Making DOS ROM for Turbo XT? \ VOGONS

Making DOS ROM for Turbo XT?

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First post, by RacoonRider

Posted on 2015-11-13, 04:23

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I've been thinking of building an XT some day, I've got quite a lot of parts already, currently the only questions are EGA monitor, XT keyboard, floppy controller and sound (if any). When I say XT, I mean a computer with two 5-inch floppy drives and no hard drive, something closer to the real XT era stuff.

I already have an hp200LX machine, which is pretty much a handheld XT:

The application manager you see on its screen, as well as DOS, fits in ROM memory, which is very convinient. Now here's the point. The following Juko ST board has an empty socket near BIOS ROM. I was wondering if I could make a bootable DOS ROM and fit it there. Is this entirely possible?

Am I missing something? Or do I have to have a board like this, with multiple ROM sockets?

Reply 1 of 5, by carlostex

Posted on 2015-11-13, 10:01

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I have a similar Juko board. The empty socket is for ROM BASIC which should start on F600h. You can always try it, but you need either to find a ROM DOS image and burn it on a compatible 27256 EPROM/EEPROM or code your own.

Even so it is not guaranteed to work. BIOSes usually look at F600h when it cannot find anything else to boot from HDD or Floppy.

Reply 2 of 5, by keropi

Posted on 2015-11-13, 10:08

keropi Offline

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there was a thread on VCF about this, it's not an easy task to do from what I gathered

http://www.vintage-computer.com/vcforum/showt … 6-DR-DOS-in-ROM
http://www.vintage-computer.com/vcforum/showt … p?15707-Rom-dos

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Reply 3 of 5, by Malvineous

Posted on 2015-11-13, 11:43

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As those threads point out, it's more about whether there's enough space in ROM than whether it will work.

For comparison, DOS 5.0's minimum boot files (io.sys, msdos.sys, command.com) are 124kB. A normal ROM chip is 64kB, although with suitable hardware (e.g. a network card with a bootrom socket) you can get them up to 128kB and perhaps more.

If your board is as @carlostex describes however, and that ROM socket sits at F600, then you only have 40kB to play with.

I think you'd at least need a board like this one, where there is one ROM socket for each 64kB address, in order to have a real chance of MS-DOS in ROM. Either that or a really early version of DOS, assuming those are much smaller (don't have access to any right now to check.) I suppose a custom cut-down build of FreeDOS could be a possibility though.