True, what many perceive as the C ABI is actually the OS ABI.
As most OSs tend to be written mostly in C, it is natural that C ABI == OS ABI.
In the few cases where the OS was written in other languages, like the mainframe systems, the early 80's OS done in Pascal dialects (Mac OS), Modula-2 (Lillith), Assembly (MS-DOS/CP-M) and many others to list here, there was no C ABI to speak of.
As most OSs tend to be written mostly in C, it is natural that C ABI == OS ABI.
In the few cases where the OS was written in other languages, like the mainframe systems, the early 80's OS done in Pascal dialects (Mac OS), Modula-2 (Lillith), Assembly (MS-DOS/CP-M) and many others to list here, there was no C ABI to speak of.