microcontroller has an extra pin that isn't used for anything. Let's use that pin! So you switch the A20 gate on/off by sending commands to the keyboard controller.
Fast forward some years. We now have a decentish version of Windows that is basically running on top of DOS. It has extensive compatibility with DOS in many ways: not only can it run (and multitask) DOS programs inside it, it can also use DOS device drivers and TSRs loaded before Windows. It does this by merrily switching back and forth between so many different CPU modes that it makes you dizzy. It needs to run with the A20 on, otherwise you would get a really funny address space where every other megabyte was inaccessible, which would be both clumsy and wasteful. And device drivers and TSRs loaded into the 64KB right above the first megabyte would also need A20 on to run. On the other hand, there might be other real mode code in the system that needs A20 to be off! So Windows could switch it on and off while running. It doesn't quite involve the keyboard but it is close... and sometimes a bad keyboard (with bad code in its microcontroller) would distract the microcontroller on the motherboard so much that the A20 switching was slow or even faulty. In that case you could fix your machine by switching keyboard!
The 8086/8088 only had a 1MB address space that wrapped around: if you used a high enough segment value with a high enough offset, you would actually refer to the beginning of memory.
The 80286 had a much larger address space: a whole 16MB! But when running in compatibility mode (where you could only refer to 1MB), the addresses no longer wrapped around. The result was that you actually had an address space of 1MB+64KB (minus 16 bytes). IBM wanted better compatibility than that so they found an unused AND gate in a TTL chip lying around somewhere and put the A20 signal from the CPU through that before it reached the bus. You could then switch A20 off at will, thus getting better compatibility. So how do you control that gate? Well, the microcontroller on the motherboard that talks to the keyboard microcontroller has an extra pin that isn't used for anything. Let's use that pin! So you switch the A20 gate on/off by sending commands to the keyboard controller.
Fast forward some years. We now have a decentish version of Windows that is basically running on top of DOS. It has extensive compatibility with DOS in many ways: not only can it run (and multitask) DOS programs inside it, it can also use DOS device drivers and TSRs loaded before Windows. It does this by merrily switching back and forth between so many different CPU modes that it makes you dizzy. It needs to run with the A20 on, otherwise you would get a really funny address space where every other megabyte was inaccessible, which would be both clumsy and wasteful. And device drivers and TSRs loaded into the 64KB right above the first megabyte would also need A20 on to run. On the other hand, there might be other real mode code in the system that needs A20 to be off! So Windows could switch it on and off while running. It doesn't quite involve the keyboard but it is close... and sometimes a bad keyboard (with bad code in its microcontroller) would distract the microcontroller on the motherboard so much that the A20 switching was slow or even faulty. In that case you could fix your machine by switching keyboard!