name      FXN4BH

         page      55,132

         title     'FXN4BH --- demo PC-DOS EXEC function'

;

; FXN4BH --- demonstrate use of the

; PC-DOS 2.0 EXEC function call 4BH

;

; Copyright (c) 1983 by Ray Duncan

;

cr       equ       0dh       ;ASCII carriage return

lf       equ       0ah       ;ASCII line feed

                             ;

cseg     segment   para public 'CODE'

                             ;

         assume    cs:cseg,ds:data,ss:stack

                             ;

demo     proc      far

                             ;at entry DS & ES = PSP

         push      ds        ;Save address for final

         xor       ax,ax     ;FAR RET to PC-DOS on stack

         push      ax

                             ;save copy of SS:SP for use

                             ;after return from overlay

         mov       cs:STK_SEG,ss

         mov       cs:STK_PTR,sp

                             ;

                             ;Reserve 1000H bytes for

                             ;this loader and release

                             ;the rest of memory for

                             ;use by the overlayed program.

         mov       bx,100h   ;ES=segment of PSP of loader

         mov       ah,4ah    ;BX=paragraphs to reserve

         int       21h

                             ;make the messages in data

                             ;segment addressable

         mov       ax,seg DATA

         mov       ds,ax

         mov       es,ax

                             ;jump if memory

                             ;de-allocation failed

         jc        ALLOC_ERR

                             ;print memory successfully

                             ;released

         mov       dx,offset MSG2

         mov       ah,9

         int       21h

                             ;

                             ;now load and execute

                             ;the overlaid program.

         mov       dx,offset PGM_NAME

         mov       bx,offset PAR_BLK

         mov       al,0

         mov       ah,4bh

         int       21h

                             ;restore stack pointers

                             ;to state before EXEC call

         mov       ss,cs:STK_SEG

         mov       sp,cs:STK_PTR

                             ;Make data segment

                             ;addressable again

         mov       ax,seg DATA

         mov       ds,ax

                             ;print message that loader

                             ;successfully regained control

         mov       dx,offset MSG3

         mov       ah,9

         int       21h

                             ;now exit to PC-DOS

         ret



alloc_err:                   ;come here if memory

                             ;cannot be released

         mov       dx,offset MSG1

         mov       ah,9

         int       21h       ;print error message and

         ret                 ;exit to PC-DOS

                             ;

demo     endp

                             ;

                             ;these two variables must

                             ;reside in Code Segment so

                             ;that they are addressable

                             ;after return from overlay.

stk_seg  dw        0         ;original SS contents

stk_ptr  dw        0         ;original SP contents

                             ;

cseg     ends



                             ;declare a stack area

                             ;for use by this loader

stack    segment   para stack 'STACK'

                             ;allow 64 bytes in this case

         db        64 dup (?)

stack    ends



                             ;declare data segment to

                             ;contain variables and tables

data     segment   para public 'DATA'

;

msg1     db        cr,lf

         db        'Unable to release memory.'

         db        cr,lf,'$'

msg2     db        cr,lf

         db        'Memory above loader released.'

         db        cr,lf,'Now loading CHKDSK.COM.'

         db        cr,lf,'$'

msg3     db        cr,lf

         db        'Loader regained control from CHKDSK,'

         db        cr,lf

         db        'now making final exit to PC-DOS.'

         db        cr,lf,'$'

;

                             ;drive, path, and name of program

                             ;to be loaded and executed.

pgm_name db        '\CHKDSK.COM',0

;

par_blk  dw        ENVIR     ;segment address of

                             ;environment descriptor

                             ;

                             ;full address of command line

                             ;to be passed at offset 80H

         dw        offset CMD_LINE     ;in overlaid

         dw        seg CMD_LINE        ;program's PSP

                             ;

                             ;full address of default

                             ;File Control Block to be

                             ;passed at offset 5CH in

         dw        offset FCB1         ;overlaid

         dw        seg FCB1            ;program's PSP

                             ;

                             ;full address of default

                             ;File Control Block to be

                             ;passed at offset 6CH in

         dw        offset FCB2         ;overlaid

         dw        seg FCB2            ;program's PSP

;

                             ;actual command line tail

                             ;to be passed to overlay

cmd_line db        4,' *.*',cr,0

;

                             ;first default FCB to

fcb1     db        0         ;be passed to overlay

         db        11 dup ('?')

         db        25 dup (0)



                             ;second default FCB to

fcb2     db        0         ;be passed to overlay

         db        11 dup (' ')

         db        25 dup (0)

;

data     ends



                             ;declare separate data

                             ;segment to contain

                             ;environment descriptor

envir    segment   para 'ENVIR'

                             ;

                             ;Search path used by PC-DOS

                             ;to look for commands or

                             ;batch files not found in

         db        'PATH=',0 ;the current directory

                             ;

                             ;Search path used by PC-DOS

                             ;to locate COMMAND.COM

         db        'COMSPEC=A:\COMMAND.COM',0

         db        0         ;extra 0 byte designates

                             ;end of environment

envir    ends



         end       demo