mboot.c32: fix register constraints bug (more cleanly)

Fix register constraints of final jump to kernel entry.
When compiled with some GCC versions, mboot.c32 would clobber the kernel
load address and try to jump to 0x2badb002.

Signed-off-by: Tim Deegan <Tim.Deegan@cl.cam.ac.uk>

1 files changed, 13 insertions, 25 deletions

diff --git a/com32/modules/mboot.c b/com32/modules/mboot.c
index 74afaa5e..3150c510 100644
--- a/com32/modules/mboot.c
+++ b/com32/modules/mboot.c
@@ -856,31 +856,19 @@ static void trampoline_start(section_t *secs, int sec_count,
         }
     }
 
-    /* Now set up the last tiny bit of Multiboot environment... */
-
-    asm volatile(
-
-        /* A20 is already enabled.
-         * CR0 already has PG cleared and PE set.
-         * EFLAGS already has VM and IF cleared.
-         * ESP is the kernels' problem.
-         * GDTR is the kernel's problem.
-         * CS is already a 32-bit, 0--4G code segments.
-         * DS, ES, FS and GS are already 32-bit, 0--4G data segments.
-         * EBX must point to the MBI: */
-
-        "movl %0, %%ebx;"
-
-        /* EAX must be the Multiboot magic number. */
-
-        "movl $0x2badb002, %%eax;"
-
-        /* Start the kernel. */
-
-        "jmp *%1"
-
-        : : "m" (mbi_run_addr), "r" (entry));
-
+    /* Now set up the last tiny bit of Multiboot environment.
+     * A20 is already enabled.
+     * CR0 already has PG cleared and PE set.
+     * EFLAGS already has VM and IF cleared.
+     * ESP is the kernels' problem.
+     * GDTR is the kernel's problem.
+     * CS is already a 32-bit, 0--4G code segments.
+     * DS, ES, FS and GS are already 32-bit, 0--4G data segments.
+     *
+     * EAX must be 0x2badb002 and EBX must point to the MBI when we jump. */
+
+    asm volatile ("jmp *%2" 
+                  : : "a" (0x2badb002), "b" (mbi_run_addr), "cdSD" (entry));
 }
 static void trampoline_end(void) {}