Programming on my UNIX V6 emulator

C

You need to write pre-K&R C which is quite a bit different from modern C. Compile your programs with cc. Yes, it is slow. Example session:

# ed test.c
?
a
main() {
    printf("Hello, world!\n");
}
.
w
38
q
# cc test.c
# a.out
Hello, world!

Assembler

The assembler is called as. Example:

        sys write; msg; 15
        sys exit
.data
msg:    <Hello, World\n>

Fortran

You can compile UNIX Fortran programs with fc. Example program:

      WRITE (6,7)
    7 FORMAT(13H HELLO, WORLD)
      STOP
      END