More BASIC-PLUS/RSTS Internals

Here are some more comments I posted on Facebook.

User programs were limited to 16 KB. There was a LENGTH command that showed the length of your program in KW, which it displayed as 8 KW max. I'm not sure why they displayed the number in words and not bytes. I think the OS and BASIC combined were 24 KB, so (64 KB total - 8 KB memory-mapped I/O - 24 KB OS and BASIC) = 32 KB. This is only enough to store 2 full-size JOBS as they called them. If the system was fully loaded with twelve full-size JOBS (.i.e 16 KB of memory and compute-bound), the system would absolutely crawl. We had two teletypes in my school. Before logging on, we'd always ask the person on the other teletype, How fast is it today?

I've often thought it would be fun to do this on a home computer from the 80s. A friend of mine had a TRS-80, and I kept thinking about getting a home computer and trying to do something like this, but it seemed like a tremendous amount of work. From the rumors I've seen, it was done by two guys in about a year, one that did BASIC from EG&H, and the other was a guy from DEC who did the OS. There's a comment below about TSS-8 on a PDP-8. The guy who did the OS on that, did RSTS. The PDP-8 was even more amazing. The PDP-8 had even less memory and a very small instruction set with quirks in memory addressing. The guy that did the the two OSs was named Mark Bramhall, and I've read a lot of his code. No doubt he was a coding genius. Google his name and DEC, and you'll see some interesting things about him. One misconception I keep seeing is that the OS was written in BASIC, and that's not true. Commonly Used Systems Programs (CUSPs) were written in BASIC. They were pretty cool programs like LOGIN, LOGOUT, and SYSTAT. SYSTAT was like ps on Linux/Unix. SYSTAT had a gazillion PEEKs. I learned about all sorts of kernel-level data structures (or as DEC called it MONITOR) by reading SYSTAT and deciphering what the PEEKs were looking at. I learned more from that than I ever learned in my college OS class.

By the way, the sources for the CUSPs are on the internet if you're ever curious about how BASIC can be used as a systems programming language.