TURBO.COM

☆ The single INT 21h. There is exactly one DOS system call instruction in all 39,731 bytes — at offset 0x0926. Every file open, close, read, write, directory listing, and memory allocation in the entire compiler + editor + runtime funnels through one dispatcher routine. This is the architectural discipline that makes 39 KB enough.

▶ How it all fits: The parser (36.1%) and editor (14.8%) together account for half the binary. The code generator is just 7.1%. The entire DOS interface is 1.2%. The software floating-point engine that implements Sin, Cos, Sqrt, Ln, Exp without an FPU: 5.8%. Everything else — the startup banner, file manager, compiler driver, error handler, keyboard input — shares the remaining ~25%.