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-Date: Tue, 11 Jan 94 13:23:11 -0800
-From: "pardo@cs.washington.edu" <pardo@meitner.cs.washington.edu>
-
->[What's needed to get `qt' on an i860-based machine?]
-
-Almost certainly "some assembly required" (pun accepted).
-
-To write a cswap port, you need to understand the context switching
-model. Turn to figure 2 in the QT TR. Here's about what the assembly
-code looks like to implement that:
-
- qt_cswap:
- adjust stack pointer
- save callee-save registers on to old's stack
- argument register <- old sp
- sp <- new sp
- (*helper)(args...)
- restore callee-save registers from new's stack
- unadjust stack pointer
- return
-
-Once more in slow motion:
-
- - `old' thread calls context switch routine (new, a0, a1, h)
- - cswap routine saves registers that have useful values
- - cswap routine switches to new stack
- - cswap routine calls helper function (*h)(old, a0, a1)
- - when helper returns, cswap routine restores registers
- that were saved the last time `new' was suspended
- - cswap routine returns to whatever `new' routine called the
- context switch routine
-
-There's a few tricks here. First, how do you start a thread running
-for the very first time? Answer is: fake some stuff on the stack
-so it *looks* like it was called from the middle of some routine.
-When the new thread is restarted, it is treated like any other
-thread. It just so happens that it's never really run before, but
-you can't tell that because the saved state makes it look like like
-it's been run. The return pc is set to point at a little stub of
-assembly code that loads up registers with the right values and
-then calls `only'.
-
-Second, I advise you to forget about varargs routines (at least
-until you get single-arg routines up and running).
-
-Third, on most machines `qt_abort' is the same as `qt_cswap' except
-that it need not save any callee-save registers.
-
-Fourth, `qt_cswap' needs to save and restore any floating-point
-registers that are callee-save (see your processor handbook). On
-some machines, *no* floating-point registers are callee-save, so
-`qt_cswap' is exactly the same as the integer-only cswap routine.
-
-I suggest staring at the MIPS code for a few minutes. It's "mostly"
-generic RISC code, so it gets a lot of the flavor across without
-getting too bogged down in little nitty details.
-
-
-
-Now for a bit more detail: The stack is laid out to hold callee-save
-registers. On many machines, I implemented fp cswap as save fp
-regs, call integer cswap, and when integer cswap returns (when the
-thread wakes up again), restore fp regs.
-
-For thread startup, I figure out some callee-save registers that
-I use to hold parameters to the startup routine (`only'). When
-the thread is being started it doesn't have any saved registers
-that need to be restored, but I go ahead and let the integer context
-switch routine restore some registers then "return" to the stub
-code. The stub code then copies the "callee save" registers to
-argument registers and calls the startup routine. That keeps the
-stub code pretty darn simple.
-
-For each machine I need to know the machine's procedure calling
-convention before I write a port. I figure out how many callee-save
-registers are there and allocate enough stack space for those
-registers. I also figure out how parameters are passed, since I
-will need to call the helper function. On most RISC machines, I
-just need to put the old sp in the 0'th arg register and then call
-indirect through the 3rd arg register; the 1st and 2nd arg registers
-are already set up correctly. Likewise, I don't touch the return
-value register between the helper's return and the context switch
-routine's return.
-
-I have a bunch of macros set up to do the stack initialization.
-The easiest way to debug this stuff is to go ahead and write a C
-routine to do stack initialization. Once you're happy with it you
-can turn it in to a macro.
-
-In general there's a lot of ugly macros, but most of them do simple
-things like return constants, etc. Any time you're looking at it
-and it looks confusing you just need to remember "this is actually
-simple code, the only tricky thing is calling the helper between
-the stack switch and the new thread's register restore."
-
-
-You will almost certainly need to write the assembly code fragment
-that starts a thread. You might be able to do a lot of the context
-switch code with `setjmp' and `longjmp', if they *happen* to have
-the "right" implementation. But getting all the details right (the
-helper can return a value to the new thread's cswap routine caller)
-is probaby trickier than writing code that does the minimum and
-thus doesn't have any extra instructions (or generality) to cause
-problems.
-
-I don't know of any ports besides those included with the source
-code distribution. If you send me a port I will hapily add it to
-the distribution.
-
-Let me know as you have questions and/or comments.
-
- ;-D on ( Now *that*'s a switch... ) Pardo