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Thread: Tandy 2000 vs Tandy 1000 SL w/NEC V30

  1. #21
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    It's a bit silly to compare a 2000 to a 5150. The former is very broken when it comes to PC compatibility and isn't an ISA machine. It's technically not even BIOS compatible as it differed in many ways from IBM's BIOS. IO.SYS was heavily modified by Tandy and included most of the BIOS functionality as a supplement. To call it MS-DOS compatible is somewhat of a stretch as Tandy only ever modified 2.1 and dropped support by the time 3, 4, 5, 6, etc were released. None of the internal nor external peripherals were mapped in the same locations, had the same interrupt numbers, nor DMA channels. Video memory was in the E and F segments. Disks were 720K. Incompatible keyboard, etc.

    The mistake of the 2000 is why Tandy quickly struck a deal with Tandon to rebadge their 5150 clones as the 1200.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  2. #22
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    It's not a "broken" machine in any sense. It served admirably in their back offices for years. It was only intended to be pc compatible, for the 500th time, at the bios and ms-dos function call level, which it was. Io.sys had to be "modified" or expressly written to take advantages of it's specific hardware. I have no clue what bios functionality as a "supplement" is supposed to mean. Most bios interrupts would produce the expected results as if it was an ibm pc. By ms-dos compatible I never ever implied vanilla dos would run. I meant ms-dos function calls were valid. This is a fact. The rest of the incompatibilities you mention are what made it incompatible. That was a given before the thread was posted.

    What I compared was a 2000s mobo with a 5150. If you're original premise was valid, and I don't hold it to be, there's less of a basis to compare a 2000 with a 1000 then an ibm pc. But to each their own.

  3. #23
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    It's a very broken machine. More than half the software written for the 5150 by 1985 would not run on a 2000. A number of public reports back that up including my own experiences with my 4 2Ks, building boards for them, exhibiting them at shows, etc. And the machine required specialized hardware versions of most of the marquee software touted for it because of the incompatibilities.

    By supplement, I mean 80% of the system BIOS was not contained in a ROM chip on the motherboard. It was contained in IO.SYS and hooked the relevant interrupt vectors when DOS loaded. The ROM BIOS has the basic startup code to setup the 80186's PCB, enough of an Int 13h disk services call to perform a reset and a read sector, enough of video BIOS to generate characters and process the teletype function, and enough of an Int 19h to get the DOS MBR off media. You can verify all this my looking at my disassembled and reverse annotated BIOS ROM for the 2K hosted on Malcolm's github repo:

    https://github.com/Tandy2K/Tandy2000...mbled/bios.asm

    Malcolm also has a nice collection of all the T2K specific software titles that have been recovered. My point is the commonality between the 5150 and the 2000 has been dramatically overstated. It doesn't extend much farther than both machine's ability to run version 2.1 of COMMAND.COM! Malcolm did turn up a the long missing demo version of Windows 1.0 for the T2K. So I guess that makes the machine 'more' compatible when it comes to Windows 1.0 apps.

    EDIT: I've had on my bucket list for some time to write a largely IBM PC compatible BIOS for the 2K so other vanilla DOS versions would run on the box. However I'm afraid it's start date has been pushed out to 2040'whatever.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  4. #24

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    Having personally owned a Tandy 2000, I have to agree the compatibility was definitely not there. The fact applications had to be specifically written for it proves that point. Was it a bad computer, not in the least. Tandy learned their lesson and the following year came out with the Tandy 1000 series and the rest is history as to how successful those computers were. Many computer vendors learned that lesson too, that if you deviated from the IBM standard, you are not going to do all that well. Just look at the DEC Rainbow, another better than IBM computer, which ended up in the same boat. Just my two cents on this subject.

  5. #25
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    Quote Originally Posted by jasa1063 View Post
    ]...deviated from the IBM standard...
    That is tricky too. There really wasn't a standard per se - or at least few companies got it right or were willing to attempt a functional copy. The PC had been out for 2 years but the T2K development likely started less than a year after the PC launch. And it was released a few months after both the XT and the first Compaq. Most of the work-a-likes came out around the same 18 months - mid 82 to late 83. The IBM standard was ratified when clones started flooding out of Taipai in the mid-80s.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  6. #26
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    Quote Originally Posted by tipc View Post
    I never said anything to the effect that it was "crippled". It's a chip with onboard peripherals, that the 8088/86/286 do not have, and you may achieve something in the way of optimization by utilizing onboard peripherals, when appropriate, as opposed to relying on external ones. Clearly Tandy could have used less external logic, or different sets of ic's.
    Could you explain how Tandy could have used less external logic? I feel like you're under the impression that the 80186 has more built into it than it actually does. It is *not* a "System on a Chip" design with a pile of unused peripherals on it; the 80186 has a DMA controller, a PIC, a timer, and a clock generator built into it, and the Tandy is using *all* of those pieces.

    Basically using the 80186 instead of an 8086 lets you get rid of about half a dozen ICs that are otherwise needed to set up a minimal x86 computer. If your intention is to build a large featureful machine with more than two DMA channels, four interrupts, or three timer channels then you'll need those additional chips that are on the 2000's motherboard to cascade everything out. Maybe the Tandy 2000 is "overbuilt" compared to a machine like the Littleboard but its architecture is perfectly valid.

    I have *no idea* where I can get a look at the Northstar Dimension's motherboard, but reading the manual for the LittleBoard is a thing I can do, and I think the difference there simply boils down to the machine being a lot less ambitious than the 2000. It does indeed get by with having only four interrupts; among other things it uses a kind of oddball 2681 DUART for its serial ports, which lets it pile two serial channels and a number of other functions onto its one interrupt line. (Instead the Tandy 2000 slaves some 8259s off its internal PIC to provide more interrupt channels, exactly the same way an AT slaves a second PIC off its first one to provide interrupts 8-15.) The Tandy 2000, by contrast, has a ***load of interrupt channels; they're documented on page 71 of the manual, and they look *nothing* like a PC's interrupt channel layout.

    (The Mindset is also a significantly different animal than the 2000; it's closer to the Littleboard in terms of piling multiple functions onto the handful of I/O channels that are available, and it does that with the help of some proprietary ASICs instead of off-the-shelf Intel parts.)

    You've made it clear that in your opinion the 2000 uses too many chips to be a "good" 80186 design, but that seems to largely be an aesthetic judgement. And if you actually look through the manual it's clear that though it shares those chips with the 5150 it implements them in entirely incompatible ways. (But ways that make sense for the 80186.)
    Last edited by Eudimorphodon; January 13th, 2020 at 11:14 AM.
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  7. #27
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    Quote Originally Posted by eeguru View Post
    EDIT: I've had on my bucket list for some time to write a largely IBM PC compatible BIOS for the 2K so other vanilla DOS versions would run on the box. However I'm afraid it's start date has been pushed out to 2040'whatever.
    So in the meantime...

    By any chance, assuming you have a working 2K handy, have you ever tried running the Topbench stub on a 2000? Supposedly it uses standard DOS I/O calls to route the core benchmark scores into a file and *may* work on it. If it does manage to work it may answer some questions. (My money is on it not quite working because of the differences in the timer functions, but my money is just a plug nickel because, well, heck if I know.)
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  8. #28
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    The two working ones are buried in storage atm. I have to pick and chose which projects get moved to active work areas. I have another NIB. And probably 2.5x more in spare parts. 2 CM-1s (1 NIB) and 1 VM-1.
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  9. #29
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    Quote Originally Posted by eeguru View Post
    The two working ones...
    None of them work. They're all broken. Remember?

  10. #30
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    Quote Originally Posted by eeguru View Post
    I have to pick and chose which projects get moved to active work areas.
    Certainly understandable; in between temporary drafts when I commandeer extra space the active portion my retro junk hobby is generally confined to about six square feet of permanent real estate.

    Figured it'd be worth checking because at this point I am genuinely curious what the results would be. In broad scope the internal operating speed of the 80186 looks to be close to that of the 80286 for a good mix of instructions, but it also looks like it takes a significant hit thanks to its multiplexed bus to anything that goes in and out. A V30 may well have a fighting chance depending on the instruction mix.
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