We'll start by noting, for the oldest among our readers, that it has nothing to do with the "high memory" concept found on early personal computers. That, of co

We'll start by noting, for the oldest among our readers, that it has nothing to do with the "high memory" concept found on early personal computers. That, of course, was memory above the hardware-implemented hole at 640KB — memory that was, according to a famous quote often attributed to Bill Gates, surplus to the requirements of any reasonable user. The kernel's notion of high memory, instead, is a software construct, not directly driven by the hardware. This is indeed a great write-up. Some old lessons I had forgotten: Physical memory needs to be mapped by a kernel data structure. As typical DRAM configurations grew, this map itself necessarily became large enough to limit the 1GB dedicated to kernel on 32-bit systems. So a subset was directly mapped, and the rest -- the "high memory" -- was mapped by a secondary structure. If I'm reading this right.