Alan tried running a newer DOS on the PCjr and it failed pretty miserably. The last officially supported DOS for the PCjr is 3.3, although 4 is reputed to work out of the box. DOS 5 does not support the PCjr out of the box, but it can be patched to run. Rather than make a new web page of this, I'm going to stick the instructions here and see if Google indexes it.

(Maybe it's time for a Wiki ...)

Anyway, here we go .. The patch is fairly simple. The standard PCjr BIOS locates the video memory starting at 112KB and it takes 16KB. That means that DOS and all of your code and TSRs have to live within the first 112KB of memory. On an expanded machine BIOS will count up to 640KB, but will still tell DOS that only 112KB is available. That is because it always puts the video memory in the same place regardless of how much memory you have. Device drivers like JrConfig move the video buffer lower and force DOS to load above it, giving DOS access to a large chunk of contiguous memory.

Here is what you need to do to get IBM DOS 5 to run:

Patch the DOS 5 boot sector to put the full memory amount in the correct location early in the boot process
Put stacks=0,0 in CONFIG.SYS to get around incompatible keyboard interrupt handlers that DOS 5 tries to use
Put JRCONFIG (or some other memory management code) in CONFIG.SYS

Let's do the patch first. Start DEBUG.COM and load the boot sector into memory:

L 0 <x> 0 1

Where <x> is the drive number to use. Use 0 for A:, 1 for B:, 2 for C:, etc.

Next, check the first three bytes:

U 0 L3

These bytes should look something like JMP 003E followed by a NOP. Assuming that they do, change the first three bytes to be a jump to 1C0:

A 0
JMP 01C0
<hit enter on a blank line to stop>

Now check the target area that we are going to put our patch code in:

D 1BF L27

Look for a string that says "Replace and press any key when ready". If there is no string there, quit now and get help. You haven't done any permanent damage yet so quitting is safe.

Assuming the string is there, change the first byte to a 0:

E 01BF 0

And now the patch code:

A 01C0
PUSH DS
MOV AX,0
MOV DS,AX
MOV AX,[415]
MOV [413],AX
POP DS
JMP 003E
<hit enter on a blank line to stop>

Finally, save the sector and quit debug:

W 0 <x> 0 1
Q

Where <x> is the drive letter that you used in the first step.

At this point the boot sector is patched and written to disk. Next setup CONFIG.SYS. The first line should be:

stacks=0,0


That line is a requirement. If you don't have it, DOS 5 will try to install its own interrupt handlers for the NMI or keyboard and those handlers are not compatible with the machine. The standard interrupt handlers in the BIOS will work just fine.

Finally, add your memory management device driver. I use JRCONFIG which can be found here: http://www.brutman.com/PCjr/downloads/jrcfg310.zip

One thing to keep in mind about DOS 5 is that it likes to count the free space on the hard drive every once in a while, whether it needs to or not. This operation takes a lot of CPU power; on a PCjr at 4.77Mhz a 100MB partition can take 10 to 15 seconds to check. You will think the machine has hung up - just be patient. One way around this is to keep your partitions small and use more than one partition.

The basic procedure is the same for all of the newer versions of DOS - find an used area with a few bytes to use for the patch, and make sure that code runs when the boot sector starts.