The first operation system on 8 bit computers (Intel 8080) was CP/M. It was the ancestor of the DOS system and actually many of the DOS functions were derived from this initial operating system. At the time CP/M was developed, there were three different software layers between the microprocessor and the user: the BIOS (or basic input-output system, it is also found today in our PC's), the Disk Operating System, and the Command(.com) processor. CP/M contained both the DOS and the command processor. CP/M was basically designed to provide a convenient interface for the user to load, save, and execute files that were stored on to an 8 inch diskette and the majority of the software functions it provided were related to these tasks (displaying and fetching characters on terminal and printer, saving and loading files to and from the disk).
Since in the days of development of this system the only language available was assembler it was written entirely in Intel 8080 ASM. To write an assembler program one does not need an operating system to be in place since there is only a two-way function between the op-codes and the numbers representing instructions. This function is simply taken from existent tables on paper.
In fact up until 1995 (before the C language come on the first palace because of its simplicity) the assembler language was widely used for written games and applications because the executable code was performing the fastest. Even today, applications written in assembler are executing faster than applications written in other languages (including C).