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Hardware hacks from Gregg Eshelman:
There are hardware hacks for the PCjr's video and audio to make it 100%
Tandy 1000 compatable. I don't remember the details, but the video hack
was pretty simple, involving cutting one trace on the motherboard,
stacking a new chip on top of one on the board and soldering in a couple
of wires. I bought a kit to do it. I never did the audio hack, only ran
into one game that absolutely had to have Tandy audio as well as video,
but there were others that had to have them both.
The PCjr used the exact same Texas Instruments audio chip as the TI-99/4A
and the Tandy 1000. Also, the PCjr Speech sidecar used TI's speech chip
but the IBM implementation was very poor compared to how TI used it with
their home computer.
The following information comes from Joel Burgess:
The "sidecars" (used for the RAM
expansion) are attachments that fit on side of the computer which
almost anyone could add on. Other sidecars included a parallel
port, speech module, and a power attachment, which was needed for
three or more sidecars as the motherboard couldn't provide enough
power). The RAM sidecars came in 128k modules, although they could
be modified to 512k (involving new DRAM chips, a soldering iron,
and patience).
Also, Video RAM on the Jr. can be selected via software. It had
16k dedicated, but you could take it up to 64k by lending
conventional RAM to the Video RAM using the pcjrmem.com driver
(always required to access RAM above 128k) with the /e switch
(followed by the desired amount of Video RAM). Another note, all
second generation Sierra games required 640k of RAM on the Jr. (as
stated on the box), as well as extra Video RAM. Sierra was one of
the few software companies that actually used the Jr.'s 16 colors,
and 3-voice sound (although as stated on the box, 640k of RAM was
a necessity).
Additional information by Michael Brutman:
Video memory on the PCJr was shared with the
system memory. It could be as low as 2KB of memory. It could be as
high as 120KB of memory too! Obviously if you do this, you have to
leave yourself enough room for the program you are running. You
can not go above 128KB, because the circuitry that does the memory
sharing only is wired to 128KB.
When the PCjr was first released, 128KB
was the limit. IBM built this into the BIOS. (The video memory is
taken from the high part of memory, assuming that high is 128KB.)
When third party vendors (and eventually IBM) started selling
memory, they had to provide device drivers to avoid the
"hole" in memory caused by the video hardware sharing
main memory with the rest of the system. And thus the need for
PCjrMem, JrConfig, and other memory device drivers. These device
drivers had to set an upper cap on the amount of memory that could
be allocated to video, otherwise you'd step on the first set of
pages used for DOS.
Wayne A. Ptaff reports:
The PCjr included (though the
"Technical Information" table doesn't mention it) a
160x200x16 video mode, allowing a _16_ (wow!) color display in
graphic mode. The CGA standard didn't include that mode. A few
games (mostly Sierra's) exploited it; I think Tandy computers also
used that chunky-pixel mode.
There were no F1-F10 keys, you had to use
a combination of "Fn" and the numeric key, just like
you'd do a Ctrl-R on a standard keyboard. For a few games relying
on F keys for control (that were still on the keyboard's left side
before the IBM AT), it was a pain in the ...
Mark Meiss adds:
A few details to add to the PCjr...
This was one of my first machines, and I always did have a strange
love-hate relationship with it.
I think that the listed clock speed is too high -- if I recall
correctly, IBM released the PCjr with the clock rate divided by 2
so as not to let the PCjr compete too strongly with the PC.
Many PC games (remember Flightmare, anyone?) were a *lot* easier
when played on the PCjr.
The original chicklet keyboard is perhaps the worst chicklet
keyboard ever designed. It had full-height keys with a
substantial amount of space between each key. Not only could
you get your fingertips stuck between keys, but the vertical
displacement of the keys was such that touch typing was an
exhausting effort.
The cartridge BASIC was incompatable with the PC's BASICA in a
number of subtle ways. As I recall, you couldn't use the PC
BASICA executable on the PCjr because it relied on ROM hooks that
were missing, but you could run the full GW-BASIC once that came
around.
PCjr did indeed have the enhanced graphics modes mentioned, but
sadly, they were not compatable with the Tandy modes of the same
color depth and resolution. Few programs ever supported the
PCjr's graphics.
Note on marketing by Jeff Harris:
IBM ran TV commercials for the PCjr that
featured a Charlie Chaplin look-a-like that did not speak while a
voice over mentioned it's features. I think some of them
even featured a little dog with Charlie.
Shanee Hill comments:
I had one of these which was purchased in '85 or so, at a substantial discount as they were being discontinued. It had been upgraded with sidecars to 640K ram and an an additional 360K floppy disk drive.
Regarding the clock speed, I believe Mr. Meiss was correct, the clock speed was lowered by IBM; however, the 3rd party manufacturer that made the upgrade sidecars provided a driver utility called speeder.sys (if I remember correctly) that corrected the clock speed and substantially improved the performance, especially for games (like G.A.T.O and MS Flight Simulator). By the way, MS Flight Sim of the time took advantage of the 16 Color Video Mode. Awesome!
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