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Filename : PRIMARY.ASM

;---------------------------------
; primary.asm by Mitch Ames

; This program checks for the existance of a secondary shell in memory,
; and returns an error level of 1 if so, 0 if not.

; The primary shell is the first program in memory which owns itself (as
; determined by the memory control block). The program compares its parent's
; PSP with that of the primary PSP to determine whether the primary or a
; secondary shell is running. Note that if this program is called directly
; from another (via function 4B) it will register the parent as a secondary
; shell.


code segment
assume cs:code,ds:code,es:code,ss:code

.radix 16

org 0100

start: mov sp,0100 ;use default DTA as stack
mov ah,52 ;find first memory control block
int 21
mov ax,es:[bx-2] ;AX points to first MCB segment
mcb: mov es,ax ;ES is memory control block segment
inc ax ;AX is memory block segment
cmp ax,es:[1] ;does block own itself?
jz shell ;first one that does is primary shell
add ax,es:[3] ;calculate address of next MCB
jmp mcb

shell: cmp ax,ds:[16] ;is primary shell
this program's parent?
mov al,0 ;if so, exit code 0
je quit
mov dx,offset msg ;else write message
mov ah,9
int 21
mov al,1 ;exit code 1
quit: mov ah,4C
int 21

msg db "Secondary shell detected.",13d,10d,"$"


code ends
end start