
;  hack24.asm  tmn  5/1/87

;  An example of a small resident critical error handler.

;  Assemble with

;    MASM HACK24;
;    LINK HACK24;
;    DEL HACK24.OBJ
;    EXE2COM HACK24
;    DEL HACK24.EXE

cseg    segment

        org 12h
han_p   dd ?

        org 100h

        assume cs:cseg, ds:nothing

entry   label far
        jmp install

;  This substitute handler doesn't do much - just exits with
;  return code 3 (DOS 3.x only) to make the DOS CALL in question
;  fail immediately with an 0053 error.  Under earlier versions
;  of DOS all we do is exit with rc=0.

dos_vsn db ?

newint24 label far
        sti
        cmp dos_vsn,3
        jb  v2x

        mov al,3
        iret

v2x:    mov al,0
        iret

end_of_res:

;  Data areas.  Everything starting here is discarded when we
;  issue the TSR call.

pat_code equ $
pat_op  db 0eah                   ; JMPF opcode
pat_off dw newint24
pat_seg dw ?
pat_size equ $-pat_code

inst_msg db 10,13,'Critical Error now Selects New Drive ...>  ',10,13
        db 0dh, 0ah, '$'

vers_msg db 'Hack24 requires DOS 2.0 or later'
        db 0dh, 0ah, '$'

        assume ds:cseg

;  Mainline

install:
        mov ah,30h                ; Save major version
        int 21h
        mov dos_vsn,al            
        cmp al,2                  ; need at least 2.0
        jae ok

        mov dx,offset vers_msg    ; complain
        mov ah,09h
        int 21h

        mov al,1                  ; exit with errorlevel 1
        mov ah,04ch
        int 21h

ok:
        mov dx,offset inst_msg    ; display installation msg
        mov ah,09h
        int 21h

;  Patch COMMAND.COM's own INT 24H handler to jump to us

        mov ax,cs                 ; we patch segment
        mov pat_seg,ax

        mov si,offset pat_code    ; ds already -> cseg
        les di,han_p
        mov cx,pat_size
        cld
        cli
        rep movsb                 ; patch DOS to jump to us
        sti

;  Now terminate and stay resident

        mov ax,offset end_of_res  ; compute paras to reserve
        add ax,0fh
        shr ax,1
        shr ax,1
        shr ax,1
        shr ax,1

        mov dx,ax                 ; preferred TSR method
        mov ah,31h
        mov al,0
        int 21h

cseg    ends
end     entry



                                                      
