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Thread: Are XT clones using 8086 processors still 100% compatible with the original PC XT?

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    Default Are XT clones using 8086 processors still 100% compatible with the original PC XT?

    How compatible are XT clones using an 8086 instead of an 8088?

    I have been told before , that early XT clones that used 8086 processors had various issues with compatibilities at first than later 8086 clones became even more compatible. I know there are differences between the 8088 and 8086 processors , like a 8 bit vs a 16 bit bus , and some 8086 xt clones only had custom bus for cards, while some did use 8 bit isa bus only, and some had both a compatible 8 bit isa bus, and a custom bus for 16 bit cards. I am also guessing programs could have ran differently due to the 8 bit vs 16 bit bus change, making some programs run much faster, which could have been an issue?

    I have been also told there really isn't much of a speed boost of using a 8086 over a 8088 at the same frequency , some article online says there is only a 30% to 60% performance increase when programs are ran on a 8086 vs a 8088.

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    Witness, say, the AT&T 6300/Olivetti M24; pretty much compatible, but faster. (8 MHz 8086). This was considered to be an advantage.

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    I think the answer to this depends on how you define "100% compatible". Strictly speaking unless you make a "cycle-exact" copy of the original XT hardware and fit it with a pirated copy of IBM's BIOS ROMs you'll probably manage to find something that doesn't run on your clone. Assuming everything else is equal (IE, they're running the same BIOS code and the hardware is identical other than the system bus is set up to allow 16 bit access to word-aligned memory locations) I can't think of any inherent reason why an 8086-based clone should be less compatible than an 8088 one for all software that's not reliant on the exact timing of software loops.

    Remember that back in the day, before 1984 or so, there were a lot of companies that were producing "MS-DOS" computers that were not strictly IBM PC/XT clones. A fair number of these were based on the 8086 instead of the 8088 because the aforementioned small speed advantage of the 8086 gave them a selling point. This could certainly contribute to the reputation of 8086 itself somehow being a major factor when it was really more due to underlying architectural differences with the system. Machines that specifically tried to be PC compatible, like the aforementioned M24/6300 or the Compaq Deskpro were very compatible with the XT, moreso than some of IBM's own machines based on 8088s. (Say the IBM Convertible or PCjr.)
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    Quote Originally Posted by Chuck(G) View Post
    Witness, say, the AT&T 6300/Olivetti M24; pretty much compatible, but faster. (8 MHz 8086). This was considered to be an advantage.
    Playing some XT class games on M24 is quite difficult as it is so fast, and M24 has no deturbo function. Example: Xonix.

    Also some floppy disk tools like IMD and Teledisk are not functioning on M24 with the onboard controller as it has some incompatiblity issues. But that is not related to the 8086 processor.

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    Also, the M24/AT&T 6300 handles word (16-bit) transfers from (8-bit) ISA cards in a different way from the IBM PC XT - these have to be split into two 8-bit transfers, but the order in which the two transfers is performed is different between the M24 and the XT. This could cause incompatibilities with some cards (or rather, with the software/firmware written for those cards). An adapter to fix the incompatibility existed - long thread here.

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    I think a better question is can a completely compatible computer be based on the 8086. The 6300 etc. aren't. "clones" in the strictest sense and jence don't enjoy complete compatibility. And an 8086 can't be dropped into a socket meant for an 8088. The 8086 has a data bus of 16 bits, the 8088 8 bits. The 8086 has a 6 byte instruction queue, the 8088 4. These are the only differences afaik. Some high powered scientific/real time/automation applications may choke due to subtle timing differences. But in general I would think the differences between the 2 chips would be transparent to software.

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    Possibly the 8086 machines got a bad rap, because many early 8086 machines had poor IBM PC compatiblity that had little to do with the CPU itself. I suspect in some cases these machines were really intended to run some other OS (like CP/M-86 or Xenix) and then minimal MS-DOS or IBM PC compatibly shoehorned in.

    For example, the Eagle 1600 used an 8086, but (and I still need to dig in to this a bit more) seems to have issues making 16-bit access on the 8-bit ISA cards, as well as a pile of other goofiness.

    Compare that to a much later 8086 clone such as the Tandy 1000SL. Never personally used one, but I would expect that to run almost all IBM PC software and even work with almost all IBM PC ISA cards.

    Now, programs such as games that required 100% accurate timing would always be a bit off using an 8086. Faster 8088 based "turbo" XTs, however, maintained compatiblity by allowing fallback to 4.77mhz mode.

    So, to answer the original question, it COMPLEATLY depends on the machine. With the exception of cycle-exact speed, compatibility depends on the implementation of the motherboard resources and I/O devices.

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    I have in no way done an exhaustive test, but my Compaq DeskPro has run everything thrown at it, from VGA, XT-IDE, networking, and EMS cards. It came a little later than, say the 6300, and Compaq prided themselves on compatibility. It does have a slow compatibility mode, but I can't attest to how exact it is. I doubt 8088 MPH!s racing the beam code would work like on an IBM XT (or Compaq Portable). But then who would buy an 8086 XT clone just to have it only run as slow as an XT?

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    Even the PS/2 Model 30 had the problem of the wrong way floppy controller. I think a combination of improved compatibility especially with video cards and the development of the AT causing developers not to search too hard for IBM XT-only solutions left the Turbo-XT (8086) field able to run nearly all DOS programs written in 1985 or later.

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    I suppose 100% IBM compatibility would include being able to run BASICA from PC-DOS, which AFAIK does not work on any clone due to the reliance on the ROM BASIC in a genuine IBM. Although also AFAIK GW-BASIC will run anything BASICA will so no real issue there.

    The Amstrad 1512/1640 were deemed to be pretty much as compatible as a clone could get (the 1512's composite sync monitor aside) and they had an 8086. Could even run IBM PC-DOS.

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