This is interesting, though it might be a bit of a cheat. He's put some data in the BIOS image to assist in decoding instructions (not to take away from how amazing this is!):
"CPU supports the full 8086/186 instruction set. Due to the complexities of the 8086’s arbitrary-length instruction decoding and flags, 8086 instructions are first converted to a simpler intermediate format before being executed. This conversion, along with instruction lengths and how each instruction modifies the flags, is assisted by some lookup tables which form part of the BIOS binary."
With that in mind, the BIOS is _hand-written_ and about 12kB (gzips to <4kB), so that's also a fantastic achievement. All-in-all, this is a wonderful entry.
The hint mentions "the importance of comments". The only comment I can spot in the actual code is here (with a couple of characters of context):
I/**/n
The code up to that point (2100 bytes total) is roughly as long as the listed output of the size tool (1979). The difference is probably due to the fact that I just counted raw bytes rather than accounting for whitespace and such. My guess, then, is that /* */ without a space hits a bug in the size tool where it probably counts that as the beginning of a comment but misses the end, causing it to count the entire remainder of the program as a comment. The compiler, meanwhile, will see that as an empty comment and continue reading code.
This was a fairly predictable consequence of providing an official size tool. Used to be, they just laid out the rules for figuring out sizes, and you had to either count manually, or build your own tool to count. Now, bugs in the size tool end up being bugs in the rules, and fun like this ensues. Whether this is good or bad is a matter of opinion. I would say it certainly makes things interesting!
Edit: HN formatting makes the inline form of that comment look like a C++/C99-style // comment. Fun!
"CPU supports the full 8086/186 instruction set. Due to the complexities of the 8086’s arbitrary-length instruction decoding and flags, 8086 instructions are first converted to a simpler intermediate format before being executed. This conversion, along with instruction lengths and how each instruction modifies the flags, is assisted by some lookup tables which form part of the BIOS binary."
With that in mind, the BIOS is _hand-written_ and about 12kB (gzips to <4kB), so that's also a fantastic achievement. All-in-all, this is a wonderful entry.