asmfuncs.asm

Next BIOS Source code : Extensible Firmware Interface

;; result in a variable since we'll need this again.
                .IF     ExceptionNumber == EXCPT32_DOUBLE_FAULT
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_INVALID_TSS
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_SEG_NOT_PRESENT
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_STACK_FAULT
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_GP_FAULT
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_PAGE_FAULT
                mov     ExtraPush, 1
                .ELSEIF ExceptionNumber == EXCPT32_ALIGNMENT_CHECK
                mov     ExtraPush, 1
                .ELSE
                mov     ExtraPush, 0
                .ENDIF

;; If there's some extra data, save it also, and modify the saved AppEsp to effectively
;; pop this value off the application's stack.
                .IF     ExtraPush == 1
                mov     eax, AppEsp
                mov     ebx, [eax]
                mov     ExceptData, ebx
                add     eax, 4
                mov     AppEsp, eax
                .ELSE
                mov     ExceptData, 0
                .ENDIF

;; The "pushad" above pushed the debug stack esp.  Since what we're actually doing
;; is building the context record on the debug stack, we need to save the pushed
;; debug ESP, and replace it with the application's last stack entry...
                mov     eax, [esp + 12]
                mov     DebugEsp, eax
                mov     eax, AppEsp
                add     eax, 12
                ; application stack has eflags, cs, & eip, so
                ; last actual application stack entry is
                ; 12 bytes into the application stack.
                mov     [esp + 12], eax

;; continue building context record
;; UINT32  Gs, Fs, Es, Ds, Cs, Ss;  insure high 16 bits of each is zero
                mov     eax, ss
                push    eax

                ; CS from application is one entry back in application stack
                mov     eax, AppEsp
                movzx   eax, word ptr [eax + 4]
                push    eax

                mov     eax, ds
                push    eax
                mov     eax, es
                push    eax
                mov     eax, fs
                push    eax
                mov     eax, gs
                push    eax

;; UINT32  Eip;
                ; Eip from application is on top of application stack
                mov     eax, AppEsp
                push    dword ptr [eax]

;; UINT64  Gdtr, Idtr;
                push    0
                push    0
                sidt    fword ptr [esp]
                push    0
                push    0
                sgdt    fword ptr [esp]

;; UINT32  Ldtr, Tr;
                xor     eax, eax
                str     ax
                push    eax
                sldt    ax
                push    eax

;; UINT32  EFlags;
;; Eflags from application is two entries back in application stack
                mov     eax, AppEsp
                push    dword ptr [eax + 8]

;; UINT32  Cr0, Cr1, Cr2, Cr3, Cr4;
;; insure FXSAVE/FXRSTOR is enabled in CR4...
;; ... while we're at it, make sure DE is also enabled...
                mov     eax, cr4
                or      eax, 208h
                mov     cr4, eax
                push    eax
                mov     eax, cr3
                push    eax
                mov     eax, cr2
                push    eax
                push    0
                mov     eax, cr0
                push    eax

;; UINT32  Dr0, Dr1, Dr2, Dr3, Dr6, Dr7;
                mov     eax, dr7
                push    eax
;; clear Dr7 while executing debugger itself
                xor     eax, eax
                mov     dr7, eax

                mov     eax, dr6
                push    eax
;; insure all status bits in dr6 are clear...
                xor     eax, eax
                mov     dr6, eax

                mov     eax, dr3
                push    eax
                mov     eax, dr2
                push    eax
                mov     eax, dr1
                push    eax
                mov     eax, dr0
                push    eax

;; FX_SAVE_STATE FxSaveState;
                sub     esp, 512
                mov     edi, esp
                ; IMPORTANT!! The debug stack has been carefully constructed to
                ; insure that esp and edi are 16 byte aligned when we get here.
                ; They MUST be.  If they are not, a GP fault will occur.
                FXSTOR_EDI

;; UINT32  ExceptionData;
                mov     eax, ExceptData
                push    eax

; call to C code which will in turn call registered handler
; pass in the vector number
                mov     eax, esp
                push    eax
                mov     eax, ExceptionNumber
                push    eax
                call    InterruptDistrubutionHub
                add     esp, 8

; restore context...
;; UINT32  ExceptionData;
                add     esp, 4

;; FX_SAVE_STATE FxSaveState;
                mov     esi, esp
                FXRSTOR_ESI
                add     esp, 512

;; UINT32  Dr0, Dr1, Dr2, Dr3, Dr6, Dr7;
                pop     eax
                mov     dr0, eax
                pop     eax
                mov     dr1, eax
                pop     eax
                mov     dr2, eax
                pop     eax
                mov     dr3, eax
;; skip restore of dr6.  We cleared dr6 during the context save.
                add     esp, 4
                pop     eax
                mov     dr7, eax

;; UINT32  Cr0, Cr1, Cr2, Cr3, Cr4;
                pop     eax
                mov     cr0, eax
                add     esp, 4
                pop     eax
                mov     cr2, eax
                pop     eax
                mov     cr3, eax
                pop     eax
                mov     cr4, eax

;; UINT32  EFlags;
                mov     eax, AppEsp
                pop     dword ptr [eax + 8]

;; UINT16  Ldtr, Tr;
;; UINT64  Gdtr, Idtr;
;; Best not let anyone mess with these particular registers...
                add     esp, 24

;; UINT32  Eip;
                pop     dword ptr [eax]

;; UINT32  SegGs, SegFs, SegEs, SegDs, SegCs, SegSs;
;; bugbug - modified segment registers could hang the debugger...  Should we
;;      attempt to insulate ourselves against this possibility?

                pop     gs
                pop     fs
                pop     es
                pop     ds
                pop     [eax + 4]
                pop     ss

;; bugbug - before restoring the last bit of stuff, we need to check and
;;              possibly move application stack top.

;; The next stuff to restore is the general purpose registers that were pushed
;; using the pushad instruction.
;;
;; The value of ESP as stored in the context record is the application ESP
;; including the 3 entries on the application stack caused by the exception
;; itself. It may have been modified by the debug agent, so we need to
;; determine if we need to relocate the application stack.

                mov     ebx, [esp + 12]  ; move the potentially modified AppEsp into ebx
                mov     eax, AppEsp
                add     eax, 12
                cmp     ebx, eax
                je      NoAppStackMove

                mov     eax, AppEsp
                mov     ecx, [eax]       ; EIP
                mov     [ebx], ecx

                mov     ecx, [eax + 4]   ; CS
                mov     [ebx + 4], ecx

                mov     ecx, [eax + 8]   ; EFLAGS
                mov     [ebx + 8], ecx

                mov     eax, ebx         ; modify the saved AppEsp to the new AppEsp
                mov     AppEsp, eax
NoAppStackMove:
                mov     eax, DebugEsp    ; restore the DebugEsp on the debug stack
                                         ; so our popad will not cause a stack switch
                mov     [esp + 12], eax

                cmp     ExceptionNumber, 068h
                jne     NoChain

Chain:

;; Restore eflags so when we chain, the flags will be exactly as if we were never here.
;; We gin up the stack to do an iretd so we can get ALL the flags.
                mov     eax, AppEsp
                mov     ebx, [eax + 8]
                and     ebx, NOT 300h ; special handling for IF and TF
                push    ebx
                push    cs
                push    PhonyIretd
                iretd
PhonyIretd:

;; UINT32  Edi, Esi, Ebp, Esp, Ebx, Edx, Ecx, Eax;
                popad

;; Switch back to application stack
                mov     esp, AppEsp

;; Jump to original handler
                jmp     OrigVector

NoChain:
;; UINT32  Edi, Esi, Ebp, Esp, Ebx, Edx, Ecx, Eax;
                popad

;; Switch back to application stack
                mov     esp, AppEsp

;; We're outa here...
                iretd
END