[Qemu-devel] Re: [Bochs-developers] [PATCH v3 5/6] Don't use unreserved memory inBIOS.

["Sebastian Herbszt" <herbszt@gmx.de>]

Gleb Natapov wrote:
> Use only first page and last page of low memory. OSes assumes that first
> page is used by bios and last page is reserved in e820 map.
> 
> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> ---
> 
> bios/rombios.c        |   11 +++++++----
> bios/rombios.h        |    1 -
> bios/rombios32.c      |   11 ++++-------
> bios/rombios32start.S |    2 +-
> 4 files changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/bios/rombios.c b/bios/rombios.c
> index 630cfd2..c6c8b19 100644
> --- a/bios/rombios.c
> +++ b/bios/rombios.c
> @@ -4547,7 +4547,7 @@ ASM_END
>                 {
>                     case 0:
>                         set_e820_range(ES, regs.u.r16.di,
> -                                       0x0000000L, 0x0009fc00L, 1);
> +                                       0x0000000L, 0x0009f000L, 1);
>                         regs.u.r32.ebx = 1;
>                         regs.u.r32.eax = 0x534D4150;
>                         regs.u.r32.ecx = 0x14;
> @@ -4556,7 +4556,7 @@ ASM_END
>                         break;
>                     case 1:
>                         set_e820_range(ES, regs.u.r16.di,
> -                                       0x0009fc00L, 0x000a0000L, 2);
> +                                       0x0009f000L, 0x000a0000L, 2);
>                         regs.u.r32.ebx = 2;
>                         regs.u.r32.eax = 0x534D4150;
>                         regs.u.r32.ecx = 0x14;
> @@ -10032,8 +10032,11 @@ rombios32_05:
>   mov gs, ax
>   cld
> 
> -  ;; init the stack pointer
> -  mov esp, #0x00080000
> +  ;; init the stack pointer to point below EBDA
> +  mov ax, [0x040e]
> +  shl eax, #4
> +  mov esp, #-0x10
> +  add esp, eax

This could be a problem if the OS decides to put the wakeup
vector just below the EBDA. But as long as the EBDA is located
above 0x9f000 it should be ok and this can be changed later.

- Sebastian