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overlay.asm
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Assembly Source File
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1989-06-07
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1KB
|
56 lines
name overlay
title 'OVERLAY segment'
;
; OVERLAY.OVL --- a simple overlay segment
; loaded by ROOT.EXE to demonstrate use of
; the MS-DOS EXEC call subfunction 03H.
;
; The overlay does not contain a STACK segment
; because it uses the ROOT segment's stack.
;
; Ray Duncan, June 1987
;
stdin equ 0 ; standard input
stdout equ 1 ; standard output
stderr equ 2 ; standard error
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
_TEXT segment byte public 'CODE' ; executable code segment
assume cs:_TEXT,ds:_DATA
ovlay proc far ; entry point from root segment
mov ax,_DATA ; set DS = local data segment
mov ds,ax
; display overlay message ...
mov dx,offset msg ; DS:DX = address of message
mov cx,msg_len ; CX = length of message
mov bx,stdout ; BX = standard output handle
mov ah,40h ; AH = fxn 40h, write file/device
int 21h ; transfer to MS-DOS
ret ; return to root segment
ovlay endp ; end of ovlay procedure
_TEXT ends
_DATA segment para public 'DATA' ; static & variable data segment
msg db cr,lf,'Overlay executing!',cr,lf
msg_len equ $-msg
_DATA ends
end