Chapter V -[bx] and loop instructions

In the front

[bx] And the description of the memory unit

• [bx] and [0] Some similar ,[0] Represents a memory unit , Its offset address is 0
• To describe a memory unit , Two messages are required ：1. Memory unit address ,2. Memory unit length .
• [bx] It also means a memory unit , Its offset address is in bx in ：mov ax,[bx] or mov al,[bx]

Loop

It's a cycle

Descriptive symbols “()”

Represents the contents of a register or a memory unit

idata

• Represents a constant

5.1 [bx]

take bx As an offset address

• mov ax,[bx]

take ax The data is sent to the indicated memory
mov [bx],ax

5.2Loop Instructions

loop  label 

Two step operation ：
1.(cx)=(cx)-1;
2, Judge cx The value in , If it is not zero, it will go to the label to execute the program , If 0 Then go down .

• Usually we use Loop Instruction to implement the loop function ,cx The number of storage cycles in .

5.3 stay Debug For tracking loop A circular program implemented by instructions

• g command : g ip Jump right after the loop

5.4Debug And assembler compiler Masm Different processing of instructions

masm incognizance [0],Debug know

5.5loop and [bx] The combined application of

S： mov al, [bx]
	mov ah, 0
	add dx, ax
	inc bx
	loop s

5.6 Segment prefix

• Instructions “mov ax,[bx]” in , The offset address of the memory unit is determined by bx give , The default segment address is in ds in
• We can display the address in the instruction accessing the memory unit, and give the segment register where the segment address of the memory unit is located
• “cs：0”、“ss：0” in cs,ss Are segment prefixes .

5.7 A safe space

• It is very dangerous to write content randomly like a piece of memory space , It may be system data or code
• In a normal PC In flight ,DOS Under way ,DOS And other legal systems are not easy to use 0：200~0:2FF Of 256 Bytes of space . So it's safe for us to use this space .

5.8 The use of segment prefixes

mov es:[bx], dl