di2589.txt 
;**************************************************************************************************
; LISTING 1 - THREE-PRIORITY-LEVEL INTERRUPT SYSTEM FOR THE 8051
;
; "Software provides interrupt system for 8051," EDN, Nov 23, 2000, pg 168

; http://www.ednmag.com/ednmag/reg/2000/11232000/designideas.htm#24di7
;
***************************************************************************************************

             ORG     0000H
             LIMP    START
             ORG     0003H
             LIMP    INTO
             ORG     000BH
             LIMP    ITO
             ORG     0013H
              LIMP    INT1
START: MOV  SP,#60H
               MOV  IP,#01H                     ;INT0 has high priority
               MOV  TMOD,#01H
               MOV  TH0,#00H
               MOV  TL0,#00H
               SETB  EA                             ;enable INT0,INT1,IT0
               SETB  EX0
               SETB  EX1
               SETB  ET0
               SETB  TR0
                       ...

INT0:             ...     .......................;INT0 interrupt service program
                      ...
                RETI
INT1:             ...                                   ;INT1 interrupt service program
                      ...
                 RETI
IT0:          CLR  TR0                             ;IT0 interrupt service program
                 PUSH  DPL
                 PUSH  DPH
                 MOV  DPTR,#GO_ON
                 PUSH  DPL
                 PUSH  DPH
                 RETI                                      ;"pseudo-RETI"
GO_ON:  NOP
                    ...
                    ...
                 MOV  TH0,#00H
                 MOV  TL0,#00H
                 POP    DPH
                 POP    DPL
                 SETB  TR0
                 RETI