;CTRL-B2.8 - TSR which hooks interrupt 1Bh (CTRL-Break response)
;            Like CTRL-B1, but stores address of old handler and calls
;            it in the new handler.

MAIN:   MOV AX,351Bh      ;Function 35h: Get interrupt vector 1Bh
        INT 21h           ;Int 21h: Function 35h
        MOV DI,O_OLDI     
        MOV [DI+2],ES     ;Save segment of old handler
        MOV [DI],BX       ;Save offset of old handler
        MOV AX,251Bh      ;Function 25h: Set interrupt vector 1Bh
        MOV DX,NEWINT     ;Offset of handler
        INT 21h           ;Int 21h: Function 25h
        MOV DX,16h        ;Set 16h paragraphs to stay resident
        MOV AH,31h        ;Function 31h: Terminate and stay resident
        INT 21h           ;Int 21h: Function 31h

NEWINT: PUSH DX           ;Save DX register
        MOV DS,CS         ;Set Data Segment to Code Segment
        MOV DX,MSG        ;Address of string
        CALL OUTSTR       ;Output string
        POP DX            ;Restore DX register
        JMP D[O_OLDI]     ;Jump to old handler
        
OUTSTR: PROC              ;Output string procedure
        PUSHA             ;Save registers
        MOV AH,0Fh        ;Function 0Fh: Get video mode (active page in BH)
        INT 10h           ;Int 10h: Function 25h
        MOV DI,DX         ;Address of start of string
        MOV AH,0Eh        ;Function 0Eh: Teletype Output
        MOV BL,0Fh        ;Text color for graphics modes
NCHAR:  MOV AL,[DI]       ;Get character
        INC DI            ;Move string pointer to next character
        CMP AL,'$'        ;Check for end of string
        JE  ESTR          ;Return if found
        INT 10h           ;Int 10h: Function 0Eh
        JMP NCHAR         ;Do next character
ESTR:   POPA              ;Restore registers
        RET               ;Return
        ENDP              ;End of procedure

MSG:    DB 'Hello World$' ;String to output, '$' terminated
O_OLDI: DW ?,?            ;Offset and segment of old handler