XT-IDE on PCjr

Hardware questions and modifications

Re: XT-IDE on PCjr

Postby Tempest » Sat Oct 27, 2012 7:57 am

So close yet so far...

I just can't seem to create a DOS 3.3 boot disk. I have the files, I'm able to transfer them to a 5.25" disk, but the PCjr says it's not a system disk. Here's what I have:

Modern PC with a 3.5 USB floppy - I use this to download files and transfer them to 3.5" floppy
Old Pentium PC with a 1.44MB 5.25" drive and a 3.5" floppy (boots into DOS 6.22) - This machine can read and write to both 3.5 and 5.25 floppies which makes it extremely useful.
PCjr with the jrIDE and 3.5" backpack drive (not running as I need the driver again)

I'm wondering if some of my problem is that the PC has a 1.44MB 5.25" drive. It has the option to format the disk as 360K which is what I use and I've used it to write 360K floppies before but still it makes me wonder. I also wonder if I need to just install DOS 3.3 on that PC to make a 3.3 bootable disk? 6.22 has the option of adding /s to the format line, but I assume it's going to make a 6.22 bootable disk which the PCjr wont understand. So...

1. Is it possible to make a DOS 3.3 bootable disk from my DOS 6.22 PC? I tried copying the DOS 3.3 boot disk files over to the floppy but the PCjr doesn't recognize it as a boot disk (non-system disk error). I'm guessing this is because some flag isn't set or something to say 'I'm bootable!'

2. If it's not possible to make a 3.3 boot disk from my 6.22 box, can I make any sort of bootable disk? Something that will at least get me into DOS and let me format that hard drive?

3. If I find a 3.5" version of DOS 3.3 (assuming that exists) and boot to it from my PC (the 3.5" is bootable, but booting from the 5.25" take some fiddling with the BIOS), can I make a 5.25" 3.3 boot disk from there?

4. Can I somehow use DOSBOX on my modern PC with the USB floppy drive to load DOS 3.3 and then make a bootable 3.5" disk that I can boot my Old Pentium with and make a bootable 5.25" DOS disk from there? Seems a bit complicated, but I'm out of ideas. :)
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Re: XT-IDE on PCjr

Postby Tempest » Sat Oct 27, 2012 1:44 pm

Ok I've got a DOS 3.3 boot disk going (I found that I had one already as it turns out). I attempted to use FDISK to make a DOS partition, and the program SAID it worked however now whenever I attempt to boot from it I just go into the built in Basic or if I boot from my disk it hangs. If i turn the hard drive off the disk boot normally though. What the heck can I do to fix this?
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Re: XT-IDE on PCjr

Postby Tempest » Sat Oct 27, 2012 3:09 pm

Ok I might actually get getting somewhere. I was able to reformat the hard drive by plugging in a different drive and then swapping it out once DOS was loaded (probably not a good thing to do, but it worked). So now I have a hard drive on my jr. with 32MB free (damn you DOS 3.3!). How do I make this drive bootable? I copied over all the files from my boot disk but it says Non-System Disk when I try to boot from it. What am I missing?

Also, how do I make a drive label?

Once I get the drive bootable I need to figure out how to make a patched DOS 5 boot disk so I can get a larger partition. One step at a time I suppose.
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Re: XT-IDE on PCjr

Postby Brutman » Sat Oct 27, 2012 4:25 pm

Here is the rough idea on how the boot process works.

A diskette is bootable if the following is true:

  • The diskette is formatted
  • A proper boot sector is written to the first sector of the diskette
  • The DOS hidden files are on the diskette in the right places
  • Command.com is on the diskette

When you use FORMAT with no parameters you get the first step. Nothing else.

When you use SYS on a formatted diskette, the boot sector is written and the hidden files are copied over. But you don't get COMMAND.COM.

When you use FORMAT with the /S parameter you get all of the steps, and as a result you get a bootable diskette.

A hard drive is just slightly different. A hard drive has to be partitioned first. When you partition a hard drive the following happens:

  • A boot sector that includes the partition table is written
  • The boot sector also has enough code to "jump" to the first sector of the partition marked ACTIVE

So the steps to preparing a hard drive to boot are:

  • Partition the hard drive (create a DOS partition)
  • Mark that partition as ACTIVE
  • Reboot from a floppy after those changes are saved.
  • FORMAT the partition

And of course, you can save yourself some steps by using FORMAT /S, which puts some bootable code at the start of the partition (I think) and copies the hidden files and COMMAND.COM there.

Now when you use FORMAT /S it is going to create a bootable drive with the same version of DOS that you are using. So if you want to make a bootable DOS 3.3 hard drive, you need to be using DOS 3.3 FORMAT and have booted with DOS 3.3. If you want DOS 5, then you need to have already booted with DOS 5.

DOS 5.x and higher are special because the code in boot sector needs to be altered to make it load on the PCjr. I posted the instructions in the forum. Create a bootable DOS 5 diskette on a normal machine, then patch the diskette. Once it is patched it will only work on the PCjr so make sure it is not your only copy. FDISK and FORMAT your hard drive as above, but then repeat the patch procedure on the hard drive boot sector to correct that boot sector and make it bootable.

Some random thoughts:

  • You can't make a bootable diskette by just copying files. You need to use SYS or FORMAT /S.
  • If you are transferring diskette images around, if the original diskette that the image was created from is bootable, then any copy of the image is bootable. That is because an image of a diskette has *all* of the sectors on the diskette, not just the files.


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Re: XT-IDE on PCjr

Postby Tempest » Sat Oct 27, 2012 6:22 pm

That worked. I didn't know you could format a hard drive with /s. My drive now boots up with DOS 3.3

I'll have to try that DOS 5 modification trick. Of course first I need to make working DOS 5 disks.

How do I create a label for the drive? Is that something that can only be done during format?

Also, how do I use the date/time chip on the jrIDE with DOS to set the time automatically?
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Re: XT-IDE on PCjr

Postby jmetal88 » Sat Oct 27, 2012 8:37 pm

Get rtc.com from here: http://www.retrotronics.org/svn/jride/t ... ebuilt/jr/

I forget for the moment what the command line usage is, but that's the utility you need to get and set the time, currently. I believe Mike wants to add BIOS support for the clock eventually, so that DOS can get and set the time by itself, but that of course depends on when he has free time to do so.
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Re: XT-IDE on PCjr

Postby Trixter » Fri Nov 02, 2012 9:52 am

While I received a jrIDE as a gift (woohoo!), I purchased a second one from alanh and will be building up a sidecar in November. However, this troubled me: "I have a DOS 5.0 disk, with the PCjr patch applied to the boot sector". This thread is very long, and Mike's page doesn't mention the boot sector patch at all. Where can I get more info on the patch? Is it just a BPB value(s) patch or is there actual new code involved? I want to build up a DOS 7.0 system with this one, plus I want to know for the future in case my DOM fails and I need to make another one.
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Re: XT-IDE on PCjr

Postby jmetal88 » Fri Nov 02, 2012 11:33 am

Trixter wrote:While I received a jrIDE as a gift (woohoo!), I purchased a second one from alanh and will be building up a sidecar in November. However, this troubled me: "I have a DOS 5.0 disk, with the PCjr patch applied to the boot sector". This thread is very long, and Mike's page doesn't mention the boot sector patch at all. Where can I get more info on the patch? Is it just a BPB value(s) patch or is there actual new code involved? I want to build up a DOS 7.0 system with this one, plus I want to know for the future in case my DOM fails and I need to make another one.


viewtopic.php?f=3&t=224

They used to be posted elsewhere, too, but I can't find the older information, now.

EDIT: You'd probably have to engineer your own patch for DOS 7. There is a patch for DOS 6.x, but I believe it is slightly different than the patch for DOS 5. I feel like DOS 5 should be adequate for anything you'd want to do on a PCjr though.
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Re: XT-IDE on PCjr

Postby Brutman » Fri Nov 02, 2012 3:57 pm

The procedure is generally the same for all versions of DOS. The offsets might change slightly, but it's easy enough to scan visually and correct.

If you read the referenced threat post it is pretty explicit about what is being done (and why).


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Re: XT-IDE on PCjr

Postby jmetal88 » Fri Nov 02, 2012 5:23 pm

Brutman wrote:The procedure is generally the same for all versions of DOS. The offsets might change slightly, but it's easy enough to scan visually and correct.

If you read the referenced threat post it is pretty explicit about what is being done (and why).


Mike


That's essentially what I meant (the offsets being different), I just didn't explain it well.
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