Assembly language learning (2)

I learned about Wang Shuang today《汇编语言》第5、6章.
I feel that these two chapters mainly deepen the for段这个概念的理解.

第5章[Bx]和loop指令,重点在于Understanding of segment addresses and offset addresses,段前缀的应用.
第6Chapters contain programs with multiple segments,重点在于Understand why segmenting is required,Learn how to write segmented assembler,Figure out how segments are stored in memory.

1.loop指令和[bx]的应用

1.1 计算ffff:0~ffff:bThe sum of the data in the cell,并将结果保存在dx中？

存在的问题：8The accumulated sum of the memory cells of the bits is placed16位的寄存器中.

解决的方法：以ax为中介,其中ah设为0,alis the value of the memory unit.

mov ax,0ffffh里面,0和h一个都不能少.

assume cs:code
code segment
    mov ax,0ffffh
    mov ds,ax
    mov ax,0
    mov dx,0
    mov bx,0
    mov cx,12
  s:mov al,[bx]
    add dx,ax
    inc bx
    loop s  
    mov ax,4c00h
    int 21h 
code ends
end

1.2 将内存ffff:0~ffff:b单元中的数据复制到0:200~0:20b中？

段前缀的应用,ds、cs、ss、es都可以使用,很灵活.

注意题目是,将内存ffff:0~ffff:b单元中的数据复制到0:200~0:20b单元中.Set the segment prefix to 0020h比0要方便.

   mov ax,0ffffh
   mov ds,ax
   mov ax,0020h
   mov es,ax
   mov bx,0
   mov cx,12
s: mov dl,[bx]
   mov es:[bx],dl
   inc bx
   loop s

2.包含多个段的程序

2.1 How to accumulate data？

That is, how some data constants should be declared,如何引用？

采用dw 0123h,0456h,0789h进行声明.

通过cs:[bx]进行定位,bx每次增加2个字节.

But just make these changes,Problems arise at runtime.Take a look at the program's memory first：

可以看出前6bytes store the value of three words,To avoid treating data as instructions,程序应该从0C53:0006开始执行.

So it needs to be used in the source programend标号,The entry address of the marked program.

2.2 利用栈,Store the data defined in the program in reverse？

首先,Declare a space ahead of time,for stack use.

另外,Set the segment address and offset address of the stack.

示例程序如下：

assume cs:code
code segment

        dw 0123h,0456h,0789h,0abch,0defh,0fedh,0cbah,0987h
        dd 0,0,0,0,0,0,0,0

start:mov ax,cs
      mov ss,ax
      mov sp,30h

      mov bx,0
      mov cx,8
   s: push cs:[bx]
      add bx,2
      loop s

      mov bx,0
      mov cx,8
   s1:pop cs:[bx]
      add bx,2
      loop s1

      mov ax,4c00h
      int 21h

code ends
end start

2.3 结构更清晰——Divide the code into multiple segments

Below is the example mentioned earlier,Don't overstate the data、Stack and code three segments.

需要注意的是：

1.在assume之后,CPU不会将CS指向code,不会将DS指向data,不会将SS指向stack.

2.endIndicates the entry of the program,这个入口将被写入可执行文件的描述信息.After the program in the executable is loaded into memory,CPU的CS：IP被设置指向这个入口,Start executing the first instruction in the program.

所以,CSThe value does not need to be set in the code,但是DS和SSThe value of needs to be explained in the code.

3.对于一个段,如果段中的数据占N个字节,则程序加载后,该段实际占有的空间为：

（N/256+1)*256

where division rounds up.

assume cs:code,ds:data,ss:stack

data segment
    dw 0123h,0456h,0789h,0abch,0defh,0fedh,0cbah,0987h
data ends

stack segment
    dw 0,0,0,0,0,0,0,0
stack ends

code segment
start:  mov ax,stack
         mov ss,ax
         mov sp,20h       
         mov ax,data
         mov ds,ax

         mov bx,0
         mov cx,8   
s:       push [bx]
         add bx,2
         loop s

         mov bx,0
         mov cx,8
s1:      pop [bx]
         add bx,2
         loop s1

         mov ax,4c00h
         int 21h
code ends
end start

Debug by yourself,Discover how segments are stored in memory,It's an interesting thing.
But the more interesting things are always behind（手动滑稽）