edit works fine for this; under DOS 4 or lesser, you're
stuck with edlin. Under Windoze, you can use the
notepad editor, which has the advantage that it prints
out the neat little headers and footers for you. Under Unix, you can
use emacs, which actually has some intelligence built in
about assembly code and can make editing it a little bit easier.Whatever your choice of editors, make sure you save the file as plain ASCII. If you are using Word Perfect or some other word processor, make sure you choose the option that causes it to save as plain ASCII and not in whatever format is native to the word processor.
Under DOS, the standard extension for an assembly language program is
.asm. Under Unix, the standard extension is
.s.