On 3/14/21 11:09 AM, Peter Corlett via cctalk wrote: I can say with a fair amount of certainty, that we at IBM knew of the existence of the LOADALL instructions including all of it's warts (and its inability to switch back from protected mode) from the earliest days. There were many heated discussions in various task forces (this was of course IBM) about the next generation OS (to become OS/2) about the '286.? First and foremost was how to be able to run DOS programs on the '286. Over very vocal opposition, management decided to use "mode switching" rather than any of the other techniques.? It should be noted, that a significant portion of us advocated abandoning the '286 in favor of the '386 to solve this problem.? The argument that management made against that approach assumed that OS/2 would be ready in 9 months and that the '386 would be late ('386 at the time was about 12-18 months away).? It turned out that OS/2 took well over 18 months to develop. At the time I was fairly familiar with the LOADALL instruction.? I had modified PC/AT Xenix to use the LOADALL instruction to allow for running Xenix programs and multiple DOS programs simultaneously.? I gave multiple demos to various folks in management but to no avail.? They had decided that mode switching as *the* way that OS/2 was going to work. I should also note, that the other way to get back to real mode from protected mode is via a triple-fault.? What gets me (and I railed on Intel when I worked there for a time) that it still existing in the architecture even though they have a machine check architecture now (which while at IBM pushed Intel to implement for the '386!). -- TTFN - Guy

On Sun, 14 Mar 2021, Liam Proven via cctalk wrote: Well, software exists that relies on the triple-fault feature for reboots including current versions of Linux (you can trigger a triple-fault in the real mode too). These days it is implemented by the southbridge catching the shutdown special cycle on PCI and asserting the reset pin to the CPU (the details might be slightly different for PCIe or HyperTransport). Back in the day I experimented with that stuff myself and all the weird ways to switch between modes with the x86, setting the IDTR in the real mode for interesting effects which would impress fellow students, etc. I ended up writing this: <http://www.rcollins.org/articles/pvi1/pvi1.html> as a result. I wrote a simple resident VM86 monitor for DOS too, just to fiddle with processor features. Also resetting the CPU with the shutdown code of 0xa put at the location 0xf of the RTC/NVRAM chip was the only way to get the family, model, and stepping ID in the EDX register for old processors that did not have the CPUID instruction (i.e. all 80386 and many 80486 implementations), unless the system BIOS clobbered it for no good reason in the short bypass code involved (sadly sometimes that did happen). I just double-checked my old DOS assembly code to see if I got the details right! NB I didn't know LOADALL would not work for switching from the protected to the real mode and did not find out about the instruction until after I already lost access to any 80286 hardware, so I never experimented with it myself. Maciej

On 3/14/21 1:42 PM, Fred Cisin via cctalk wrote: When they stopped selling Apple II's when Lisa was released.

On 3/14/21 11:36 AM, Guy Sotomayor via cctalk wrote: > I can say with a fair amount of certainty, that we at IBM knew of the > existence of the LOADALL instructions including all of it's warts (and > its inability to switch back from protected mode) from the earliest days. ca. 1980, we were in search of a decent 16-bit processor for upward migration. Code base compatibility wasn't much of a concern--we were resigned to the prospect that everything had to be recoded, so code migration wasn't a concern. We played a bit with the 8086, but couldn't see a path forward in the immediate future. So we in engineering decided that the best CPU was the Moto 68K and we wrapped up a test board and started cutting some code for it. Bill Davidow was on our BOD and when he got wind of our efforts, he nearly went through the roof. When we defended our decision by saying that the 8086 was limited in possibilities, with the rumored iAPX432 nowhere near reality, so the 68K was the logical next step. Zilog had their Z800, but after looking at the cost of the MMU and the fact that it, too was a segmented memory CPU and slower than the 68K, we never got any farther than talking about it. We did have an Onyx box for software development in the lab, however. Bill put us on the OEM pre-release steppings for both the 186 and the 286. We got the 186 going long before the 286 (Intel, IIRC, had taken on the job of writing the Xenix kernel for Microsoft). Davidow's stubbornness cost us months of product delay in getting a multi-user system out the door. We certainly were not advised about the LOADALL instruction at that time. Nor, I suspect would it have made much of a difference. In our system, the 186 did the I/O heavy lifting for the 286. My impression was that the design for the 286 never envisioned the need to switch back and forth between real and protected mode. We never did. --Chuck