65X-DOS/doc/syscalls.md

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# System Calls
The system API is exposed to user code using the COP interrupt.
The high byte of the C accumulator is loaded with a function number;
the other registers are loaded with call-specific data (or nothing),
and any return values are placed in the same registers.
What follows is a list of the system calls, their numbers, and
register arguments they take, and what if any values they return.