[PATCH 03/18] x86/boot: use %ecx instead of %eax

[Daniel Kiper <daniel.kiper@oracle.com>]

Use %ecx instead of %eax to store low memory upper limit from EBDA.
This way we do not wipe multiboot protocol identifier. It is needed
in reloc() to differentiate between multiboot (v1) and
multiboot2 protocol.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
 xen/arch/x86/boot/head.S |   27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index c99f739..6180783 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -86,14 +86,14 @@ __start:
         jne     not_multiboot
 
         /* Set up trampoline segment 64k below EBDA */
-        movzwl  0x40e,%eax          /* EBDA segment */
-        cmp     $0xa000,%eax        /* sanity check (high) */
+        movzwl  0x40e,%ecx          /* EBDA segment */
+        cmp     $0xa000,%ecx        /* sanity check (high) */
         jae     0f
-        cmp     $0x4000,%eax        /* sanity check (low) */
+        cmp     $0x4000,%ecx        /* sanity check (low) */
         jae     1f
 0:
-        movzwl  0x413,%eax          /* use base memory size on failure */
-        shl     $10-4,%eax
+        movzwl  0x413,%ecx          /* use base memory size on failure */
+        shl     $10-4,%ecx
 1:
         /*
          * Compare the value in the BDA with the information from the
@@ -105,21 +105,22 @@ __start:
         cmp     $0x100,%edx         /* is the multiboot value too small? */
         jb      2f                  /* if so, do not use it */
         shl     $10-4,%edx
-        cmp     %eax,%edx           /* compare with BDA value */
-        cmovb   %edx,%eax           /* and use the smaller */
+        cmp     %ecx,%edx           /* compare with BDA value */
+        cmovb   %edx,%ecx           /* and use the smaller */
 
 2:      /* Reserve 64kb for the trampoline */
-        sub     $0x1000,%eax
+        sub     $0x1000,%ecx
 
         /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
-        xor     %al, %al
-        shl     $4, %eax
-        mov     %eax,sym_phys(trampoline_phys)
+        xor     %cl, %cl
+        shl     $4, %ecx
+        mov     %ecx,sym_phys(trampoline_phys)
 
         /* Save the Multiboot info struct (after relocation) for later use. */
         mov     $sym_phys(cpu0_stack)+1024,%esp
-        push    %ebx
-        call    reloc
+        mov     %ecx,%eax
+        push    %ebx                /* Multiboot information address */
+        call    reloc               /* %eax contains trampoline address */
         mov     %eax,sym_phys(multiboot_ptr)
 
         /* Initialize BSS (no nasty surprises!) */
-- 
1.7.10.4