Int 13h buffer 64k boundaries

On 04/19/2018 12:14 PM, Fred Cisin via cctalk wrote: It's not so much expertise, but where you start your investigations. Right when I peered into the 5150, I saw the 8237 DMA controller (first cousin to the 8257) and recognized it from my 8-bit (8085) days. It was immediately obvious that IBM had taken a bunch of legacy 8 bit peripheral chips and shoved them into the PC. In fact, the 5150 was surprising in that how primitive the engineering was--something you didn't expect from a high-tech pioneer like IBM. So the DMA address space had to be 16 bits with simple bank select--using a disk controller chip that was design to be used with 8 inch drives. The Technical Reference BIOS listing confirmed the suspicion that the 5150 implementation couldn't cross 64K banks. It had nothing to do with DOS, per se. At the same time the PC debuted, we were working with early steppings of the 80186, which did feature two channels of 20-bit address DMA--and 16 bit bus width to boot. So, at the time, looking at the 5150, it was an overpriced primitive implementation using a 1970s CPU. Many people at the time thought it would be less popular than the 5100. Rather than buy my first 5150, I was strongly drawn to the NEC APC. For about the same price as an outfitted 5150, you could buy a true 16 bit box with 8" disk drives and really nice graphics that was built like a battleship. The only problem is that nobody had ever heard of it. But IBM had the golden reputation. Many people at the time, particularly the older ones, didn't talk about "computers" so much as "IBM machines". --Chuck