-#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.
- # This stack frame must match the one set up by thread_create().
+ #
+ # 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()
+ # in size.
pushl %ebx
pushl %ebp
pushl %esi
pushl %edi
# Get offsetof (struct thread, stack).
- .globl thread_stack_ofs
+.globl thread_stack_ofs
mov thread_stack_ofs, %edx
# Save current stack pointer to old thread's stack, if any.
movl SWITCH_CUR(%esp), %eax
- test %eax, %eax
- jz 1f
movl %esp, (%eax,%edx,1)
-1:
# Restore stack pointer from new thread's stack.
movl SWITCH_NEXT(%esp), %ecx
popl %ebp
popl %ebx
ret
+.endfunc
- .globl switch_entry
+.globl switch_entry
+.func switch_entry
switch_entry:
- # Discard thread_switch() arguments.
+ # Discard switch_threads() arguments.
addl $8, %esp
# Call schedule_tail(prev).
pushl %eax
- .globl schedule_tail
+.globl schedule_tail
call schedule_tail
addl $4, %esp
# Start thread proper.
ret
+.endfunc