The use of assembly language [BX] and LOOP

Example 1. Transfer data 0~63(3FH) to memory 0:200~0:23F in sequence

Analysis: 1. 0:200~0:23F is actually the same memory space as 0020:0~0020:3F .

            2. Use BX to store the offset address,The dx register is used as a container for storing intermediate variables (source data, constants 0-63 ) to write to memory.

The corresponding program code:

assume cs:code
code segment
  mov ax,0020h
  mov ds,ax
  mov bx,0
  mov ax,0
  mov cx,64
s:mov ds:[bx],ax
     inc bx
  inc ax
  loop s
 
  mov ax,4c00h
  int 21h

code ends
end

After compiling and linking, an executable file is generated, loaded with debug, and its operation is tracked, as shown in the figure:

Example 2. Transfer data 0~63(3FH) to memory 0:200~0:23F in sequence, the program can only use 9 instructions, 9 instructions include "MOV AX 4C00H" and "INT 21H" (return instruction)

Analysis: Data 0-63 are 64 consecutive numbers, and 0-3fH are also 64 consecutive numbers. You can use a bx variable to represent both the offset address and the increment of the number

The program code is as follows:

assume cs:code
code segment
mov ax,0020h
mov ds,ax
mov bx,0
mov cx,64
s:mov ds:[bx],bx
inc bx
loop s

mov ax,4c00h
int 21h
code ends
end

After compiling and linking, an executable file is generated, loaded with debug, and its operation is tracked, as shown in the figure: