A question about PPU's scanline(HELP ME!)

Do you know how they do in rendering one scanline of Emulator?

As my analysis we can not interrupt one scanline rendering,
in other word the line is not scanned by per pixel.
So we can only wait until we finish rendering this scanline.

I want to use multi-Thread to do so,
becuase I can not let the CPU work during the PPU rendering.
But I've found they don't use it, espesically the DOS program.
There's no thread under DOS......

Can you help me?

MS-DOS is a single-processing operating system. Use Windows or GNU/Linux or *BSD or Solaris if you want to have your multiple threads run on multiple processors or multiple HT functional units of newer Pentium 4 CPUs.