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IBM PCjr STRIPPED BARE: We tear down the machine Big Blue would rather you forgot
30th anniversary voyage of the 8088-powered 'Peanut'
Mmmm... 64KB in only eight chips!
On the right is the internal power supply. Yes, the power supply is not shielded off, but is a card that plugs into the logic board.
How could the PCjr have such a small power supply? Well, it also included an external component that embodies the term "power brick" almost literally.
Moving over to the right, we find the PCjr's floppy disk drive. The floppy drive was an addition to later models; some early PCjrs have a blank plate and relied entirely on the aforementioned cartridges and external tape drive. (If you actually used one of those and lived to tell, by the way, we'd love to hear about it in Comments.) The only wired power connections running from the power supply on our unit were to the floppy disk and fan.
One thing we picked up on over the course of our teardown was that the most demanding hardware component on the system was in fact the 1.2MB floppy drive, which required a card full of controller circuitry.
The PCjr's fan is placed behind the floppy drive and blows air into its circuitry and over the half of the logic board containing the CPU and other chippery. For those who have grown up in the era of giant heat sinks and case fans big enough to cool a living room, this is throwback. But at this point in the PC era, the CPU was not yet the power and cooling hungry beast it has morphed into.
In fact, when you strip the PCjr down to its motherboard, you might be hard pressed to immediately pick out where the CPU is located (it's on the left underneath the floppy drive.)
A closer look reveals something a bit surprising about the PCjr. Like the original IBM PC, the jr runs software written for the 8088 processor design from Intel [PDF]. In our model, however, we found this...
Yes, in 1982 when IBM first released the PC, they were worried that Intel would not be able to keep up with demand for new processors. As such, the company asked that the 8088 be licensed out to other chipbuilders. Among them was AMD, which had previously been producing Z80 family processors. The 8088 powering our PCjr was among the first x86 processors that AMD would make: it had 16-bit registers, an external 8-bit data bus and could address up to 1MB of memory.
Running at a 4.77MHz, the PCjr's clock speed was comparable to other PCs of the day. Because the PCjr had no dedicated memory controller, the machine's first 128KB block of RAM was refreshed by the VGA (that's video gate array in this era) graphics chip, which typically paused execution as it regularly swept through all the available memory addresses, not just the video frame buffer. Ideally, your code and video memory should be in separate blocks with their own controllers to avoid this, but the PCjr threw it all together under one controller, hence the contention. (Add-on packs of 128KB RAM had their own separate DRAM refresh circuitry.)
That bastion of veracity, Wikipedia reckons one in four clock cycles were lost due to this necessary refresh by the VGA component. Divide 4.77 by four and multiply by three, and you come up with the PCjr's effective clock rate: 3.58MHz. We, however, could find no reference to this particular limitation in the IBM PCjr Technical Reference; the document does acknowledge that the video electronics governed the RAM refresh, though.
We'll let you argue this one out on your own – but one thing is certain, however: pokey performance was a major complaint from users of the PCjr.
Seated on the logic board were eight relatively inexpensive, slow, and low-power Mitsubishi M5K4164ANP-15 chips (or equivalent), each providing 64 kilobits of DRAM, ganged together to provide – as you might guess – 64 kilobytes of memory.
That other card plugged into the logic board? It's the additional 64KB of memory that you got when you ordered the high-end PCjr configuration. In another delightful cost-cutting measure, the PCjr shipped without direct memory access. I guess at that point you'd have already resigned yourself to maddeningly slow performance, so what's a few more seconds of load time here or there?
This additional 64KB of DRAM, plus a 1.2MB floppy and parallel port – we be stylin' (click to enlarge).
Another way that the PCjr cut corners when compared to, say, the IBM PC was that it didn't have a dedicated graphics card with dedicated graphics memory. As a result, junior's graphics controller had to share system memory with the PCjr's compute chores. Cheaper, yes. Lousier, also yes.
This Motorola CRT controller is a long way from an Nvidia GeForce GTX 880M (click to enlarge)
That Motorola CRT controller, above, is said to be the same chip used in, among others, the Apple II and the original IBM PC.