Legacy PC design misery

Summary

I've spent chunks of the last couple of days fighting a problem that's existed for about 25 years. The 8086 was a 16-bit processor with a 20-bit address space, limiting the maximum physical address that could be accessed to 1MB. However, quirks of the segmented memory system meant that addresses greater than 1MB could be constructed - these would wrap around to the bottom of memory. Because loading the segment registers was a time consuming operation, some programmers used this behaviour as a performance optimisation.The 80286 introduced 24 bit address space. Unfortunately, this meant that the addresses that previously wrapped to the bottom of memory now pointed at real addresses - not ideal if you were expecting the old behaviour. IBM fixed this by tying the 21st address line (A20 - they're zero indexed) through an and gate, with the default behaviour being to keep it tied at 0 and thus maintaining the old wraparound behaviour. Applications that wanted to access the full address space needed to enable the A20 logic gate. IBM didn't want to add any extra hardware to their system if they could avoid it, so tied the other side of the and gate to a spare pin on the keyboard controller. By writing a couple of bytes to the keyboard controller, your PC-AT stopped pretending to be an XT and gave you access to all of the insanely expensive RAM it had stuffed in it. Hurray!PCs have been emulating this behaviour since the AT was first cloned. Of course, this being the PC industry, many have got it wrong. There's a set of approaches for controlling the A20 gate that may work, varying in terms of performance and desirability. Most hardware will give the desired result (ie, I have no desire to run DOS executables from 1982, make my A20 work damnit) using any of the various methods of A20 enabling. Some hardware doesn't. The most common method used in bootloaders (where we still have access to system BIOS services) is to call int 15h with an ax of 0x2401, which asks the BIOS to e...