;
; File:
;			   entry.asm
; Description:
;		       System call entry code
;
;			Copyright (c) 1998
;			Pasquale J. Villani
;			All Rights Reserved
;
; This file is part of DOS-C.
;
; DOS-C 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; either version
; 2, or (at your option) any later version.
;
; DOS-C is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
; the GNU General public License for more details.
;
; You should have received a copy of the GNU General public
; License along with DOS-C; see the file COPYING.  If not,
; write to the Free Software Foundation, 675 Mass Ave,
; Cambridge, MA 02139, USA.
;
; $Header:   C:/dos-c/src/kernel/entry.asv   1.0   07 Feb 1998 20:42:08   patv  $
;
; $Log:   C:/dos-c/src/kernel/entry.asv  $
;  
;     Rev 1.0   07 Feb 1998 20:42:08   patv
;  Modified stack fram to match DOS standard
; $EndLog$

		page	60,132
		title	System call entry code

IFDEF ??version
_TEXT		segment	byte public 'CODE'
DGROUP		group	_FIXED_DATA,_DATA,_BSS,_BSSEND	; small model
		assume	cs:_TEXT,ds:DGROUP,ss:DGROUP
_TEXT		ends

_FIXED_DATA	segment para public 'DATA'
_FIXED_DATA	ends

_DATA		segment word public 'DATA'
_DATA		ends

_BSS		segment word public 'BSS'
_BSS		ends

_BSSEND		segment byte public 'STACK'
_BSSEND		ends

ELSE
_TEXT		segment	byte public 'CODE'
_TEXT		ends

_FIXED_DATA	segment para public 'DATA'
_FIXED_DATA	ends

_DATA		segment word public 'DATA'
_DATA		ends

CONST		segment word public 'CONST'
CONST		ends

_BSS		segment word public 'BSS'
_BSS		ends

_BSSEND		segment byte public 'STACK'
_BSSEND		ends

DGROUP		group	CONST,_DATA,_BSS,_BSSEND	; small/tiny model
		assume	ds:DGROUP, ss:DGROUP
ENDIF

		include	stacks.inc

_TEXT		segment	byte public 'CODE'
		extrn	_int21_syscall:near
		extrn	_int25_handler:NEAR, _int26_handler:NEAR
		extrn	_set_stack:FAR
		extrn	_restore_stack:FAR
		extrn	_api_sp:WORD
		extrn	_api_ss:WORD
		extrn	_disk_api_tos:WORD
		public	_cpm_entry
		public	_int20_handler
		public	_int21_handler
		public	_low_int25_handler
		public	_low_int26_handler
		public	_int27_handler


;
; MS-DOS CP/M style entry point
;
;	VOID FAR 
;	cpm_entry(iregs UserRegs)
;
; This one is a strange one.  The call is to psp:0005h but it returns to the 
; function after the call.  What we do is convert it to a normal call and 
; fudge the stack to look like an int 21h call.
;
		assume	cs:_TEXT
_cpm_entry	proc	far
		; Stack is:
		;	return offset
		;	psp seg
		;	000ah
		;
		push	bp		; trash old return address
		mov	bp,sp
		xchg	bp,2[bp]
		pop	bp
		pushf			; start setting up int 21h stack
		;
		; now stack is
		;	return offset
		;	psp seg
		;	flags
		;
		push	bp
		mov	bp,sp		; set up reference frame
		;
		; reference frame stack is
		;	return offset		bp + 6
		;	psp seg			bp + 4
		;	flags			bp + 2
		;	bp		<---	bp
		;
		push	ax
		mov	ax,2[bp]	; get the flags
		xchg	ax,6[bp]	; swap with return address
		mov	2[bp],ax
		pop	ax		; restore working registers
		pop	bp
		;
		; Done. Stack is
		;	flags
		;	psp seg (alias .COM cs)
		;	return offset
		;
		cmp	cl,024h
		jbe	cpm_error
		mov	ah,cl		; get the call # from cl to ah
		jmp	short _int21_handler	; do the system call
cpm_error:	mov	al,0
		iret
_cpm_entry	endp


;
; Terminate the current process
;
;	VOID INRPT far 
;	int20_handler(iregs UserRegs)
;
		assume	cs:_TEXT
_int20_handler	proc	far
		mov	ah,0		; terminate through int 21h
_int20_handler	endp


;
; MS-DOS system call entry point
;
;	VOID INRPT far 
;	int21_handler(iregs UserRegs)
;
		assume	cs:_TEXT
_int21_handler	proc	far
		;
		; Create the stack frame for C call.  This is done to 
		; preserve machine state and provide a C structure for 
		; access to registers.
		;
		; Since this is an interrupt routine, CS, IP and flags were 
		; pushed onto the stack by the processor, completing the 
		; stack frame.
		;
		; NB: stack frame is MS-DOS dependent and not compatible
		; with compiler interrupt stack frames.
		;
		PUSH$ALL

		;
		; Create kernel refernce frame.
		;
		; NB: At this point, SS != DS and won't be set that way 
		; until later when which stack to run on is determined.
		;
		mov	bp,DGROUP
		mov	ds,bp
		mov	bp,sp
		;
		; Push the far pointer to the register frame for 
		; int21_syscall and remainder of kernel.
		;
		push	ss
		lea	ax,word ptr [bp]
		push	ax
		call	near ptr _int21_syscall
		;
		; Clean up stack.  It's int2_syscall responsibility to 
		; restore user stack.
		;
		pop	cx
		pop	cx
		;
		; Recover registers from system call.  Registers and flags 
		; were modified by the system call.
		;
		POP$ALL

		;
		; ... and return.
		;
		iret	
_int21_handler	endp



;
; Terminate the current process
;
;	VOID INRPT far 
;	int27_handler(iregs UserRegs)
;
		assume	cs:_TEXT
_int27_handler	proc	far
		;
		; First convert the memory to paragraphs
		;
		add	dx,0fh		; round up
		rcr	dx,1
		shr	dx,1
		shr	dx,1
		shr	dx,1
		;
		; ... then use the standard system call
		;
		mov	ax,3100h
		jmp	short _int21_handler	; terminate through int 21h
_int27_handler	endp

;
; I really do need to get rid of this because it's the only thing stopping 
; us from being ROMABLE.
;
stkframe        dd      ?

_low_int25_handler  proc far
		sti
		pushf
		push    ax
		push    cx
		push    dx
		push    bx
		push    sp
		push    bp
		push    si
		push    di
		push    ds
		push    es
		
		mov     word ptr cs:stkframe, sp     ; save stack frame
		mov     word ptr cs:stkframe+2, ss

		cld
		mov     ax, seg DGROUP
		mov     ds, ax

		mov     word ptr DGROUP:_api_sp, offset DGROUP:_disk_api_tos
		mov     word ptr DGROUP:_api_ss, ds

		call    far ptr _set_stack

		push    word ptr cs:stkframe+2
		push    word ptr cs:stkframe
		call    _int25_handler
		add     sp, 4

		call    far ptr _restore_stack

		pop     es
		pop     ds
		pop     di
		pop     si
		pop     bp
		pop     bx      ; pop off sp value
		pop     bx
		pop     dx
		pop     cx
		pop     ax
		popf
		retf            ; Bug-compatiblity with MS-DOS.
		                ; This function is supposed to leave the original
		                ; flag image on the stack.
_low_int25_handler  endp


_low_int26_handler  proc far
		sti
		pushf
		push    ax
		push    cx
		push    dx
		push    bx
		push    sp
		push    bp
		push    si
		push    di
		push    ds
		push    es
		
		mov     word ptr cs:stkframe, sp     ; save stack frame
		mov     word ptr cs:stkframe+2, ss

		cld
		mov     ax, seg DGROUP
		mov     ds, ax

		mov     word ptr DGROUP:_api_sp, offset DGROUP:_disk_api_tos
		mov     word ptr DGROUP:_api_ss, ds

		call    far ptr _set_stack

		push    word ptr cs:stkframe+2
		push    word ptr cs:stkframe
		call    _int26_handler
		add     sp, 4
		
		call    far ptr _restore_stack

		pop     es
		pop     ds
		pop     di
		pop     si
		pop     bp
		pop     bx      ; pop off sp value
		pop     bx
		pop     dx
		pop     cx
		pop     ax
		popf
		retf            
_low_int26_handler  endp

_TEXT		ends
		end