Correction: 68K Mac OS calls were A-line traps — in other words, they had opcodes of the form `$Axxx`. To the processor, they're unimplemented instructions that each take an exception through the same vector. The exception handler is the Mac OS trap dispatcher.

`TRAP` is a different instruction, with opcodes `$4E4x`. Each one gets its own exception vector.

It's not just trap calls, though — sometimes applications write directly to the sound buffer or use hardware page flipping.

As I recall MacOS system calls were done through invalid instructions which would cause the CPU to "trap" (raise an interrupt). Giving rise to the question Mac extension writers asked of each other: "How many traps did you patch?"