-#include "switch.h"
+#include "threads/switch.h"
-.globl switch_threads
+#### 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 switch_threads(), 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
+.func 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
+ # [SysV-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
popl %ebp
popl %ebx
ret
+.endfunc
.globl switch_entry
+.func switch_entry
switch_entry:
# Discard switch_threads() arguments.
addl $8, %esp
# Start thread proper.
ret
+.endfunc