I found the code surprisingly readable.
Most variables names are what I expected them to mean despite their shortness: pc, sp, bp are registers, a is the accumulator, fd is a file descriptor (of the input file, what else?), tk is for the token, t is temporary, etc... For the less obvious ones, it is usually not that hard to infer their meaning from either the code or comments.
Because yes, they are comments, not many, but they are helpful. For example, the VM has unusual instructions (for me) like LEV and ADJ, and they are commented. The "obvious" ones like MUL and SHR are not.
The variable names are not "needlessly cryptic". I've seen (and written, not proud of it) a lot of needlessly cryptic variable names, and believe me, these are crystal clear by comparison. Here, there is a clear influence from assembly mnemonics that really helps understanding.
Now on the why. This is minimalism, and minimizing the number of comments and variable name length is part of it. It is actually a very interesting exercise.
The golden rule in making understandable code is making it as short as possible. There is a limited amount of space on your screen and in your mind, and the shorter your code is, the more you can see/understand at once. Of course, too much is too much, you don't want to do things IOCCC style, and striking a balance is difficult. So once in a while, reading or writing very compact code can help you understand where shaving off characters is fine and where it really hurts understanding.
Most variables names are what I expected them to mean despite their shortness: pc, sp, bp are registers, a is the accumulator, fd is a file descriptor (of the input file, what else?), tk is for the token, t is temporary, etc... For the less obvious ones, it is usually not that hard to infer their meaning from either the code or comments.
Because yes, they are comments, not many, but they are helpful. For example, the VM has unusual instructions (for me) like LEV and ADJ, and they are commented. The "obvious" ones like MUL and SHR are not.
The variable names are not "needlessly cryptic". I've seen (and written, not proud of it) a lot of needlessly cryptic variable names, and believe me, these are crystal clear by comparison. Here, there is a clear influence from assembly mnemonics that really helps understanding.
Now on the why. This is minimalism, and minimizing the number of comments and variable name length is part of it. It is actually a very interesting exercise. The golden rule in making understandable code is making it as short as possible. There is a limited amount of space on your screen and in your mind, and the shorter your code is, the more you can see/understand at once. Of course, too much is too much, you don't want to do things IOCCC style, and striking a balance is difficult. So once in a while, reading or writing very compact code can help you understand where shaving off characters is fine and where it really hurts understanding.