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

OS執行兩個程式(程式A和程式B)
A執行1ms後，要換B時

$\to$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

   • ​​​​​​JMP 1234H
     ​​​​​​JMP BX
     ​​​​​​JMP [BX]
     

3. Condition jump instruction

4. Condition jump
   • JC Lable offset is specified
   • JB Lable
     
5. Branch program structure
   ​​​​IF xxx THEN
   ​​​​    then statement
   ​​​​ELSE
   ​​​​    else statement
   

   
   
   
   

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

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

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

Applications

• Block-move program (區塊移動)

ADDR：address

mov si, BLK1ADDR
mov di, BLK2ADDR

BLK1到BLK2移動了n
DS * 16 + SI

$\to$ DS * 16 + DI

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

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

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

---

;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

LOOPS and LOOP-handling instructions

CX：Count Register
LOOP

$\to$ CX != 0
LOOPE/LOOPZ

$\to$ CX != 0 && ZF = 1
LOOPNE/LOOPNE

$\to$ CX != 0 && ZF = 0

Example (比較Block-move prog)

這裡用的是LOOP，所以沒有DEC CX，因為LOOP會自己幫CX減一。

Example

Strings and String-handling instructions

Basic string insruction

• block move program using the move-string instruction
  
  

一二行就是得這樣寫

• block scan operation using the SCASB instruction
  
  

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
  
  

String的每一個Byte都設為05

• REP – repeat instruction
  

  

• 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