Assembly Source series of FXN

This is the past DOS era of the compilation of source code, although has passed, but for the study of the assembly is still helpful, assembly language is just a basic programmer language, most people can grasp, not necessarily in-depth research.

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
; 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
;d E-allocation failed
JC Alloc_err
;p rint 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 the 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
;P rint 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;p rint 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 to overlay.
Stk_seg DW 0; original SS contents
Stk_ptr DW 0; original SP contents
;
Cseg ends
;d Eclare a stack area
; for use by this loader
Stack segment para stack ' stack '
; Allow bytes in the case
DB DUP (?)
Stack ends
;d Eclare 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, ' $ '
;
;d rive, 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 ' command line
; To is passed at offset 80H
DW offset Cmd_line in overlaid
DW seg cmd_line;p rogram ' s PSP
;
; Full address of default
; File control blocks to IS
;p at offset 5CH in
DW offset FCB1; overlaid
DW seg FCB1;p rogram ' s PSP
;
; Full address of default
; File control blocks to IS
;p at offset 6CH in
DW offset FCB2; overlaid
DW seg FCB2;p rogram ' s PSP
;
; actual command line tail
; To is passed to overlay
Cmd_line db 4, ' *.* ', cr,0
;
; the FCB to
FCB1 DB 0 passed to overlay
DB one DUP ('? ')
DB DUP (0)
; second default FCB to
FCB2 DB 0 passed to overlay
DB one DUP (')
DB DUP (0)
;
Data ends
;d eclare 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