---
tags : 微算機與組合語言
---
###### 國立臺北大學資工系江宥旻
8088/8086 programming - control flow instructions and program structures
===
Introducion
---
This chapter will cover following instructions
+ Flag-Control instructions
+ Compare instructions
+ Control Flow and the Jump instructions
+ Subroutines and subroutine-handling instructions
+ Loop and Loop-Handling instructions
+ String and string-Handling instructions

Flag-control instructions
---


> LAHF和SAHF在OS很重要

Example

Save flag to MEM1 and reload the flags with the contents of MEM2

LAHF
MOV [MEM1], AH
MOV AH, [MEM2]
SAHF

:::info
OS執行兩個程式(程式A和程式B)
A執行1ms後，要換B時
$\rightarrow$把A存在memory裡，換B執行
:::

> SR：Status Register

Compare instrution
---

CMP
+ Determine the relationship between two numbers
+ No operands are affected excepts for flags
+ 只會改變Flag不會更改到操作數
+ Basically a subtraction operation


Example

Please show that all of flags

MOV AX, 1234H
MOV BX, 0ABCDH
CMP AX, BX

AX = 0001001000110100
BX = 1010101111001101
CMP = 0110011001100111 (CMP是把AX減去BX來看AX和BX的大小關係，不會存在任何地方)



Control flow and the JUMP instructions
---

Jump instrutions
+ Unconditions jump
+ Jmp instruction
+ 

+ Condtion jump
+ Jxx instruction (有很多，用xx代替)
+ The condition refers to specific status flag
+ 


1. When the jump instruction is executed, **IP is reload** with a new value
+ IP+2 plus the signed displacement
2. Intrasegment jump(同區段跳躍) v.s. intersegment jump(跳區段跳躍)
+ 

+ Short-label, near-label, Memptr 16 or register 16 operand
+ 若無法被64K涵蓋就需要跳區段
+ ```asm
JMP 1234H
JMP BX
JMP [BX]

3. Condition jump instruction

![](https://i.imgur.com/VcyhaCY.jpg =500x)

4. Condition jump
    + JC Lable `offset is specified`
    + JB Lable
    ![](https://i.imgur.com/tBYh8NY.png)
5. Branch program structure

IF xxx THEN
then statement
ELSE
else statement

    ![](https://i.imgur.com/JhP5fOG.png =400x)
    ![](https://i.imgur.com/n14BaHz.png =300x)
    ![](https://i.imgur.com/IkkMzW6.jpg =300x)

The loop program structure - Repeat-until and While-Do
---

REPEAT
Statement 1;
Statement 2;
...
Statement N;
UNTIL <condition>

![](https://i.imgur.com/0cDXbvs.png =400x)
![](https://i.imgur.com/VCioi6L.png =200x)![](https://i.imgur.com/pEP0DjN.jpg =300x)

While <expression>
Statement 1;
Statement 2;
...
Statement N;

![](https://i.imgur.com/eYHwbJZ.png =400x)![](https://i.imgur.com/iadKZD4.png =200x)

Applications
+ Block-move program (區塊移動)

![](https://i.imgur.com/M0XrOkR.png =300x)![](https://i.imgur.com/zSgL9Hs.png =200x)

:::success
ADDR：address

mov si, BLK1ADDR
mov di, BLK2ADDR

BLK1到BLK2移動了n
DS * 16 + SI $\rightarrow$ DS * 16 + DI
:::
![](https://i.imgur.com/Gox1u89.png =200x)一個一個Byte移動

Subroutines and subroutine-handling instructions
---
1. Subroutine
    + Procedure
2. Call subroutine
    1. IP or CS and IP must be modified
    2. Call instruction
        + Go to subroutine
    3. Ret instruction
        + Return from subroutine 

![](https://i.imgur.com/0NscN1f.jpg =300x)![](https://i.imgur.com/RwGtA6k.png =300x)

1. Intrasegment call (in same segment)
    + modify IP only
2. Intersegment call (cross over different segment)
    + Modify IP and CS
3. RET
    + return from subroutine

Example

call 1234h ;Near-proc -> modify IP
call bx ;Near-proc -> modify IP
call [bx] ;Near-proc -> modify IP
call DWROD PTR [di] ;Far-proc -> modify IP and CS

![](https://i.imgur.com/6A0eViB.png =400x)

**Push and Pop instruction**
+ Compiler自己做的
+ 防止主程式call到副程式時，更改到Reg的value
    + 副程式會用到的Reg，主程式會先存原本的值，避免被更改到

![](https://i.imgur.com/IeMKhVX.png =200x) ![](https://i.imgur.com/wjaeZpa.png =400x)

---

;Subroutine: SQUARE
;Description: BX = squre of BL

SQUARE PORC NEAR
PUSH AX ;Save the register to be used
MOV AL, BL ;Placce the number in AL
IMUL BL ;Multiply with itself
MOV BX, AX ;Svae the result
POP AX ;Restore the register used
RET
SQUARE ENDP

+ Push ax
    1. sp - 1 <- ah
    2. sp - 2 <- al
    3. sp <- sp - 2
+ Pop ax
    1. al <- sp
    2. ah <- sp + 1
    3. sp <- sp + 2

---
針對Flag的Push and Pop
+ PUSHF
    + Push flag register on the top of the stack
+ POPF
    + Pop flag register from the top of the stack

![](https://i.imgur.com/bMGqjBx.png =500x)

LOOPS and LOOP-handling instructions
---
![](https://i.imgur.com/T5ANr3J.png)
:::success
CX：Count Register
LOOP $\rightarrow$ CX != 0
LOOPE/LOOPZ $\rightarrow$ CX != 0 && ZF = 1
LOOPNE/LOOPNE $\rightarrow$ CX != 0 && ZF = 0
:::

Example (比較Block-move prog)

![](https://i.imgur.com/MOBgWCc.png =400x)

> 這裡用的是LOOP，所以沒有DEC CX，因為LOOP會自己幫CX減一。

Example

![](https://i.imgur.com/1mNVn34.png =200x)


Strings and String-handling instructions
---

**Basic string insruction**
![](https://i.imgur.com/piAhLK5.png)

![](https://i.imgur.com/kFOaDyy.jpg =300x)

+ block move program using the move-string instruction
![](https://i.imgur.com/rGlxzNd.png =300x)
> 一二行就是得這樣寫

+ block scan operation using the SCASB instruction
![](https://i.imgur.com/I79FDX3.png =300x)
> 字元比對：在一個區塊找有沒有AL的值

+ Initialing a block of memory with a store string operation
![](https://i.imgur.com/te8JGmt.png =300x)
> String的每一個Byte都設為05

+ REP – repeat instruction
![](https://i.imgur.com/w6a9LaC.png =300x)

+ Initialing a block of memory by repeating the STOSB instruction 

MOV AX, 0
MOV DS, AX
MOV ES, AX
MOV AL, 05
MOV DI, 0A000H
MOV CX, 0FH
CLD
REP STOSB ;重複CX次

REP STOSB 
1. AGAIN STOSB
2. LOOP AGAIN


## **程式可以去參考emu86的sample code**

參考資料
+ Barry B. Bery, “The Intel Microprocessors,” 8th Edition, 2009, Prentice Hall.
+ Walter A. Triebel, Avtar Singh, “The 8088 and 8086 Microprocessors – Programming, Interfacing, Software, Hardware, and Applications,” 4th Edition, 2003, Prentice Hall.
+ 國立臺北大學資工系張玉山教授ppt