Unidentified PC DOS 1.1 Boot Sector Junk Identified

The linker had a bad habit of leaving unused memory uninitialized. Every time you linked a program, the binary would be different. Functionally the same, but they wouldn't compare byte for byte. So my manager asked me to make sure the linker zeroed out all unused memory.

After linking a program, the linker printed a message something like this:

  8412 bytes used
   439 bytes free

My manager knew what a hassle this task was, and he was fairly chill, so just for fun I added a temporary message meant for his eyes only:

  8412 bytes used
   439 bytes free and it's pretty fucking clean

Unfortunately he wasn't the first to see the message. His manager was giving a demo to a customer and they saw the message. Oops.

Later we wanted to add a "weak external" feature to the assembler and linker, something like the "weak reference" that a few modern languages have. If the external was missing, then instead of failing the link step, it would leave a zero that you would check at runtime.

The regular external directive was called EXTERN, and the tradition was for these names to be very short. I thought of calling it WEXTERN but then decided to call the feature a "secondary external", so the directive was SEXTERN!

The 386 was the first x86 processor to introduce four "Control Registers", CR0-CR3. The first one, CR0, was technically already introduced with the 286, but then still named "Machine Status Word" and manipulated with its own set of different , explicit instructions (that still survive to this day).

Now, if you look at Intel's documentation of the 386, or any later CPU in the line--including today's Intel CPUs in Laptops, Desktops, and Servers--you will see that the second of those registers, CR1, is entirely "reserved". No single bit in it can be accessed, neither reading or writing.

This is bizarre not only because CR1 was introduced by the 386 along with CR2 and CR3 (which are defined and common), but also because the successor, the 486, introduced a new Control Register, CR4, instead of starting to use reserved bits in CR1. This is despite CR4 sharing its characteristics with CR0 (and unlike e.g. CR2): It's mostly a bit field for global processor state. So while you could have assumed that CR1 existed as planned "overflow" to add new control bits to once CR0 became full, the seemingly simple addition of CR4 in the immediate successor goes against that theory.

Decades ago, I even wrote to the 386's chief architect to try to settle that question... he must have not understood my question, because he just replied that according to Intel documentation, that register is "reserved".

But incidentally, a while ago I talked about that mystery with the author of this article, and received the most plausible theory so far: It turns out that an early pre-release document shows that the 386 was originally planned to contain an on-chip cache. Evidently, that part of the plan must have been scrapped, because the 386 shipped without an on-chip cache, which was only added with the 486 (maybe they ran into problems with implementing a cache controller, or maybe at the time so much on-chip SRAM would have made for a prohibitive price point).

It is therefore not unlikely that CR1 was once meant, and in prototypes maybe even did, control the cache. Once that cache was removed, CR1 was just made "reserved", and not repurposed in the 486 and later CPUs out of an abundance of caution for compatibility: Accessing CR1 reliably causes an Undefined Opcode trap, and maybe some important software at the time relied on that in a bad way.

I highly recommend his site too --- there's plenty of boot-sector analyses there, explanations of the PC boot process, and other low-level information presented in fast-loading, script-free ad-free HTML from someone whose goal isn't to make a quick $$$, but rather to share information freely. In other words, a great resource of the "document web".

As for this article, it also reminds me of "junk DNA" sequences, which could have similar relevance to biological historians.

Edit: Photos of Commodore computers look really close to what I remember, but not exactly. Also, Latvia (where I live) just recently had regained independece from USSR and it was a time when the market was flooded with clones for everything. E.g. instead of NES kids had famiclones like Zhiliton, UFO or in my case, I think it was Dendy.

Edit: Floppy reader was in-built, not external, as far as I remember.

Edit: Sorry, more like 30-year mystery. Somewhere around beginning of 90s.

Edit: I think it also had game Alley Cat.

Edit: Thank you everyone for chiming in! This inspired me and gave extra keywords for further investigation and narrowing down the exact model / make. Gonna continue searching on Sunday.