How to load and then patch in 16 bit environments?

Hi,
Thanks for your help.
I fixed this part as follows:

      ParamBlock LABEL WORD
                 WORD  0
                 DWORD CmdLine
                 DWORD DfltFCB,DfltFCB
      LoadSSSP   DWORD ?
      LoadCSIP   DWORD ?

Then I used this code to change and then enter to the second program:

 mov bx,SEG ParamBlock ;Loading the Child Process
      mov es,bx
      mov bx,ParamBlock
      lds dx,PgmName
      mov al,01h
      mov ah,4bh
      int 21h
      
      
      mov es,WORD PTR cs:[LoadCSIP] ;Trying to change the twentieth Byte in 
      mov si,20h                    ;the second program 
      mov BYTE PTR es:[si],'$'
      
      mov ss,WORD PTR cs:[LoadSSSP]   ;Trying to go to the second program and 
      mov sp,WORD PTR cs:[LoadSSSP]+2 ;executing it
      jmp DWORD PTR cs:[LoadCSIP]
      

      mov ah,4ch
      int 21h

Unfortunately,It doesn't work.
I have some questions to be able to understand the concept.
When the second program is loaded,where is it located?
Is it right after the stack segment of the first program?
If it's so,I should be able to search in the memory for the bytes I want.
But I need an algo to search in memory.

ss of the parent program is the last segment wihch I should use and add sp to it to get the last address in the memory.
After this address normally the first segment of the child program should be loaded.

How can I code an algo to search in this area?
Also what is the last address in the memory?
I mean how far shall I do search in memory to find the desired bytes.

Is there any way to use SCASB instruction to find the place in memory?
In fact,let me know what should be put in ES: DI and AL and CX to be able to use SCAS instruction.

Regards,
Zest.