FreeDOS on IBM PCjr
mk1...@hotmail.com
Brian
Ralf A. Quint
>Has anyone run FreeDOS on a PCjr? I know this may seem kind of an odd question, however, I am currently operating with a PCjr with a Racore Drive II expansion with a modified 3.5" drive on it, and 736k RAM...now....this issue...the Racore has a switch on the back to make some PC programs accessible on the PCjr. The FreeDOS 7 will work when in "PC" mode. However, in "jr" mode, I get a lot of multi-colored screen garble and a system lockup, probably due to the jr's memory hole of the 1'st 128k. so basically...nothing can be put into the 1'st 128k and would need to be able to be loaded outside the 1'st 128k of memory. Any ideas? Anyone ever work with this?
>
>Brian
>
Well, as hardly someone has a PCJr these days anyway (someone had once
promised me to give me one, but the machine unfortunately never showed
up), it is hard to tell what's wrong. I had only brief contact with a
couple machines back in the '80s and remember that they were not quite
"PC compatible", so that could have a lot of different reasons, the
memory layout is defintely a very likely source for the problem.
Where are you located, just in case there would be a possibilty to
examine that machine a bit more detailed?
Ralf
Thomas Mueller
Has anyone run FreeDOS on a PCjr? I know this may seem kind of an odd question, however, I am currently operating with a PCjr with a Racore Drive II expansion with a modified 3.5" drive on it, and 736k RAM...now....this issue...the Racore has a switch on the back to make some PC programs accessible on the PCjr. The FreeDOS 7 will work when in "PC" mode. However, in "jr" mode, I get a lot of multi-colored screen garble and a system lockup, probably due to the jr's memory hole of the 1'st 128k. so basically...nothing can be put into the 1'st 128k and would need to be able to be loaded outside the 1'st 128k of memory. Any ideas? Anyone ever work with this?
> Brian
How much hard drive on the PCjr? I think in the days of the PCjr, PC hard
drives didn't exist, the first 10 MB hard drive was a few years in the future,
and 64 KB was considered a lot of RAM. What is the Racore Drive?
I never worked with the PCjr. What kind of OS did it run?
Michael B. Brutman
The PCjr is a cult item - a horrible little machine that makes
everything a struggle. And hence the fun in owning one. :-)
The video is mostly CGA compatible. It has some extensions and the
hardware that drives it is different, so it is BIOS compatible but not
register compatible with PC CGA cards. It still shows up at B800:000 in
the memory map, just like a CGA card.
The video steals it's memory from the first 128K of system memory. On a
normal PCjr, you only have 64K or 128K of memory, so the stealing comes
from the top of memory. On a 128K system you wind up with 112K for DOS
at 16K for video. On a 64K system you wind up with just 48K, and a 16K
video buffer.
The amount of memory that the video steals is variable. It ranges from
2KB to 96KB. This supports higher resolution graphics modes (320x200x16
and 640x200x4), and multiple pages - the number of pages depends on how
much you allocate. The BIOS routines to do the allocation grow the
memory range downward from the 128K mark, but the hardware is more
flexible - it can come from any 16K chunks in the first 128K.
The trick for an OS is to leave the first 16K of memory available for
BIOS, DOS, etc. Then allocate some memory for the video buffer. Then
load the OS after that point. (Say at the 32K or 48K mark, which leaves
the first 16K for the system and 16 or 32K for the video.) Under DOS
2.1 through DOS 6.2 you can do this with a device driver. (JRCONFIG is
the device driver of choice, although several existed.)
I haven't seen the problem first hand, but I imagine that FreeDOS has no
idea of how weird the Jr is, and it's just assuming that the low memory
is all available. Which will cause it to load code or data over the
video buffer. Which works fine, until somebody modifies the video
buffer with a PRINTF. :-) Then it becomes self-modifying code.
Somebody running ELKS (stripped down Linux) ran into this problem. The
solution was to play with the video hardware registers to move the video
memory down as low as possible (16K to 32K), then load the kernel above
that. Basically the same deal as the device drivers that existed for DOS.
I've got the Tech Ref for the Jr and enough sample code to muddle
around. If you guys are amenable to it, I wouldn't mind trying to make
the mods myself. (I have to look and see what your programming
environment is - I'm probably not setup or smart enough to just jump
right in.) All with your able guidance of course ...
Mike
Michael B. Brutman
Thomas Mueller wrote:
Most Jr's don't have hard drives. Here are the specs from the factory:
Intel 8088 or compatible (AMD was a second source)
64 or 128K of RAM - nothing else is supported out of the factory
Diskette drive (optional) - 2nd diskette drive was not supported
CGA compatible graphics built in on the motherboard
Serial port, joysticks, light pen, and other ports built in
Three voice sound using a TI sound chip
In reality most Jrs have more memory, and many have a second diskette
drive. Memory could go up to 640K. 736 was possible if you take into
account that a Jr can't run a monochome adapter, so that part of the
memory map can be used for DOS as well. (It's contiguous with the first
640K.)
Racore (also known as Rapport) and other companies made expansion
chassis's that could optionally expand the memory, add a second diskette
drive or a hard drive, a parallel port and a clock/calendar.
Brian's system in particular is rather 'hooked' - 736K of RAM, 2
diskette drives (360K and 720K), and a parallel port hard drive. The
hard drive is supported by a DOS device driver.
More information about the Jr than you ever wanted to know:
http://mail.magnaspeed.net/~mbbrutman/PCjr/pcjr.html
Life was fun on a 128K system. And hence the reason for keeping these
old beasts running.
Mike
Brian Heise
top of the original PCjr case and then the cover of the original case
went on top of the exapansion unit. The unit was designed to add a
second floppy drive, and later HDD like you mentioned. The expansion
unit also allowed for a memory board to be installed into it, allowing
for extra RAM to be installed. In mine for example, I have the base
memory of 128k, a memory card in the Racore of 512k = 640k total and
then a modified 128k IBM memory sidecar allowing DOS to recognize a
total of 736k.
The HDD I am using is run through the parallel port. It is an EIDE
setup and Im playing with 1.6gb. Definitely nice for larger programs.
The OS that was used most commonly in the earlier days of the jr was
DOS 2.1 . This eventually made way for DOS 3.2 and 3.3 and in order
to get the full potential out of the HDD and to run Windows 3.0, a
patched version of DOS 5.0 works just dandy.
Brian
Dosius
Never on a PCjr, but I did run 2024f and a few newer versions
successfully on a Tandy 1000 HX.
The memory above 128K might be inaccessible without the proper IBM
device driver (should have come with the PCjr).
-uso.
Dosius
>
> How much hard drive on the PCjr? I think in the days of the PCjr, PC hard
> drives didn't exist, the first 10 MB hard drive was a few years in the future,
> and 64 KB was considered a lot of RAM. What is the Racore Drive?
Racore would be an expansion unit (sidecar)? PCjr came with 64K
expandable to 128, to 256 with a sidecar, or more.
>
> I never worked with the PCjr. What kind of OS did it run?
Standard IBM PC DOS 2.1 (3.30 works as well). Used to have one of
them. Then it suddenly died on me :(
-uso.
AppzJunkie
The Jr was around when Harddrives were 20Mb big ! And memory at 512Kb and
640Kb were considered overkill by Bill Gates himself...
It was running PC-Dos 2.10 or MS-Dos 2.11 (it contained a patch to deal
with the PCJr wierdness...).
Bob
"Thomas Mueller" <tmue...@bluegrass.net> wrote in news:b2vai4$1h82oi$1@ID-
49635.news.dfncis.de: