BLIS/COBOL was an operating system that, unusually, was written in COBOL. It is the only such system to gain reasonably wide acceptance. It was optimised to compile business application written in COBOL. BLIS was available on a range of Data General Nova and Data General Eclipse 16-bit minicomputers.
Originally, most operating systems were written in assembly language for a particular processor or family of processors. Non-assembler operating systems were comparatively slow, but were easier for revision and repair. One of the reasons for the C programming language's low-level features, which resemble assembly language in some ways, is an early intent to use it for writing operating systems. Similar goals led to IBM's development of PL/S. The high-level nature of COBOL, which created some problems for operating system development, was partially addressed in BLIS, since it was deliberately optimised for COBOL.