[SeaBIOS] [RFC PATCH v3 0/3] Add segment limit checks to emulator

[Kevin O'Connor]

On Sat, Jul 24, 2010 at 11:45:22AM -0400, Kevin O'Connor wrote:
> On Mon, Jul 12, 2010 at 04:13:06PM +0300, Avi Kivity wrote:
> > Does SeaBIOS use big real mode now?
> 
> SeaBIOS calls option roms in big real mode.  This is required by the
> relevant specs.
> 
> See the transition16big function in src/romlayout.S.  It briefly jumps
> to an address at 0xffxxx during the transition to real-mode.  At a
> quick glance, it looks like it could probably be changed to not use a
> code address >64K.

I put together a SeaBIOS patch so it does not use code addresses >64K
in big real mode - in case anyone wants to test it.  Note, this only
reduces the use of code addresses >64K - SeaBIOS will still try to use
data addresses >64K (eg, in option rom PMM code).

-Kevin


diff --git a/src/misc.c b/src/misc.c
index 354df87..108c332 100644
--- a/src/misc.c
+++ b/src/misc.c
@@ -156,8 +156,8 @@ u64 rombios32_gdt[] VAR16VISIBLE __aligned(8) = {
     GDT_LIMIT(BUILD_BIOS_SIZE-1) | GDT_CODE | GDT_BASE(BUILD_BIOS_ADDR),
     // 16 bit data segment base=0x0 limit=0xffff (SEG32_MODE16_DS)
     GDT_LIMIT(0x0ffff) | GDT_DATA,
-    // 16 bit code segment base=0 limit=0xffffffff (SEG32_MODE16BIG_CS)
-    GDT_LIMIT(0xfffff) | GDT_CODE | GDT_G,
+    // 16 bit code segment base=0xf0000 limit=0xffffffff (SEG32_MODE16BIG_CS)
+    GDT_LIMIT(0xfffff) | GDT_CODE | GDT_G | GDT_BASE(BUILD_BIOS_ADDR),
     // 16 bit data segment base=0 limit=0xffffffff (SEG32_MODE16BIG_DS)
     GDT_LIMIT(0xfffff) | GDT_DATA | GDT_G,
 };
diff --git a/src/romlayout.S b/src/romlayout.S
index 54e5a4d..a469596 100644
--- a/src/romlayout.S
+++ b/src/romlayout.S
@@ -105,7 +105,7 @@ transition16big:
         movw %ax, %fs
         movw %ax, %gs
 
-        ljmpl $SEG32_MODE16BIG_CS, $(BUILD_BIOS_ADDR + 1f)
+        ljmpw $SEG32_MODE16BIG_CS, $1f
 
         .code16gcc
 1: