Patching IBM DOS 5 for the PCjr

[Trixter]

More details, such as what you've tried and what isn't working, is necessary to troubleshoot your issue.

[ZiggyTheHamster]

What was happening is that it would hang, sometimes with graphic corruption, when it started to boot. I assumed it was just a boot loader issue, but it actually turned out that files on the disk were corrupted too. Something's not right with this CF card because another one works fine (I should have tried one earlier). Which is a shame, too, because this one was a SanDisk Ultra and was supposed to be highly recommended for use in retro systems, and the one that actually works is a trash class Toshiba CF card I picked up for $1.

[Trixter]

Yeah, there are some combinations of CF cards + adapters that don't work, unfortunately. It's best to either use an industrial DOM 8GB or smaller, or just keep trying CF cards from different manufacturers, usually 2G or smaller.

[MattCarp]

I’m curious, why wasn’t this a problem for pre version 5? Did earlier versions of DOS detect PCjr machines and use the value from 0415?

[vonhelmst]

Does this also work for MS DOS 6.22?
I get an error after line D 1BF L27.

Tried with BOM, CF and 2.5 HD.
All recognized by JR-IDE BIOS.

When attempting to boot off C: it hangs at Starting MS DOS 6.22.

Thanks in advance for any assistance here.

[vonhelmst]

I also tried the PatchDOS utility.
It succesfully patched my 5.25 6.22 boot disk and it does boot OS off floppy.
I get the following error when attemlting to patch the BOM, CF and 2.5 HD.
A fatal error has been encountered:
Boot sector read was not successful.

I did the boot sector patch, fdisk w.active part, format /s prior to the above.

Thanks again

[alanh]

The only time I was able to successfully boot 6.22 is when I completely replaced the first 128K visible to the CPU with off-board memory and gave all the 128K on-board exclusively to the video gate array. But this isn't easy to do and breaks all sorts of software including the JR's own video BIOS. You can read more about the details in my long rant here:

viewtopic.php?f=1&t=1005#p7249

I think it has something to do with 6.22 requiring more initial memory than the 48 KB lower continuous before encountering the power-on mapping of the video frame buffer. If that is true, it might be possible to load 6.22 without my intrusive (and incompatible) mods by fixing up the BDA to show 736 KB memory and relocating the video frame buffer to +112 KB before the initial DOS entry point. You could do both in similar patch code that is posted here. Haven't tried and now my Jr is packed away for a while. The Amiga 2K needed the bench space!

[vonhelmst]

Update-
I was able to boot off 6.22 (both CF and 2.5 SSD) using my secondary system with JRExcellerator installed, PC memory mode enabled.
alanh, thank you for your reply.

[Brutman]

I've posted detailed instructions on how to patch DOS 5 and above for the PCjr at http://www.brutman.com/PCjr/docs/Patchi ... e_PCjr.pdf.