diff options
Diffstat (limited to 'qt/README.PORT')
-rw-r--r-- | qt/README.PORT | 112 |
1 files changed, 0 insertions, 112 deletions
diff --git a/qt/README.PORT b/qt/README.PORT deleted file mode 100644 index d56300923..000000000 --- a/qt/README.PORT +++ /dev/null @@ -1,112 +0,0 @@ -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 |