Best DOS to use on the Pcjr?

[Tempest]

Bump my ancient thread.

Are there patched DOS 5 disk images somewhere? In addition to trying DOS 3.31 I'd like to see if 5.0 is any good. I assume it's only available on 3.5" floppies (I have 3.5" drive so that should be ok).

[arcadeshopper]

https://web.archive.org/web/20190415030 ... /pcjr.html

if you are looking for those files..they are backed up there
I got my jr booting from the dos5 disk, fdisked a 1gb partition and formatted and it boots IF I boot the floppy first.. otherwise it won't boot.. hmm

[Brutman]

Somehow I missed this thread.

I've done the DOS 5 patch many times but I've never really written a good howto for it. I think Trixter wrote a program but I prefer to do it manually because that technique applies to any version of DOS.

If the boot sector of your hard drive is patched, or was created using a format.com that was patched, then it should be bootable. But the PCjr also requires STACKS=0,0 to prevent DOS from installing it's own keyboard handler. See viewtopic.php?f=3&t=224 for the original thread where I discuss the patch.

[alanh]

It really should just be done in the option BIOS. My $.02

[Brutman]

It's possible to do that, but I would want it controlled by a switch somewhere. Maybe it's time to think about finishing the project and using some of the FLASH area for configuration flags.

Moving the contents of 0x415 to 0x413 would allow us to skip patching the later versions of DOS. But I'm loathe to change the personality of the machine too much. With the current setup, every PCjr with a jrIDE behaves the same as PCjrs that use a different expanded memory solution. I think there is value to that consistency.

[Brutman]

It's actually worse than filling in 0x413 correctly. DOS also has to load above the video buffer. Jrconfig moves the video buffer first, then fixes things up so DOS loads above the video buffer. And then you have to decide how much video buffer should be reserved and how much of the slow RAM should be hidden by a RAM disk, or not hidden if you are trying to get the maximum amount of memory.

I think the minimalist approach in BIOS is the correct way to do it.

[alanh]

It's the BIOS responsibility to tell DOS where the top of usable RAM ends via the BDA. The Jr system BIOS doesn't scan past 640KB. An option ROM on an adapter providing extra memory above 640K and fully aware of the system BIOS scan limitations should verify it's upper memory works correctly and (IMO) should fix-up the BDA to reflect it's tested configuration. I primarily want to eliminate the double boot which is only necessary because DOS reads the available memory from the BDA only once before JRCONFIG.SYS loads.

Also any .SYS file loaded from CONFIG.SYS can tell DOS how many pages of RAM past it's load address DOS should keep allocated as part of that driver. JRCONFIG.SYS always computes the difference from its load address to absolute 128KB and reserves that much. This effectively extends the footprint of JRCONFIG.SYS all the way to the first 128KB boundary in all situations. I don't recall it even moving the frame buffer at all. I'm pretty sure it leaves it at the end of the first 128KB so the space in the dummy DOS RAM reservation will be continuous. It always reserves the remainder of the first 128KB so that nothing will ever load into slow RAM. Then it provides a second loadable driver that will install the RAM disk I/O handlers for that region if the user wants the RAM disk function.

I have a rewritten BIOS as a starting point. I'll dust it off this week and assess where it's at.

[Brutman]

JRConfig.Sys can reserve the memory and not use it, make it available as a RAM disk, or move the video window downward if just reserving the space or using it as a RAM disk is not desirable. That's how one used to be able to max out RAM on a more limited system; you could still load DOS into the first 128K, above the video buffer. It would be slower but on a 256K system it was often needed. Being able to move the video window is one of it's key features. I've even used it to provide only a 4K video window (80x25 text) when I really needed the maximum amount of RAM.

This really isn't a matter of can the BIOS do it ... it is a matter of should the BIOS do it. Assume you have a PCjr that can run DOS 5 unmodified. Now it's not compatible with any other PCjr in the house, and you still need two sets of boot floppies. The second boot required by jrconfig also doesn't take a lot of time or break anything so I don't understand the need to fix it.

[Trixter]

My $0.02: Considering that 40:13 and 40:15 are located in the BIOS Data Area, the BIOS should be putting the correct value there, as other BIOSes do.

[Brutman]

The difference between 0x413 and 0x415 is trivial. It's the movement of the video buffer (optional) or the fixup of DOS where jrConfig earns it's keep.

The real issue here is boot diskettes from one PCjr not working on another PCjr. That doesn't seem worth the hassle of fixing the jrConfig "double boot", which is really a trivial problem.