source: bootcd/isolinux/syslinux-6.03/core/callback.inc @ e16e8f2

;; -----------------------------------------------------------------------
;;
;;   Copyright 1994-2009 H. Peter Anvin - All Rights Reserved
;;   Copyright 2009 Intel Corporation; author: H. Peter Anvin
;;
;;   This program is free software; you can redistribute it and/or modify
;;   it under the terms of the GNU General Public License as published by
;;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
;;   Boston MA 02111-1307, USA; either version 2 of the License, or
;;   (at your option) any later version; incorporated herein by reference.
;;
;; -----------------------------------------------------------------------

;;
;; callback.inc
;;
;; Callbacks from 32-bit mode to 16-bit mode
;;

;
; 16-bit intcall/farcall handling code
;

;
; 32-bit support code
;
                bits 32
                section .text

;
; Intcall/farcall invocation.  We manifest a structure on the real-mode stack,
; containing the com32sys_t structure from <com32.h> as well as
; the following entries (from low to high address):
; - Target offset
; - Target segment
; - Return offset
; - Return segment (== real mode cs == 0)
; - Return flags
;
                global core_farcall:function hidden
core_farcall:
                mov eax,[esp+1*4]               ; CS:IP
                jmp core_syscall

                global core_intcall:function hidden
core_intcall:
                movzx eax,byte [esp+1*4]        ; INT number
                mov eax,[eax*4]                 ; Get CS:IP from low memory

core_syscall:
                pushfd                          ; Save IF among other things...
                inc dword [CallbackCtr]
                push ebx
                push ebp
                push esi
                push edi
                push dword [CallbackSP]

                cld

                movzx edi,word [word RealModeSSSP]
                movzx ebx,word [word RealModeSSSP+2]
                sub edi,54              ; Allocate 54 bytes
                mov [word RealModeSSSP],di
                shl ebx,4
                add edi,ebx             ; Create linear address

                mov esi,[esp+8*4]       ; Source regs
                xor ecx,ecx
                mov cl,11               ; 44 bytes to copy
                rep movsd

                ; EAX is already set up to be CS:IP
                stosd                   ; Save in stack frame
                mov eax,.rm_return      ; Return seg:offs
                stosd                   ; Save in stack frame
                mov eax,[edi-12]        ; Return flags
                and eax,0x200ed7        ; Mask (potentially) unsafe flags
                mov [edi-12],eax        ; Primary flags entry
                stosw                   ; Return flags

                mov bx,.rm
                jmp enter_rm    ; Go to real mode

                bits 16
                section .text16
.rm:
                mov ax,sp
                add ax,9*4+4*2
                mov [CallbackSP],ax
                pop gs
                pop fs
                pop es
                pop ds
                popad
                popfd
                retf                            ; Invoke routine

.rm_return:
                ; We clean up SP here because we don't know if the
                ; routine returned with RET, RETF or IRET
                mov sp,[cs:CallbackSP]
                pushfd
                pushad
                push ds
                push es
                push fs
                push gs
                mov ebx,.pm_return
                jmp enter_pm

                ; On return, the 44-byte return structure is on the
                ; real-mode stack, plus the 10 additional bytes used
                ; by the target address (see above.)
                bits 32
                section .text
.pm_return:
                movzx esi,word [word RealModeSSSP]
                movzx eax,word [word RealModeSSSP+2]
                mov edi,[esp+9*4]       ; Dest regs
                shl eax,4
                add esi,eax             ; Create linear address
                and edi,edi             ; NULL pointer?
                jnz .do_copy
.no_copy:       mov edi,esi             ; Do a dummy copy-to-self
.do_copy:       xor ecx,ecx
                mov cl,11               ; 44 bytes
                rep movsd               ; Copy register block

                add dword [word RealModeSSSP],54
                                        ; Remove from stack

                pop dword [CallbackSP]
                dec dword [CallbackCtr]
                jnz .skip
                call [core_pm_hook]
.skip:
                pop edi
                pop esi
                pop ebp
                pop ebx
                popfd
                ret                     ; Return to 32-bit program

;
; Cfarcall invocation.  We copy the stack frame to the real-mode stack,
; followed by the return CS:IP and the CS:IP of the target function.
; The value of IF is copied from the calling routine.
;
                global core_cfarcall:function hidden
core_cfarcall:
                pushfd                          ; Save IF among other things...
                inc dword [CallbackCtr]
                push ebx
                push ebp
                push esi
                push edi
                push dword [CallbackSP]

                cld
                mov ecx,[esp+9*4]               ; Size of stack frame

                movzx edi,word [word RealModeSSSP]
                movzx ebx,word [word RealModeSSSP+2]
                mov [word CallbackSP],di
                sub edi,ecx             ; Allocate space for stack frame
                and edi,~3              ; Round
                sub edi,4*3             ; Return pointer, return value, EFLAGS
                mov [word RealModeSSSP],di
                shl ebx,4
                add edi,ebx             ; Create linear address

                mov eax,[esp+5*4]       ; EFLAGS from entry
                and eax,0x202           ; IF only
                stosd
                mov eax,[esp+7*4]       ; CS:IP
                stosd                   ; Save to stack frame
                mov eax,.rm_return      ; Return seg:off
                stosd
                mov esi,[esp+8*4]       ; Stack frame
                mov eax,ecx             ; Copy the stack frame
                shr ecx,2
                rep movsd
                mov ecx,eax
                and ecx,3
                rep movsb

                mov bx,.rm
                jmp enter_rm

                bits 16
                section .text16
.rm:
                popfd
                retf
.rm_return:
                mov sp,[cs:CallbackSP]
                mov esi,eax
                mov ebx,.pm_return
                jmp enter_pm

                bits 32
                section .text
.pm_return:
                mov eax,esi
                ; EDX already set up to be the RM return value
                pop dword [CallbackSP]
                dec dword [CallbackCtr]
                jnz .skip
                call [core_pm_hook]
.skip:
                pop ebx
                pop ebp
                pop esi
                pop edi
                popfd
                ret

                section .bss16
                alignb 4
                global core_pm_hook
CallbackSP      resd 1                  ; SP saved during callback
CallbackCtr     resd 1

                bits 16
                section .text16