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Thread: Model 4 Newbie Question - Model III Compatibility Mode

  1. #1

    Default Model 4 Newbie Question - Model III Compatibility Mode

    Guys, I know that the M4 has that Model III compatibility mode. I also know that with the M4P, you can force it by pressing "3" or "F3" during boot.
    How about the M4? Are there such boot key combinations to boot in Model III mode? What I found online is that you need to boot TRSDOS 1.3 for Model III.
    Is that all that is required. What if I want to fire up something else than TRSDOS 1.3, but still want Model III compatibility? Thanks!

  2. #2
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    Quote Originally Posted by LambdaMikel View Post
    Guys, I know that the M4 has that Model III compatibility mode. I also know that with the M4P, you can force it by pressing "3" or "F3" during boot.
    How about the M4? Are there such boot key combinations to boot in Model III mode? What I found online is that you need to boot TRSDOS 1.3 for Model III.
    Is that all that is required. What if I want to fire up something else than TRSDOS 1.3, but still want Model III compatibility? Thanks!
    So I don't know about key combinations, but it seems like the M4 figures it out on the fly. Meaning, you need to boot to some type of Dos, and that dos kinda determines what Model you're emulating. LSDos 6 will only work on a Model 4, so it boots into that mode. When you use LDos 5.x, it will emulate a model 3. I think the same goes for whatever TRS-DOS you are using. Just boot to a Model 3 or Model 4 TRS-DOS and you'll be in that mode. At least, that's been my experience.
    -- Brian

    Systems: Amstad PCW 8256, Apple IIe/II+/GS/Mac+/Mac 512k, Atari 800/520STFM, Commodore 64/128/Amiga 3000/PET 4032/SX-64, IBM PS/1 2121-B82, Kaypro II, Osborne 1, Tandy 1000 SX, TI-99/4A, Timex Sinclair 1000, TRS-80 Color Computer 3/Model 4 GA

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    The desktop Model 4 has ROMs that start the system up in Model III mode; the 4P, on the other hand, needs to load a ROM image ( Typically MODELA/III if I remember correctly, at least for English language) into RAM, then it executes the startup code in the ROM image.

    The desktop 4 will boot any Model III DOS just like a III will.
    --
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

  4. #4

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    The TRS-80 Model 4, boots either in Model 3 or Model 4 mods according to the version of LDOS/TRSDOS/NEWDOS diskette you boot with, no special keys to press aside reset
    Model 3 mode basic is BREAK-Reset combinaison (also know as cassette mode).
    The TRS-80 Model 4P was build without the Model 3 ROM, why you need to load it during dos boot sequence. (modela/iii file on disk)
    Last edited by Patrick Bureau; February 12th, 2020 at 03:01 AM. Reason: typos and various adjustements
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    The technical reference manual states that the DOS command is "SYSTEM (slow)" for slower mode and "SYSTEM (fast)" for faster mode. I tested this command with LS-DOS 6.3.1; with each invocation DOS accessed the system disk, did not generate an error and returned a prompt. No message was issued, however. Issuing a DIR command thereafter seemed to update the screen at a slower pace in the slow mode.

    YMMV.

    -CH-

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    Yeah, that command only controls the clock speed. The differences between Model III and Model 4 mode are more substantial than just the clock speed; there's the memory map (Model 4 mode is 64K RAM with the possibility of 32K-bank bank-switching if you have 128K installed, ROM mapped out; video RAM is not mapped-in by default (it and the keyboard together can be mapped-in to two different locations); keyboard matrix is not mapped-in by default) as well as 80x24 video instead of 64x16 video.
    --
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

  7. #7

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    Thanks to all for the explanation. It would be interesting to learn how exactly the M4 figures out when it has to go in M3 mode, and how this is accomplished on a technical level. I guess M3 mode is the default, and M4 is enabled by calling some specific M4 initialization routine that is only in the bootloader / bootsector of TRSDOS 6.x, and hence specific to the M4?

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    Quote Originally Posted by LambdaMikel View Post
    Thanks to all for the explanation. It would be interesting to learn how exactly the M4 figures out when it has to go in M3 mode, and how this is accomplished on a technical level. I guess M3 mode is the default, and M4 is enabled by calling some specific M4 initialization routine that is only in the bootloader / bootsector of TRSDOS 6.x, and hence specific to the M4?
    Maybe this helps?

    http://www.classiccmp.org/cpmarchive...odel4info.html

    The Model 4 looked a lot like the Model 3, and even contained the Model 3 ROMs. If you turned the machine on without a diskette in it, it would boot up just like a Model 3. The magic happened when you booted up a Model 4 DOS. The machine swapped out the boring old ROM for 64K of RAM, allowing it to run CP/M as well as TRSDOS 6.x or Model 4 DOSPLUS. The screen and keyboard were still memory mapped, but it was a "hidden" map. An output port set whether the system's RAM or the screen and keyboard were mapped into upper memory, although it wasn't documented how to do that yourself. The idea behind the DOS was to let the operating system handle all that for you, so the same program would run on any LS-DOS 6.x based machine.
    -- Brian

    Systems: Amstad PCW 8256, Apple IIe/II+/GS/Mac+/Mac 512k, Atari 800/520STFM, Commodore 64/128/Amiga 3000/PET 4032/SX-64, IBM PS/1 2121-B82, Kaypro II, Osborne 1, Tandy 1000 SX, TI-99/4A, Timex Sinclair 1000, TRS-80 Color Computer 3/Model 4 GA

  9. #9

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    A very good and informative read, thanks for the pointer!

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