On Apr 26, 11:51 pm, "Helge Kruse" <spamt...@[EMAIL PROTECTED]
> wrote:
> Reply inline.
>
> "Mike Gonta" <spamt...@[EMAIL PROTECTED]
> wrote in
messagenews:770a0df6-a9d3-46c6-b5d2-76ea248fc4fc@[EMAIL PROTECTED]
> > On Apr 26, 10:25 am, "Helge Kruse" wrote:
>
> >> I wrote a x86 boot loader a long time ago. One step is to jump form
16 bit to 32 bit segment before entering protected mode.
> >> Is there any way to write a clean jump to 32 bit segment?
>
> > ;assemble with FASM
> > use16
> > 66 31 C0 xor eax, eax
> > 8E D8 mov ds, ax
> > 0F 01 16 18 00 lgdt [gdt]
> > 0F 20 C0 mov eax, cr0
> > 24 EF and al, 0EFh
> > 0F 22 C0 mov cr0, eax
> > EA 30 00 08 00 jmp 8:protected_mode
>
> I dont know the syntax 8:Label. Is 8 the selector offset?
> In that case I would expect a far jump. This would need 16 bit selector
offset and 32 bit offset, wouldn't it? But I think EA is the
> opcode for a near jump.
Actually, I take my words back. You can force TASM (and probably MASM
as well) to generate this kind of far instruction without using DB
0EAH,... But you need to jump through the hoops.
Here's an example:
CODE SEGMENT PARA PUBLIC "CODE" USE16
ASSUME CS:CODE
FXN PROC NEAR
NOP
JMP FAR PTR LOAD_CS_8
FXN_NEXT = $ - FXN
RET
FXN ENDP
CODE ENDS
CODE8 SEGMENT BYTE PUBLIC "CODE" USE16 AT 8
ASSUME CS:CODE8
ORG FXN_NEXT
LOAD_CS_8 PROC FAR
LOAD_CS_8 ENDP
CODE8 ENDS
END FXN
The disassembly:
C:\>debug FARJ.EXE
-u
15C5:0000 90 NOP
15C5:0001 EA06000800 JMP 0008:0006
15C5:0006 C3 RET
Alex
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