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Re: OS calls.

  Line F is used on 030 machines for floating point, I think; and it's used
  by early TOS versions (the ones that don't run on 030's :-)) internally
  by the AES to call the VDI.

Right. Line-F is part of the coprocessor calling interface, it's used on
68020 and up for any coprocessor - I suppose math and MMU are the only
ones that ever existed in widespread use.

  I would be quite surprised if the actual calling mechanism (trap vs.
  line A or line F) turned out to be a significant factor in system call
  timings. I would think that the context save/restore code would be the
  most significant part (at least for MiNT). Anything that we could do
  to speed those up, and/or to generally speed up the kernel's handling
  of system calls once control reaches the kernel, would be a big win.

According to my 68020 user's manual, the Trap #x, Line A, and Line F
instructions all have identical timing characteristics. I don't think this
line of investigation is going to go anywhere.