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;
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; jmemdosa.asm
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;
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; Copyright (C) 1992, Thomas G. Lane.
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; This file is part of the Independent JPEG Group's software.
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; For conditions of distribution and use, see the accompanying README file.
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;
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; This file contains low-level interface routines to support the MS-DOS
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; backing store manager (jmemdos.c).  Routines are provided to access disk
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; files through direct DOS calls, and to access XMS and EMS drivers.
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;
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; This file should assemble with Microsoft's MASM or any compatible
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; assembler (including Borland's Turbo Assembler).  If you haven't got
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; a compatible assembler, better fall back to jmemansi.c or jmemname.c.
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;
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; To minimize dependence on the C compiler's register usage conventions,
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; we save and restore all 8086 registers, even though most compilers only
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; require SI,DI,DS to be preserved.  Also, we use only 16-bit-wide return
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; values, which everybody returns in AX.
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;
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; Based on code contributed by Ge' Weijers.
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;
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JMEMDOSA_TXT	segment byte public 'CODE'
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		assume	cs:JMEMDOSA_TXT
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		public	_jdos_open
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		public	_jdos_close
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		public	_jdos_seek
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		public	_jdos_read
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		public	_jdos_write
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		public	_jxms_getdriver
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		public	_jxms_calldriver
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		public	_jems_available
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		public	_jems_calldriver
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38 
;
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; short far jdos_open (short far * handle, char far * filename)
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;
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; Create and open a temporary file
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;
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_jdos_open	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	cx,0			; normal file attributes
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		lds	dx,dword ptr [bp+10]	; get filename pointer
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		mov	ah,3ch			; create file
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		int	21h
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		jc	open_err		; if failed, return error code
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		lds	bx,dword ptr [bp+6]	; get handle pointer
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		mov	word ptr [bx],ax	; save the handle
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		xor	ax,ax			; return zero for OK
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open_err:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jdos_open	endp
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73 
;
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; short far jdos_close (short handle)
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;
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; Close the file handle
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;
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_jdos_close	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	bx,word ptr [bp+6]	; file handle
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		mov	ah,3eh			; close file
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		int	21h
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		jc	close_err		; if failed, return error code
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		xor	ax,ax			; return zero for OK
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close_err:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jdos_close	endp
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105 
;
106 
; short far jdos_seek (short handle, long offset)
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;
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; Set file position
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;
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_jdos_seek	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	bx,word ptr [bp+6]	; file handle
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		mov	dx,word ptr [bp+8]	; LS offset
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		mov	cx,word ptr [bp+10]	; MS offset
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		mov	ax,4200h		; absolute seek
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		int	21h
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		jc	seek_err		; if failed, return error code
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		xor	ax,ax			; return zero for OK
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seek_err:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jdos_seek	endp
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;
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; short far jdos_read (short handle, void far * buffer, unsigned short count)
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;
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; Read from file
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;
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_jdos_read	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	bx,word ptr [bp+6]	; file handle
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		lds	dx,dword ptr [bp+8]	; buffer address
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		mov	cx,word ptr [bp+12]	; number of bytes
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		mov	ah,3fh			; read file
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		int	21h
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		jc	read_err		; if failed, return error code
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		cmp	ax,word ptr [bp+12]	; make sure all bytes were read
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		je	read_ok
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		mov	ax,1			; else return 1 for not OK
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		jmp	short read_err
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read_ok:	xor	ax,ax			; return zero for OK
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read_err:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jdos_read	endp
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176 
 
177 
;
178 
; short far jdos_write (short handle, void far * buffer, unsigned short count)
179 
;
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; Write to file
181 
;
182 
_jdos_write	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	bx,word ptr [bp+6]	; file handle
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		lds	dx,dword ptr [bp+8]	; buffer address
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		mov	cx,word ptr [bp+12]	; number of bytes
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		mov	ah,40h			; write file
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		int	21h
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		jc	write_err		; if failed, return error code
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		cmp	ax,word ptr [bp+12]	; make sure all bytes written
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		je	write_ok
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		mov	ax,1			; else return 1 for not OK
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		jmp	short write_err
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write_ok:	xor	ax,ax			; return zero for OK
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write_err:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jdos_write	endp
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214 
 
215 
;
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; void far jxms_getdriver (XMSDRIVER far *)
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;
218 
; Get the address of the XMS driver, or NULL if not available
219 
;
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_jxms_getdriver	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov 	ax,4300h		; call multiplex interrupt with
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		int	2fh			; a magic cookie, hex 4300
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		cmp 	al,80h			; AL should contain hex 80
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		je	xmsavail
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		xor 	dx,dx			; no XMS driver available
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		xor 	ax,ax			; return a nil pointer
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		jmp	short xmsavail_done
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xmsavail:	mov 	ax,4310h		; fetch driver address with
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		int	2fh			; another magic cookie
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		mov 	dx,es			; copy address to dx:ax
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		mov 	ax,bx
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xmsavail_done:	les 	bx,dword ptr [bp+6]	; get pointer to return value
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		mov	word ptr es:[bx],ax
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		mov	word ptr es:[bx+2],dx
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		pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop	bp
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		ret
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_jxms_getdriver	endp
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255 
 
256 
;
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; void far jxms_calldriver (XMSDRIVER, XMScontext far *)
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;
259 
; The XMScontext structure contains values for the AX,DX,BX,SI,DS registers.
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; These are loaded, the XMS call is performed, and the new values of the
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; AX,DX,BX registers are written back to the context structure.
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;
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_jxms_calldriver 	proc	far
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		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		les 	bx,dword ptr [bp+10]	; get XMScontext pointer
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		mov 	ax,word ptr es:[bx]	; load registers
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		mov 	dx,word ptr es:[bx+2]
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		mov 	si,word ptr es:[bx+6]
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		mov 	ds,word ptr es:[bx+8]
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		mov 	bx,word ptr es:[bx+4]
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		call	dword ptr [bp+6]	; call the driver
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		mov	cx,bx			; save returned BX for a sec
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		les 	bx,dword ptr [bp+10]	; get XMScontext pointer
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		mov 	word ptr es:[bx],ax	; put back ax,dx,bx
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		mov 	word ptr es:[bx+2],dx
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		mov 	word ptr es:[bx+4],cx
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		pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		pop 	bp
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		ret
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_jxms_calldriver 	endp
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296 
 
297 
;
298 
; short far jems_available (void)
299 
;
300 
; Have we got an EMS driver? (this comes straight from the EMS 4.0 specs)
301 
;
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_jems_available	proc	far
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		mov	ax,3567h		; get interrupt vector 67h
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		int	21h
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		push	cs
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		pop	ds
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		mov	di,000ah		; check offs 10 in returned seg
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		lea	si,ASCII_device_name	; against literal string
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		mov	cx,8
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		cld
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		repe cmpsb
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		jne	no_ems
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		mov	ax,1			; match, it's there
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		jmp	short avail_done
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no_ems:		xor	ax,ax			; it's not there
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avail_done:	pop	ds			; restore registers and exit
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		pop	es
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		pop	dx
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		pop	cx
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		pop	bx
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		pop	di
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		pop	si
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		ret
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ASCII_device_name	db	"EMMXXXX0"
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334 
_jems_available	endp
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337 
;
338 
; void far jems_calldriver (EMScontext far *)
339 
;
340 
; The EMScontext structure contains values for the AX,DX,BX,SI,DS registers.
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; These are loaded, the EMS trap is performed, and the new values of the
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; AX,DX,BX registers are written back to the context structure.
343 
;
344 
_jems_calldriver	proc far
345 
		push	bp			; linkage
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		mov 	bp,sp
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		push	si			; save all registers for safety
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		push	di
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		push	bx
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		push	cx
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		push	dx
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		push	es
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		push	ds
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		les 	bx,dword ptr [bp+6]	; get EMScontext pointer
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		mov 	ax,word ptr es:[bx]	; load registers
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		mov 	dx,word ptr es:[bx+2]
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		mov 	si,word ptr es:[bx+6]
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		mov 	ds,word ptr es:[bx+8]
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		mov 	bx,word ptr es:[bx+4]
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		int	67h			; call the EMS driver
361 
		mov	cx,bx			; save returned BX for a sec
362 
		les 	bx,dword ptr [bp+6]	; get EMScontext pointer
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		mov 	word ptr es:[bx],ax	; put back ax,dx,bx
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		mov 	word ptr es:[bx+2],dx
365 
		mov 	word ptr es:[bx+4],cx
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		pop	ds			; restore registers and exit
367 
		pop	es
368 
		pop	dx
369 
		pop	cx
370 
		pop	bx
371 
		pop	di
372 
		pop	si
373 
		pop 	bp
374 
		ret
375 
_jems_calldriver	endp
376 
 
377 
JMEMDOSA_TXT	ends
378 
 
379 
		end