Intel chips use the LITTLE ENDIAN convention. Data transfer between RAM and CPU registers attaches the LOW byte of RAM to the LOW 8 bits of a register. Please refer to the Instruction Set of Intel 8086.
In all of these assignments you are given that bytes in memory starting at address 200 are
 
Adress : 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 20A, 20B..
Value : 12, 34, 56, 78, AB, CD, EF, 12, 34, 56, 78, 90
 
CODE for executed
 
             MOV  BX, 200           
             MOV  AX, [BX]   
             MOV  CX, [BX + 1]
             CMP  AX, CX  
             JB   cxismax       
             SUB  AX, CX
             MOV  DX, AX
             JMP  storeresult 
cxismax:     SUB  CX, AX
             MOV  DX, CX
storeresult: MOV  [BX+5], DX