Compiling source series of Calc

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.

PAGE, 132
TITLE CALC
Cgroup GROUP codeseg
Codeseg SEGMENT PARA Public ' CODE '
Assume Cs:cgroup,ds:cgroup,es:cgroup
Public CALC
ORG 100H
CALC PROC FAR
JMP START
;---------------------------------------------------------------------;
; ;
; DATA area;
; ;
;---------------------------------------------------------------------;
DB ' INTERRUPT number = '
Int_number DB 61h
Screen_handle DW 0001h
Message DB ' Pemath isn't resident ', 13,10
Message_len EQU $-message
TAG DB ' Pemath '
Tag_len EQU $-tag
;---------------------------------------------------------------------;
; ;
; CODE area;
; ;
;---------------------------------------------------------------------;
START:
;---------------------------------------------------------------------;
; TEST for PRESENCE of CALCULATOR;
;---------------------------------------------------------------------;
SUB Ax,ax
MOV Es,ax
SUB BH,BH
MOV Bl,int_number
SHL bx,1
SHL bx,1
MOV DI,ES:[BX]
MOV Es,es:[bx+2]
ADD di,4
LEA Si,tag
MOV Cx,tag_len
Repe CMPSB
JE Call_calc
MOV Bx,screen_handle
MOV Cx,message_len
LEA Dx,message
MOV ah,40h
INT 21h
JMP Short Calc_exit
;---------------------------------------------------------------------;
; Call CALCULATOR;
;---------------------------------------------------------------------;
Call_calc:
MOV Al,int_number
MOV BYTE PTR Int_code,al
DB 0CDh; Int
Int_code:
DB 00h
NOP
NOP
Calc_exit:
INT 20h
CALC ENDP
Codeseg ENDS
End CALC