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This page is for answering some of the more common
questions
about
the Jr. If your question isn't covered here, please feel free
to
email me (mbbrutman at gmail.com)
or use the web forum (https://www.brutman.com/forums).
This page is broken up into several sections:
Storage
Hardware
Questions:
Q: Can I add another diskette drive to a PCjr?
Yes, there are several options.
- The PCjr diskette drive controller can be modified to
trick
it
into supporting two disk drives. It requires some soldering
work
and a software patch to a boot diskette to "convince" DOS that two
floppy drives are available. This technique has the
interesting
side effect of making both floppy disks spin, even when only one of
them is being
accessed. However, the price is right and this was a
relatively
common modification.
Once the modification was done the drive cable would be replaced and a
second drive would be attached. The second drive has to
sit outside of the system, usually in its own enclosure complete with
a separate power supply. (The separate power supply is needed
because the PCjr barely has enough power inside it for its own uses.)
The software patch applied to the DOS boot diskette is crude - it just
bumps the count of drives on the system. BIOS only looks for
one
drive,
so BIOS doesn't tell DOS if more than one drive is available.
This
can also be done by a device driver, which is a more elegant solution.
(The device drivers came later.)
A later version of this modification appeared in Home Computer
Magazine, September 1984 issue. This version of the
modification
cured the problem where the two drives would spin even when only one
was being accessed. I haven't seen it in person - if you have
this issue please contact me.
- You can replace or augment the standard drive
controller
with a
third party drive controller. An example of this approach is
a
small
card made by Creative
Firmware, which used the modem slot and
inserted
itself in between the drive controller and the first floppy drive.
It
has the same interesting side effect as the hardware modification
described
above - both diskette drives spin when one is in use.
Other
cards may have been available.
- You can try to find a PCjr with an expansion chassis.
Several companies (Legacy
, Rapport
, PC
Enterprises
) made these and they usually held extra memory, a second diskette
drive, or even a hard drive. (The hard drive options are
rare.)
The expansion chassis usually had extra circuitry to "help"
the
drive controller handle an extra diskette drive without resorting to
the
modification used in option 1. PCjrs with an expansion
chassis
are hard to find, but you may stumble into one.
- You might be able to find an external floppy disk
drive
that
interfaces through the parallel port. You would not be able
to
boot from this floppy, as the machine would not know that it was there
until a device
driver was loaded. The Microsolutions Backpack line of drives generally
work.
Assuming that you use a diskette controller approach and not the
parallel port approach, you have a choice of several diskette drives to
use.
- The standard diskette drive is a 5.25" double density
diskette
drive. These store 360KB, just like the standard diskette
drive.
These can be obtained used.
A word of caution - any genuine 5.25" double density diskette drive
that you find in the year 2002 is probably close to 16 years old.
It may not spin at the correct speed or be reliable.
Use
caution, and be aware that you may need to try several drives.
Another word of caution - do not mistake a double density drive with a
high density drive. Double density drives were used on the
PC,
XT, and PCjr. High density drives were introduced with the
PC AT, and they can store up to 1.2MB of data on a diskette.
The
PCjr controller can not use the high density drives; the controller
just isn't capable of doing it. A high density drive might
connect and work properly as a low density drive, but you may have
problems with head positioning and track width. My advice -
don't
even try it.
- A 720KB 3.5" diskette drive can be connected.
These are the
original 3.5" drives, and they are only double density. Newer
3.5" drives store 1.44MB, or even 2.88MB. The PCjr can not
use
these newer drives, as that requires a better drive controller than the
PCjr has. This type of diskette drive is relatively rare, as
1.44MB version supplanted them very quickly.
If you connect this drive to a PCjr the PCjr will think that
it has a standard 5.25" diskette drive - electrically they are the
same. The PCjr BIOS will also only use the first 40 tracks of
the
drive because that is all a 5.25" diskette drive is supposed to have.
The drive actually has 80 tracks, which allows it to store
twice
as much as a 5.25" diskette drive. To use the other 40 tracks
you
would need to install a DOS device driver (DRIVER.SYS) to tell DOS the
true capacity of the drive. (This works under DOS 3.2.)
You must use 720KB diskettes (double density) if you do this.
1.44MB diskettes are not compatible with the 720KB drives,
and
you may experience data loss if you try to use them.
Rumor has it that the PCjr BIOS can support up to three diskette
drives, assuming you have a controller that you can connect them too.
I have tried connecting newer 1.44MB floppy drives to the PCjr and the
worked. You can only use them with 720KB diskettes, which
makes
them behave like double density drives. The good news is that
you
don't have to go rummaging around trying to find the relatively rare
720KB drives if a standard 1.44MB floppy drive will work. I
used
a 1.44MB drive from a Compaq 4/33i (1992 vintage) and a Sony 1.44MB
drive (2000 vintage) and both behaved well. I had problems
reading the floppy under Windows 98, but Linux did just fine with
it. I suspect it is because I wrote a 360KB diskette image
onto
the drive, which set the boot sector up to make it look like a 360KB
diskette. Windows 98 was able to read the directory but
nothing
else - Linux did everything correctly.
If you use the parallel port solution, you can probably
add
any
floppy drive that you want, as long as you can run the drivers.
This include high density drives. Keep in mind that
the
standard parallel port can send eight bits at a time, but it can only
read four bits at a time - writes will be twice as fast as reads, and
reads will be very slow! (See the section on the
bi-directional
parallel port modification below for a way to improve this.)
See the section entitled "
The
Diskette Drive " for more information on the PCjr diskette
drive.
Q: Can I use a high density diskette drive on a PCjr?
It depends.
The PCjr controller card can handle 360KB 5 1/4 inch diskette drives
and 720KB
3 1/2 inch diskette drives.
The PCjr controller card can not generate the proper
data for
high
density diskette drives. (These are the 1.2MB and 1.4MB
types.) I think that the data rate on the controller card has
to
be significantly faster than what the PCjr can do. This is
not a
design flaw on the part of the PCjr; Only the PC AT had high
density drives at that time, and it was considered a heavy duty piece
of equipment.
You can connect a high density drive to the PCjr, but
you will
have
to use double density media to make it usable. (Double
density
media forces the drives to operate in double density mode, which the Jr
can use.) Connecting a 1.44MB 3.5 inch diskette drive works
well
with 720KB double density diskettes. Connecting a 1.2MB 5.25
inch
drive is not recommended because of track width differences, even if
double density media is used.
PC
Enterprises had two solutions for running a high
density
diskette
drive
on the Jr. One solution used the slow, low data rate
controller
built
into the Jr. It read and wrote 1.2MB diskettes, but at a
slower
rate
than a real high density controller. (They must have spun the
disk
more slowly to compensate for the low data rate.) Later on
they
developed
a new controller that worked at a faster data rate.
Parallel port solutions - anything goes, as long as you
can
get the
drivers to work.
Q: Can I add a hard drive to a PCjr?
Yes. (And read the part about jrIDE - this is the one you want!)
One option is the traditional hard drive upgrade for a machine.
You get a standard hard drive that you can boot from, just
like a
PC, XT or AT. There were several flavors of this option.
One flavor of this option was an expansion chassis that included the
hard drive and room for other options, such as a parallel printer port,
diskette drive, extra memory, clock/calendar, etc. The
expansion
chassis usually sat on top of the PCjr, making the PCjr look like a
"double decker." The expansion chassis used a special sidecar
that connected it to the system bus on the right side of the machine.
Companies such as Racore, Legacy
,
and PC
Enterprises sold this type of solution. The expansion
chassis usually had it's own power supply. Hard drives were
usually limited to low profile models no more than an inch tall, as the
expansion chassis was usually not very tall.
Another flavor of this option was an externally mounted hard drive in
another box. The other box might have had the full system bus
brought
to it by a sidecar, or it may have been connected via a SCSI bus,
making
it a storage solution only.
PC
Enterprises had several solutions. The two that I
know of
were based on
SCSI. The first used a Seagate chipset. The second
was
based
on a Future Domain chipset, and was known as the TMC 850Jr.
The TMC 850jr is amazing - it was a real,
honest-to-goodness
SCSI
adapter capable of controlling seven devices. It also patched
the
BIOS of the PCjr to allow booting directly from the hard drive, without
first booting from a floppy. DOS recognizes a SCSI hard drive
attached to it with no problems. It truly is an amazing
adapter.
The Future Domain DOS ASPI drivers seem to work on the PCjr,
although they are not needed for operating a hard drive.
(They
are needed for SCSI CD-ROM support.) I have tested several
SCSI
hard drives, a NEC Multispin CD-ROM, and an Iomega Zip 100 SCSI drive
with the TMC 850jr.
(Not to short sell the Seagate based adapter - it was
probably
pretty decent too. I just haven't seen one, so I can't rave
about
it.)
A company called RIM also had a SASI/SCSI based solution. The
upgrade consisted of a small interface card that installed in the modem
slot and a larger SASI/ST506 bridge board that sat in an external
chassis. The bridge board would then permit the use of a relatively
standard MFM hard drive, such as the Seagate ST-225. The RIM solution
does not have a BIOS extension so the machine is not bootable with it.
However, they sold a cartridge known as the "HARDBIOSjr" that added the
necessary BIOS extensions to make it a bootable solution.
The ultimate solution for the PCjr is a new project called
jrIDE. jrIDE adds an IDE interface, memory, and a real time clock. It
fits in an existing PCjr sidecar and lets you add a relatively modern
large capacity drive to the PCjr. Although most of the drive will not
be used by the PCjr, these drives are cheap, newer, and should have a
fairly long lifespan compared to the old MFM clunkers from the mid
1980s.
The jrIDE is a new project, designed in late 2011 and first functional
in early 2012. For more information on the jrIDE see http://www.brutman.com/jrIDE .
The other major option is to use a hard drive or other
storage
device that
is attached via parallel port. These are generally slower and
not
as
well integrated as the solutions described above; the machine still has
to
boot from a floppy diskette and you need to load device drivers.
Most PCjrs have a parallel port side car, so this is a
feasible
option. The only barrier is finding working device drivers
for
DOS. (Most of the devices do have device drivers that work
under
DOS.) Tested devices include hard drives, parallel-to-SCSI
adapters, parallel port CD-ROMs, SparQ removable storage, and
the
Iomega Zip drive. Instructions for the SparQ can
be
found at (www.micro-zone.com
).
If you are going to do the parallel port route, see the
section
on
the bi-directional
parallel port modification below for a way to improve the speed of the
parallel
port.
Hard drive vendors for the PCjr from the ancient past:
Q: Can I use a CD-ROM/Iomega Zip/SparQ/Syquest drive
on a
PCjr?
Yes.
People have reported success with the following types of devices on the
parallel port:
- Iomega Zip drives
- Hard drives using a parallel-to-IDE converter.
- Backpack CD-ROMs
- CD-ROMs using parallel-to-SCSI converters
- SparQ removable storage device
And of course, there are the SCSI solutions. SCSI on a PCjr
is
rare though, so don't hold your breath. The following devices are
known to work with the TMC 850Jr, which is the only SCSI adapter I've
been able to test on a Jr:
- Various hard drives
- NEC Multispin 2x CD-ROM
- Iomega SCSI Zip 100
The trick to parallel port devices is having DOS device drivers.
If you have a working device driver, almost anything is possible.
One particularly notable device driver is PalmZip
by Klaus Peichl. This
device driver for the standard parallel port Zip 100 works on every
version of DOS from 2.1 to 5 (and probably 6.x as well). The speed
isn't great, but it works well and has never crashed my machines or
dropped a byte of data.
Let me know what types of removable storage you've used!
Q: How about Compact Flash?
Yes! By this point you should have figured out that the parallel port is the key.
In particular, the Datafab MDCFE-SR is known to work well on the PCjr.
There is a slight problem - it needs power supplied to it and the
mechanism they provide is to use a vampire tap from a standard keyboard
port. You will have to cobble together some form of alternative power
source for it. Performance was adequate - about 40KB/sec for reads and
writes on a bi-directional parallel port.
Compact Flash is also a possibility if you have a jrIDE. (Compact Flash
with an adapter can be used to emulate an IDE hard drive.)
Memory, Speed
kits,
Processors, etc.
Q: How much memory can I add to a PCjr?
A PCjr has a minimum of 64KB of memory, which is soldered onto the
motherboard. (Eight chips, each 64 kilobits - no parity
protection in this setup.) To get to 128KB, you need the
internal
expansion card. To go beyond that, you need to start adding sidecars or
other boards.
Some of the sidecars were sold as 128KB units, which
would
bring
memory up to 256KB. The IBM and Microsoft Booster sidecars
could
be modified to go up to 512KB, giving the PCjr up to 640KB of memory.
The way the PCjr memory
map
is laid out, it is possible to get up to 736KB on a PCjr
system.
Of course you need that much memory installed and some software patches
to make DOS recognize the extra memory.
Remember that on a PCjr, the graphics
subsystem
"steals" memory from the system to use for the video display.
Therefore, you are usually short 16KB or 32MB on a PCjr.
Device
drivers to use extra memory usually reserve the unused memory in the
first 128KB for performance reasons; that means on a 640 KB system you
have at best 628KB or 608KB using some slow memory, and at worst only
512KB is
you reserve the slow memory for a RAM disk. This is usually
enough
for most old software.
Here are some memory expansion vendors for the PCjr from
the
ancient
past:
- Microsoft ("Microsoft
Booster " sidecar came with 128KB (optional), a mouse, and a
clock
chip)
- Tecmar ("Jr Captain" and "Jr Cadet")
- Impulse
- IBM (sidecar)
- Rapport/Racore (in an expansion unit)
- HotShot by ES Quality Products (internal daughtboard)
Q: What
speed
modifications can I make to
a PCjr?
There are several things that you can do to speed up a PCjr.
The first improvement that you can make is to add memory
via a
sidecar, and use software to reserve the first 128KB of memory for DOS,
a ram disk, and the video graphics memory. This forces your
programs to run above the 128KB mark, which makes them faster because
the access to this memory is faster. See the section on " The Memory
Subsystem "
for the details on why this is so.
The second thing that you can do is to replace the Intel
8088
microprocessor with the NEC V20 microprocessor. The NEC V20
was
an Intel 8088 clone that ran a little faster. Some software
was
not compatible with the NEC V20, most of it being games.
(Broderbund's Lode Runner is one of them.) The NEC
V20 can
give you a noticeable improvement; some users have reported their PCjrs
running almost 15% faster. (Obviously it depends on the
benchmark.) It's going to be a little hard finding a NEC V20
now
though .. at least you don't have to worry about voiding your
warrantee. ;-)
Another option was the PC-SPRINT kit. PC-Sprint was
designed
by Douglas
A. Severson in 1985 and it was intended for the PC and the XT, however
it
works on the PCjr as well. It involves replacing the
microprocessor with a version that is rated for 8Mhz, and changing the
8245 timing chip. The effect is to give you a 7.33Mhz system.
The memory and the disk drives will still be at their same
old
speed, but processor operations will be much faster.
Another option was the jrExcellerator, a board sold by PC
Enterprises.
The jrExcellerator was a daughtboard board that plugged into
the
8088
socket on the motherboard. It came with a replacement NEC
V20.
With
the switch flipped on, the Jr would run at 9.54Mhz. It also
had
an
onboard BIOS chip that would allow the Jr to recognize and use extra
memory
without a device driver.
Q: Can I change the clock crystal in a PCjr to speed it
up?
No. The PCjr uses the clock crystal for a lot of
things.
Changing it by even a few percent will leave you with a dead screen and
a dead keyboard.
However, you can use the PC-Sprint modification which is
almost
equivalent.
Q: Can I add an 8087 math coprocessor to the PCjr?
Yes. There is no socket on the motherboard for the 8087, so
you
just can't plug the chip in. A company named TIAC
Manufacturing
(from Canada) made a daughterboard that used the 8088 socket, and had
the 8088 and 8087 sit on the daughterboard. You will need to
find
one of these (or construct one!) to add an 8087 math
coprocessor.
The original cost of the TIAC jr-87 in late 1984 was $90, without the
8087 chip. Legacy technologies also made an 8087
daughterboard.
Be aware that you may be stymied by software. An
8087 on a
PCjr would be extremely rare, and if the software recognizes that it is
running on a PCjr it may not even bother to look for an 8087.
(A
Racore PC ID cartridge could get around a software limitation like
that.) You will have to run a program to change the equipment
word in memory to note the presence of an 8087, as the PCjr BIOS won't
look for one and won't set the equipment word right if one is present.
Parallel
Ports
Q: What kind of printer port does the PCjr use?
The parallel port sidecar uses a fairly standard parallel port
implementation that was common on early PCs. The parallel
port
can write 8 bits at a time and read 4 bits using control lines.
It is implemented in
TTL logic with approximately 13 chips. The output is a
standard
DB25
connector. The port and IRQ line are not adjustable, meaning
that
two parallel port sidecars can not be used on the same machine. (And of
course there are ways around this ... keep reading.)
Q: Can I use a bi-directional printer port?
Yes. There was a pretty common hack to modify the parallel
port
to turn it into a PS/2 compatible bi-directional printer port.
This isn't any use for printers, but is great for storage
devices
connected via the parallel port.
The hack involves cutting a pin on two ICs used in the parallel printer
port, and connecting them together via wire. After that -
voila,
you have a bi-directional printer port. The trick is knowing
the
two pins. :-) Click here
for the details. The great thing about a bi-directional parallel port
is that it is much faster than a standard parallel port, especially
when using a device like a hard disk or Ethernet adapter on the port.
Q: Can I have more than one parallel port on a PCjr?
Yes. The parallel port sidecar sold by IBM does not have an
adjustable address. However, other parallel ports may.
The
Racore expansion unit features jumpers that can set the parallel port
address; other vendors may have the same feature.
If not, you can always modify the IBM sidecar to change the addresses.
Click here
for the details.
Q: Can I use better types of parallel ports on a PCjr?
(EPP,
ECP?)
With some work, yes.
An EPP parallel port uses a few extra I/O registers to work, but those
are readily available on the PCjr and do not conflict with other
hardware. An ECP parallel port will not work though, as that requires
DMA capability.
The tricky part is getting the parallel port onto the system. EPP
parallel ports were invented long after the PCjr failed as a machine.
To get an EPP parallel port onto a PCjr you need to use an 8 bit ISA
bus to PCjr adapter. The pinout for such an adapter can be found here.
If you are connecting a hard disk or other non-printer peripheral
through a parallel port, this can be a much faster solution than the
standard parallel port or a bi-directional parallel port. With my hard
disk setup I have seen that sequential disk reads are three times
faster than on a normal bi-directional parallel port, and sequential
disk writes are 2 times faster.
Monitors
Q: What options do I have for hooking up a monitor to
the
PCjr?
There are three options available:
- Use an RGB monitor, such as the PCjr Color Monitor
or an
IBM
Color Graphics Display (with an adapter cable). This provides
the
best color output.
- Use a composite monitor. A monochrome composite
monitor is
excellent for text applications. A color composite monitor is
harsh on the eyes in comparison to an RGB monitor, but usable.
- Use a television. This requires an
adapter. This is
not going to be a display that you want to look at for hours on end,
especially in 80 column mode. In fact, 80 column color text
is
just about unreadable. Televisions do not have the bandwidth
or
clarity to display text at 80 columns.
Q: Can I use an old CGA monitor on the PCjr?
Yes. IBM sold an adapter to go from the rectangular BERG type
connector on the PCjr to a standard CGA type connector. If
you
can not find such an adapter, constructing one should not be too hard -
just find the pinouts, and hack away.
Q: Can I use an old monochrome monitor on the PCjr?
It depends.
If you are talking about a high quality monochrome
monitor
(TTL
style), such as one designed to be used with a Hercules card, forget
it. The PCjr has CGA only (a superset actually), and it can
not
drive such
a monitor. Even if you could connect a monochrome adapter onto the PCjr
using an ISA bus adapter the PCjr does not have the BIOS programming
necessarily to set the card up and make it produce output.
If you are talking about a lower quality (but still
good)
monochrome
monitor that uses the composite video output of the PCjr, then yes, you
can connect it. You will have CGA graphics, but in 16 shades
of
grey. I used an amber monochrome monitor on my PCjr for
years,
and I loved it.
Q: I'm having diskette read errors - help!
Is your monitor sitting within six inches of the diskette
drive?
Or on top of it? This is a no-no ... IBM often
showed
pictures of PCjr's with the PCjr Color Monitor proudly sitting on top
of it. Yet there is a warning in the technical reference
guide
that says the diskette drive should be at least six inches away from
the monitor. Monitors generate a lot of RF noise.
Move your
monitor and try again.
If it's not your monitor causing the problem, you either
have
a bad
diskette or a bad diskette drive. Try a new/different
diskette.
Rick Ryan (rryan@ryannet.com) adds an interesting point:
"I was the technical director for a PC retailer
when
these
things came out. Customers who thought they could get the
better
PC/XT Color Display instead of the PCjr Color Display found their
floppy drives stopped working if the larger display was placed on top
of the PCjr."
Apparently the PCjr Color Monitor was safe to sit on top of
a PCjr, but the bigger IBM Color displays were not.
PCjr
Software
Questions
Q: A game from an IBM PC seems to run, but the screen
is all
messed up. Why?
Welcome to the wonderful world of not being exactly compatible with the
IBM PC.
The graphics
subsystem of the PCjr is compatible with the CGA standard at
a high
level (BIOS), but software that directly accesses registers or memory
is not guaranteed to work. Software might do this for
performance
reasons, which is good, except that it limits portability.
Sometimes the bad program can be patched to interact
correctly
with
the video hardware. I will be digging this patch out and
making
it available soon. (Email me if "soon" hasn't arrived yet for
you.) If these patches don't work, you're out of luck.
Q: I have two cartridges in my PCjr, and it doesn't
work. If
I pull one cartridge out it works. Why?
Cartridges are ROMs, and they have to tell the PCjr BIOS where to
locate them in the memory map. There are a limited number of
locations that they can go to. If two cartridges are inserted
and
they both require the same memory locations, then they can not work.
See "Cartridge
Slots
" for more details.
Q: I have a cartridge without a label - how do I find
out
what it
does?
The easiest way is just to try it:
- Some carts can be used after DOS is booted.
BASIC,
ColorPaint, Lotus 1-2-3 and others do this.
- Some carts take control of the machine as soon as the
machine
boots up. Most games do this. Having a disk in the
drive
doesn't matter, as the machine doesn't even try to boot from the disk.
- Some carts require that the machine does not have a
disk in
it
when it boots. These carts "hook" Cassette BASIC to start, so
if
the machine boots from the disk the hook won't run, because Cassette
BASIC
didn't start. Spinaker titles tend to do this. (I
would
consider
it a bug - the cart should have just taken the machine over, not let it
have a chance to boot from the disk first.
And then there are the mystery carts that seem to do nothing.
These are challenging.
Some carts acted silently: They enhanced the BIOS of the machine in
some way that is hard to detect. Some examples of these carts:
- Racore's PC ID cartridge: This cartridge changed the
ID
byte in
the BIOS to make the PCjr report as a PC.
- QuickSilver cartridge: This cartridge speeds the boot
process.
(The difference is very obvious.)
- Keyboard Buffer cartridge: This cartridge enhanced
the BIOS
to
make the machine handle the keyboard better when the disk was running.
If you can't tell what a cartridge does, then you have to look inside
it. (Not physically!) The way to look inside a
cartridge is
to use a cartridge "ripper", which will tell you what address the
cartridge is using and give you the memory contents of the cartridge.
(Remember, a cartridge is just ROM.) You can then
disassemble the contents
of the cartridge to find out what it does.
A thorough discussion of cartridge 'ripping' can be found here. My
cartridge ripper (PCJRCART) can be downloaded by going to the downloads page or by
clicking here.
Another PCjr cartridge ripper is included in the tools
distributed with
Tand-Em, the Tandy 1000 emulator. (The Tandy 1000 was very
close
to the Jr.)
Q: I've got extra memory on my system, but if I run
CHKDSK
to see
how much is available it doesn't show it?
A few possible answers:
- When you started the machine, did you watch it
count/test
your
extra memory? It should all be there.
- Assuming that you got past the last question, did
you load
a
device driver to make DOS recognize the extra memory? If you
didn't, you will need to. DOS on a PCjr will not recognize
more
than 128KB of memory. Look for a device driver called
"JrConfig." (JrConfig can be found on the downloads
page.)
- Have you taken the video buffer into account?
PCjr's
"steal" memory from the main memory to use for the video
buffer.
The amount of the theft is usually 16KB or 32KB.
- Did you load other device drivers (i.e.: ram disk)
that
are
using the missing memory?
- Did you load a terminate-and-stay-resident (TSR)
utility
that
might be using memory before running chkdsk?
If you are comfortable with DOS, then these questions should just serve
as reminders for you. If you are not comfortable with DOS and
you
have no idea what I'm talking about, you will need to find a
DOS reference manual and do some reading. Don't be insulted -
DOS
skills are obsolete, and only old-timers are going to know it.
Q: What versions of DOS will run on a PCjr?
Officially:
- DOS 2.1 (this version was shipped with the PCjr)
- DOS 3.0 to 3.3 (these versions officially support the PCjr )
Unofficially:
- DOS 4 might work, although IBM did not officially support it
- DOS 5 and DOS 6 can be made to work on systems with more than 128K of RAM with a software patch
Don't bother with:
- DOS 1.x or DOS 2: They are not aware of the PCjr's hardware differences and you will have a bad time.
- FreeDOS: At a minimum FreeDOS needs to be patched to deal with the memory hole created by the video buffer.
Q: Can I run Windows on my PCjr?
Yes. I have a reliable report, and have seen a screen shot!
Your Jr needs to be running DOS 5 or better, 640KB of RAM, and some
sort of hard disk support. That will let you run Windows 3.0.
The hard disk support can be provided using the parallel port
if
a better solution isn't available.
Q: Can I run Unix/Linux or OS/2 on my PCjr?
No. Please be serious. ;-) Those are real operating
systems. The PCjr can run DOS and maybe Windows, which imply
that those are toy operating systems. ;-)
You know, come to think of it, somebody has probably
ported
Linux to the PCjr ... ;-)
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