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