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[pintos-anon] / src / threads / switch.S

diff --git a/src/threads/switch.S b/src/threads/switch.S
index fe06d77f9be7480f84fff623008f435fd6cf0eae..ba699988de02005bbe8aef62d6e40b723f4c7956 100644 (file)
--- a/src/threads/switch.S
+++ b/src/threads/switch.S
@@ -1,9 +1,25 @@
 #include "switch.h"
 
+#### struct thread *switch_threads (struct thread *cur, struct thread *next);
+####
+#### Switches from CUR, which must be the running thread, to NEXT,
+#### which must also be running thread_switch(), returning CUR in
+#### NEXT's context.
+####
+#### This function works by assuming that the thread we're switching
+#### into is also running switch_threads().  Thus, all it has to do is
+#### preserve a few registers on the stack, then switch stacks and
+#### restore the registers.  As part of switching stacks we record the
+#### current stack pointer in CUR's thread structure.
+
 .globl switch_threads 
 switch_threads:
        # Save caller's register state.
-       # Note that the SVR4 ABI allows us to destroy %eax, %ecx, %edx.
+       #
+       # Note that the SVR4 ABI allows us to destroy %eax, %ecx, %edx,
+       # but requires us to preserve %ebx, %ebp, %esi, %edi.  See
+       # [SV-ABI-386] pages 3-11 and 3-12 for details.
+       #
        # This stack frame must match the one set up by thread_create().
        pushl %ebx
        pushl %ebp