Classic Computers Emulated Part 5: The IBM PC/AT

 

IBM Introduces a New Standard.....

The IBM PC/AT

The IBM PC/XT was never meant to be long-term products for Big Blue. The PC/XT was essentially a 8-bit computer that had a 16-bit CPU shoved into it. 8-bit computing could only go so far. IBM realized this, so they started to work on their successor to the PC/XT, the PC/AT. Yes, I know it's not a very imaginative name. IBM could never come up with awesome names for their products. However, this computer was a massive upgrade to that of the PC/XT in multiple ways. The PC/AT was a true 16-bit computer. This was highlighted in the name. AT was an acronym for Advanced Technology, which it would have been for 1984. The machine had both a 16-bit motherboard and processor. It introduced EGA graphics, which allowed for higher resolutions and more on-screen colors displayed at once. Also, the motherboard had 16-bit ISA expansion slots. Backward compatible with the 8-bit standard, these new slots offered greater performance for adapters that took advantage of them. Released in August 1984, like its predecessor, the XT, the PC/AT took the business world by storm. 

The IBM PC/AT was powered by the Intel 286. Released in 1982, the processor could address up to 16MB's of RAM, which was much more than the 1MB memory limitations of the Intel 8088/8086. The processor was designed with multitasking and multi-user support in mind. The processor incorporated mechanisms for protected mode, which allowed programs to run in memory segments that were "protected". Programs couldn't overwrite eat others memory segments. This meant that there was memory protection for programs, and multitasking could take place without system crashes. Programs overwriting each other memory could lead to system crashes. These features enabled the UNIX operating system to be ported to the IBM PC/AT in the form of Microsoft Xenix. Xenix was Microsoft brand of the UNIX operating system that was licensed from AT&T. The PC/AT version of the OS used the protected mode feature of the 286. Multitasking and multi-user supported was provided by the 286 advanced capabilities. 

The 286 also provided a "real" mode. Real mode was a feature that allowed the 286 to run DOS programs in compatibility mode. Essentially, the computer with the 286 acted like a faster DOS PC. Real mode still restricted DOS to 640KB of RAM, even it the computer had more memory. Again, this was done to provide compatibility with DOS, which was essential at this point, since MS-DOS was the dominate OS running of IBM Compatibles. Since the 286 was designed right before the DOS takeover, it's engineers felt that it wasn't essential to use protected and real mode at the same time. Therefore, it wasn't possible to use both modes at the same time. That meant that if you used the PC/AT, you either had to run it as a advance workstation running Xenix, but loose compatibility with DOS, or run DOS, and not use the advance features that the 286 offered. Eventually solutions came up to remedy this problem, but it would be a issue that would follow this platform for the next couple of years.

The EGA (Enhanced Graphics Adapter) video standard was introduced alongside the PC/AT. This adapter was a massive upgrade compared to CGA. EGA offered a resolution that could go up to 640 by 350, 16 colors on-screen. Designed with "business graphics" in mind, this card had slow video timings. Therefore, it was not really geared towards gamer though. However, throughout the cards existence, EGA was widely supported by the game developers, and many games supported the graphics modes offered by the card.  

Along with the faster processor and newer video adapter, IBM introduced a new 1.2MB floppy disk drive. It was in the 5-1/4 inch floppy form factor, and was backward compatible with the 320/360KB disk. Also introduced was a new 20MB Hard Disk drive as well. IBM released PC-DOS Version 3.0 to support the new disk media. 

The IBM PC/AT, like the XT, was a massive success in the marketplace. Despite it's massive price tag, $5,000 starting price, this machines made rapid inroads in the corporate market. With a price tag like that though, this machine or its clones wouldn't be found in the home setting for the next couple of years. The system shipped with a 20MB Hard-Disk pre-installed as well. Many of the first shipping units had issues with the hard-disk. They were constantly failing. Many of the first system had issues with the floppy-disk drives as well. Also, because the BIOS took awhile to perform it POST (Power-On-Self-Test), the computer was perceived to be slower than a lot of the clones on the market. Despite that, the PC/AT found a lot of heavy use in many industries. It was used by CAD designers, office-works, and it in networking environments as well. Because the system could run Microsoft Xenix, the PC/AT could be a capable UNIX workstation as well, providing multi-user, networking support, along with multitasking capabilities.

Because DOS ran in real mode on the 286 processor, the OS didn't have any access to the advanced capabilities of the 286. With that said, environments were released that tried to give DOS multitasking capabilities. One such environment was Desqview. When running on the 286, Desqview was still limited by the 640KB barrier imposed by the combination of DOS and the IBM's memory segmentation of the 8088, or in the 286's case, compatibility mode. Desqview allowed users to multitask DOS programs, within the memory constraints of course.
IBM released their own multitasking environment for DOS. Called TopView, it could also multitask DOS programs like Desqview. Also, like Desqview on the 286, TopView was also restricted to the 640KB memory barrier imposed on it.
The EGA video standard was introduced alongside EGA. Many DOS programs were updated to support the new video standard. AutoCAD, Lotus 123, Microsoft Word and Chart, and various other applications. Mouse System, a company that sold mouse's, released PC -Paint for mouse users to play with. Programmed by John Bridges and Douglas Wolfgram, the program was often bundled with their mouse's. The first version of PC-Paint supported only CGA, while version 2 was updated to support the new EGA graphics adapter. It had a user interface similar to MacPaint on the Macintosh, with the menu bar at the top of the screen, paint window, and tools on the side window. Mouse users felt at home.

IBM new standard would revolutionize the computer industry. PC/AT users were often times the early adopter of new technologies, like networking, the VGA video standard, 32-bit processors, the Creative Labs SoundBlaster audio adapters, etc. Technologies which would eventually take the PC to new heights.

PCEM
I decided to use PCEM to emulate the IBM PC/AT. The emulation of the computer is very solid. I had no issues running Desqview or IBM's Topview on the emulated computer, or any DOS program for that matter. The emulation is almost carbon copy to the real thing. Granted though, I only used the PC/AT configuration as a DOS PC. I gave the virtual machine only 1MB of RAM to play with. DOS isn't going to need any more memory anyway, as it still could only use 640KB of RAM. Also, giving the machine 2 or more MB's of RAM causes the BIOS memory check to take a very long time to check the PC's memory. Wars have been fought in less time than it took the PC/AT to finish a memory check. I equipped the PC/AT VM with a 30MB Hard-Disk image that was formatted with IBM PC-DOS Version 3.10 from 1985. I haven't attempted to run Microsoft Xenix on it. Most people who used this machine back in the day used it simply as a faster DOS PC. Running this emulated computer made me feel like a office worker from the 80's. IBM introduced a very advanced computing standard, but as reflected in the next article, they would struggle to remain the market leader of the standard that they created. Big Blue is starting to have the computing blues.
Work Cited:

PCEM - https://pcem-emulator.co.uk/

Intel 286 - https://en.wikipedia.org/wiki/Intel_80286

IBM Personal Computer/AT - https://en.wikipedia.org/wiki/IBM_Personal_Computer/AT

IBM PC DOS - https://en.wikipedia.org/wiki/IBM_PC_DOS

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