Busses so slow that you couldn’t even clear the display. Yes, this indeed allowed the 80386 to run real mode programs. A triple fault shuts down the processor. Thanks to MCGA, adlib, soundblaster (even early versions) to play Commander Keen, Leisure Suit Larry, Wolfenstein 3D, Stunt Island, X-Wings, Loom, Indiana Jones, Doom, Ultima Underworld, …. Not until after the release of Doom was the average PC upgraded enough to rival the Amiga 500 at games, but on the Amiga it was still easier to plug in two joysticks or even mice (i.e. The Amiga was so far ahead of early DOS machines, it was pathetic. To determine if access is allowed, the processor looks at RPL, the provilege level given for the segment in the descriptor (DPL), and the privilege level the processor is currently running at (CPL). The check is max(CPL, RPL) <= DPL. 2-player Lemmings or Settlers). I wasted a huge amount of time getting this to work. MS DOS was basically gifted to me when I had a 286. My personal stuff was always on MS stuff because vendors gifted me free copies at every turn. Was fun way to detect maliciously acting programs and even detecting new/unknown computer viruses - including ways to recover files from infections. They tell me that when you have an input to some unit (e.g., the selector input to the MMU) that is always going to be shifted a fixed amount, you can build that in essentially for free. I guess I don't regret it, because it was more fun than trying to install fucking Linux, but overall it really was time wasted. I can’t believe it beat good clean 68K machines with good frame buffer and a fast bus… but that failure is another story. I had a SCSI and RAM expansion for my Amiga 500. The full DOS extender was DOS/4G. No, config.sys was a thing from day 1. Also the Amiga 3000 released in 1990 was considered to the best overall computer of its time. I've wondered if the 16 byte paragraph size came from prejudices of engineers who were involved with the iAPX 432, an "object oriented" processor that was canceled after Intel realized it had significant, architecture-level performance flaws. Even at the time, most of the good games used DOS4GW, so you only needed 500K free. The more loaded in the lower 4 GB, the less available memory is allocated to each process. The 68k was a more forward-looking ISA for sure. Type MEM. I was *so* happy when Microsoft released a recompiled version of Excel, and I could simply use the protected memory modes of Windows 3.0. edit: corrected terminology page window to page frame. Regarding memory management, then it was not untill MS-Dos 6.2x that it all began to get easier. Which meant it had its own file drivers and was, in effect, a full 32-bit OS chain-loaded from DOS. In consumer computers of 1987, that didn't buy it much. I remember when I discovered that microsoft memory manager was not the only solution. Of course, PCs were much more powerful than, say, an Amiga 500. And to add insult to injury, there was this strange 640 KiB memory limitation. > None of these segmented memory issues and 640K or 16M limitations existed on that superior processor of the day. But the Amiga offered a flat memory address, while a DOS program could only access memory using cumbersome 64 KiB segments. In protected mode, like in real mode, the 286 used a segmented addressing scheme. I don't know off the top of my head whether Watcom C/C++ ever targetted the 16-bit Phar Lap or Rational Systems extenders. The truth is of course a little more complicated than that. I never bothered loading KEYB.COM, because that took up nearly 30K, and it hardly seemed worth it when most of the keys were the same... and one day I suppose the wind changed, because here I still am.). [0] http://www.tenberry.com/dos16m/faq/basics.html. Or did CPU engineers restrict their designs to match the segmented demands of the programming languages? Thom Holwerda 2018-06-10 OS News 31 Comments. If they are C++ programers, I hope they consider passing it by reference. Which is why I find Windows 95 a truly fascinating OS: A window to the future built on DOS. He was comparing the memory model of 68k vs. x86. It's possible, but only if you try! Running MS-Dos 5 and down was a nightmare. There was far more and better multimedia software available for the Amiga 500 and handy utilities on Aminet during the early 90s. I can't take someone seriously who writes "Of course, PCs were much more powerful than, say, an Amiga 500.". That's where IBM positioned the 5150 it in the market in 1981. Fast, no. But entering Protected Mode on the 286 is a one way ticket: if you want to switch back to Real Mode you have to reset the CPU! And yet the idea of a ram-disk in the way the Amiga had it. Transparent, yes. The documentation I read at the time claimed that this was to make sharing pieces of segments easier (or perhaps they wanted to encourage people to make zillions of very small segments, e.g., one per procedure). Pinball fantasies, dreams and illusions were still better on the Amiga. If T is 1, it uses a table called the Local Descriptor Table (LDT). The T bit selects which is used. I shoot people for less. 15 Mb XMS MS-DOS 6.0 : 3.07, recognizes max. > Windows 95 was still an extender started from DOS and using its file access subsystem. "A solution might have been to switch back and forth from Protected Mode. Uh... the 68000 had, in fact, exactly 16MB of addressible space (no MMU and a 24 bit address bus). QEMM https://en.wikipedia.org/wiki/QEMM was wonderful. I started on 8088 learning IBM DOS and getting stuff for MS DOS to run on it was generally painless except for some rare exceptions. So we had to tweak. What in the hell about this post got it downvoted? IIRC, Doom's use of Watcom C validated that you could write performant games without using very much assembler, but BP wasn't up to the task. v8086 mode, like many aspects of the x86 architecture, is a big mess. Trust me, it wasn’t a conscious decision, they simply didn’t understand the difference. The more things change the more they stay the same I guess. The whole post is about inferior features of x86. But not in this case, the menu system was not there on day 1. There is a single GDT shared by all code on the system. and powering down the machine when shutting down 95, hiding the fact that DOS was still there. Cue Germanglish rambling about how the PC was effectively just a CPU (and not a very good one) with slow peripherals attached while the Amiga is a perfect jewel of cooperating subsystems with an ISA that's like poetry from the angels. Oh no. That OS/2 2.0 debacle is one of my favorite topics. It wasn't supposed to be a world-beater in the late 80's, it was designed to be a better/cleaner and much cheaper PDP 11. Getting Ultima VII to successfully run, which required its Voodoo Memory Manager [0], was probably how I became comfortable hacking computer settings. Excel 2.1 and Word for Windows 1.0 were written to different EMS standards, however – 3.2 and 4.0 respectively. the Amstrad PC3086 in 1990). Until the arrival of Windows, that is. A "huge" pointer had a selector and offset, and would include the selector in pointer arithmetic, so that you could increment through to the next segment. Everything was a hack and a kludge. I am sorry to say, that the PC did not beat the Amiga untill somewere around 1994. The RL bits specify the Requested Privilege Level (RPL). Look to the '020 machines Sun and others were shipping for examples that make it more attractive. The descriptor parts of segment registers effectively had fixed contents derived from what was loaded into the selector parts, rather than from descriptor tables. Of course, by the time 486, SVGA, etc. Same thing with my first 386SX, got Windows FWG 3.11 for free. If I understand it correctly it uses Virtual Mode which was added in 386. Which is why I find Windows 95 a truly fascinating OS: A window to the future built on DOS. Instead the BIOS would jump to a specific memory location. The descriptors in the GDT and LDT were 8 bytes long, so the physical address of the descriptor is 8 * selector + base_of_descriptor_table.


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