X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Fswitch.S;h=feca86cd16d02e3d6f31d1dde25cff379c0861a4;hb=64e74e3536b95c4d0a46aec56bc373ad0f6d470b;hp=fddd821afc8ef296fb22d800768e743577be89c5;hpb=44d0fa6a2b24a84e5eb0d54959ed91c1d4f15343;p=pintos-anon

diff --git a/src/threads/switch.S b/src/threads/switch.S
index fddd821..feca86c 100644
--- a/src/threads/switch.S
+++ b/src/threads/switch.S
@@ -1,25 +1,40 @@
-#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
@@ -31,17 +46,20 @@ switch_threads:
 	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).
+	# Call thread_schedule_tail(prev).
 	pushl %eax
-	.globl schedule_tail
-	call schedule_tail
+.globl thread_schedule_tail
+	call thread_schedule_tail
 	addl $4, %esp
 
 	# Start thread proper.
 	ret
+.endfunc