A special version of TeX called INITEX is used to compile macro
libraries and dump them in format (FMT) files. PLAIN.FMT and LPLAIN.FMT
are the format files for plain TEX and LATEX.
For example to build a new version of LATEX you need LPLAIN.TEX,
LATEX.TEX, LFONTS.TEX, HYPHEN.TEX , the string pool file TEX.POOL, plus
all the TFM files called by LFONTS. Just run INITEX LPLAIN. LPLAIN
calls all the other files. If all the right files are there it will
dump out LPLAIN.FMT. There is also a version called SLITEX (SPLAIN)
for generating slides or posters