The 8080 and Z-80 had 16-bit registers and could do some limited 16-bit arithmetic. The 8088 (used in the first IBM PC) used an 8-bit bus to communicate with m

The 8080 and Z-80 had 16-bit registers and could do some limited 16-bit arithmetic. The 8088 (used in the first IBM PC) used an 8-bit bus to communicate with memory and I/O. The 8086 had a 16-bit bus. CompuPro had an 8086 S-100 board, which may have been the first target for DOS, but I cannot recall for certain. Both 8086 and 8088 ran in what is now called "real mode", which allows direct access to only 64k of RAM (just like the 8080 and Z-80). To access more than 64k at a time, you had to use segment registers. CP/M 3 (x80) employed bank-switching, which was a similar technique, but done with hardware that was external to the CPU instead of built-in. There really wasn't much difference between the Z-80 and the 8086 in terms of performance. Yeah, the 8086 had a 16-bit bus, but the instructions were 16-bits too, so running a program from non-cached RAM (because there wasn't any such thing as RAM cache then) was about the same. The 8088 (at 4.77MHz) would under-perform when compared with a (4MHz) Z-80, because they both used an 8-bit bus, but the Z-80 needed only one memory fetch per instruction, vs. two for the 8088. https://en.wikipedia.org/wiki/X86_memory_segmentation https://en.wikipedia.org/wiki/X86_memory_segmentation I do not fail to realize anything, but I'm wondering why you are so passionate about trying to disprove that SCP/Tim Patterson ripped off DR/Gary Kildall's code to produce QDOS when there is so much strong evidence that he did.