Adding A Pause Between Items in Config.NT / Config.Sys

I wanted to debug startup of a 16-bit DOS driver on 32-bit Windows 10 with NTVDM, however attempts to attach debugger / Time Travel Debugging Trace to NTVDM startup process was triggering access violations and causing NTVDM.exe to crash. Once NTVDM had started I could attach debugger fine, but was missing the driver startup code I wanted to capture.

MS-DOS 6.00 added a feature where F8 could be pressed to run autoexec.bat/config.sys entries one line at a time, but I haven’t found an alternative that works with c:\windows\system32\config.nt in Windows.

In this case using Microsoft Macro Assmbler built this driver with the following commands:

masm wait.asm
link wait
exe2bin wait.exe wait.sys
xcopy wait.sys C:\windows\system32

The code is here, this can also be used a template for a simple MS-DOS driver.

; *******************************************************************
; * Press Any Key To Continue DRIVER                                *
; *******************************************************************

cseg        segment para    public  'code'
wait        proc    far
            assume  cs:cseg,es:cseg,ds:cseg

; *******************************************************************
; * MAIN PROCEDURE CODE                                             *
; *******************************************************************

begin:

; *******************************************************************
; * DEVICE HEADER - REQUIRED BY DOS                                 *
; *******************************************************************

next_dev    dd  -1              ; no other device drivers
attribute   dw  8000h           ; character device
strategy    dw  dev_strategy    ; address of 1st dos call
interrupt   dw  dev_interrupt   ; address of 2nd dos call
dev_name    db  'WAIT$ '      ; name of the driver

; *******************************************************************
; * WORK SPACE FOR THE DEVICE DRIVER                                *
; *******************************************************************

rh_ofs      dw  ?               ; request header offset
rh_seg      dw  ?               ; request header segment
msg1        db  'Waiting...'
            db  0dh,0ah,'$'
seconds     db 0   
counter     db 0  
crlf	    db 0dh,0ah,'$'
; *******************************************************************
; * THE STRATEGY PROCEDURE                                          *
; *******************************************************************

dev_strategy:                   ; first call from DOS
    mov     cs:rh_seg,es        ; save request header ptr segment
    mov     cs:rh_ofs,bx        ; save request header ptr offset
    ret

; *******************************************************************
; * THE INTERRUPT PROCEDURE                                         *
; *******************************************************************

dev_interrupt:                  ; second call from DOS
    cld                         ; save machine state on entry
    push    ds
    push    es
    push    ax
    push    bx
    push    cx  
    push    dx
    push    di
    push    si

; perform branch based on the command passed in the req header

    mov     al,es:[bx]+2        ; get command code
    cmp     al,0                ; check for 0
    jnz     exit3               ; no - exit go to error exit
    rol     al,1                ; get offset into table
    lea     di,cmdtab           ; get address of command table
    mov     ah,0                ; clear hi order
    add     di,ax               ; add offset
    jmp     word ptr[di]        ; jump indirect

; command table
;       the command code field of the static request
;       field contains the function to be performed

cmdtab  label   byte            ;
        dw      init            ; initialization

; *******************************************************************
; *     LOCAL PROCEDURES                                            *
; *******************************************************************

initial proc    near
    lea     dx,msg1             ; initialization
    mov     ah,9                ; message
    int     21h                 ; dos call
    mov     al,30               ; number of seconds to wait
    call    sleep
    ret                         ; return
initial endp

; *******************************************************************
; *     DOS COMMAND PROCESSING                                      *
; *******************************************************************

;command    0   initialization

init:   call    initial         ; display a message
        lea     ax,exit         ; get end address (offset)
        mov     es:[bx]+0eh,ax  ; store offset address
        push    cs              ; get end
        pop     ax              ; address (segment)
        mov     es:[bx]+10h,ax  ; store in break address
        jmp     exit2

; *******************************************************************
; *     ERROR EXIT                                                  *
; *******************************************************************

; Set the done flag, error flag, and unknown command error code

exit3:  mov     es:word ptr 3[bx],8103h
        jmp     exit1                   ; restore environment

; *******************************************************************
; *     COMMON EXIT                                                 *
; *******************************************************************

; common exits fall thru code
;   2 sets status to done and no error
;   1 restore callers es:bx
;   0 restore machine state and exit

exit2:                                  ; set done flag and no error
        mov     es:word ptr 3[bx],0100h
exit1:  mov     bx,cs:rh_ofs            ; restore req hdr to bx and es
        mov     es,cs:rh_seg            ; as saved by dev_Strategy
exit0:  pop     si                      ; restore all registers
        pop     di
        pop     dx
        pop     cx
        pop     bx
        pop     ax
        pop     es
        pop     ds
        ret
exit:

; *******************************************************************
; *     END OF PROGRAM                                              *
; *******************************************************************


wait    endp

sleep proc    near

wait_for_al_seconds:      
wait_loop:
    push ax               ; save our counter (al)
    mov     [counter],al
loop_top:
    mov ah,2
    int 1ah             ; get time

    mov ah, [seconds]   ; retrieve last good value
    cmp ah, dh          ; is it same as last good value?
    jz  loop_top         ; yup, ignore it, loop again!

    mov [seconds], dh   ; save seconds

    ; display counter - can handle range of 0-99
    mov	al, [counter]   ; retrieve counter
    cbw                 ; set AH to 0
    mov  dl, 10
    div  dl             ; Divides AX by 10: quotient in al, remainder in ah
    add  ax, "00"
    mov  dx, ax         
    mov  ah, 02h        ; Display 1st digit of counter
    int  21h
    mov  dl, dh          
    int  21h            ; Display 2nd digit of counter

    
    lea     dx,crlf             ; display carriage return
    mov     ah,9                
    int     21h                 

    pop ax              
    dec al              ; decrease al by one (does not set flags!!)
    or al,al            ; set flags
    jnz wait_loop       ; al=0?  nope, around we go again!

    ret                 ; 
sleep   endp


cseg        ends
            end     begin

; that's all folks!

Now we can add line to C:\Windows\System32\config.nt to load our driver where we want it to pause:

DEVICE=%SystemRoot%\System32\wait.sys

To test all existing ntvdm.exe process must be terminated, as config.sys is only loaded when a new ntvdm.exe instance is created. Now when launching a 16-bit DOS application you will see a count down for 30 seconds when this line of config.nt has been hit: