X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=src%2Fthreads%2Fswitch.S;h=244249e07c063594fc3f385937781d22b0bd2371;hb=3f08530b1f8640d3fdc05477e44d0e81c0199ed3;hp=c077e46d590202ef5156ab0d9084d1ea28e6afe2;hpb=637d20eadbc4cb26b67335c282b34e20a2738fb6;p=pintos-anon

diff --git a/src/threads/switch.S b/src/threads/switch.S
index c077e46..244249e 100644
--- a/src/threads/switch.S
+++ b/src/threads/switch.S
@@ -1,10 +1,28 @@
-#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
@@ -16,10 +34,7 @@ switch_threads:
 
 	# 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
@@ -31,10 +46,12 @@ switch_threads:
 	popl %ebp
 	popl %ebx
         ret
+.endfunc
 
 .globl switch_entry
+.func switch_entry
 switch_entry:
-	# Discard thread_switch() arguments.
+	# Discard switch_threads() arguments.
 	addl $8, %esp
 
 	# Call schedule_tail(prev).
@@ -45,3 +62,4 @@ switch_entry:
 
 	# Start thread proper.
 	ret
+.endfunc