Instruction System - Program Control Instruction & Program Structure Design

1. Instruction System -Program Control Instruction & Program Structure Design 计算机学院 李征 Tel：13882153765 Email：lizheng@cs.scu.edu.cn OICQ: 1340915

2. Program Control Instruction • Program control instructions can change the program flow with or without conditions. • These instructions are used for branch or cycle program structure design.

3. About Status Flags • All program control instructions do not affect status flags. • Program transfer instruction with conditions may judge status flags.

4. 1) Program Transfer without Condition • Direct transfer in segment: • JMP label (Symbol DISP) • DISP is provided by instruction itself, and it is complement code (signed). • DISP can be 8-bit (near transfer) or 16-bit (far transfer).

5. Direct transfer in segment • JMP L1 • MOV AX，0 • … • L1: MOV AX，0FFFFH • … • After execution, what is (AX)?

6. Direct transfer in segment • What does this DISP mean? • DISP = EA of destination address – EA of JMP instruction • DISP means the byte distance from ‘JMP’ to destination address. • The calculation of DISP is performed in assembling procedure.

7. Direct transfer in segment • Destination EA > EA of JMP：DISP > 0 • Destination EA < EA of JMP：DISP < 0 • Function of JMP：IP <= （IP）+ DISP

8. Direct transfer in segment • （IP）+DISP • This addition is considered as signed operation. • DISPmay be positive or negative, but (IP) is always positive. • The 16-th bit (out of word range) of (IP)is considered as sign，it is always 0. • The 16-th bit of DISPis extended sign from the 15-th bit.

9. Direct transfer in segment • Example: (IP) =1100101001101011 • DISP = 11100110 • (IP) + DISP： • 0 11001010 01101011 • + 1 11111111 11100110 • 0 11001010 01010001 • The addition result is always positive. After this operation, (IP) should be considered as unsigned again.

10. Direct transfer in segment • If DISP is in range from -128 to 127, it is generated as 8-bit. (near transfer) • If DISP is out of range from -128 to 127, it is generated as 16-bit. (far transfer)

11. 1) Program Transfer without Condition • Indirect transfer in segment: • JMP 16-bit register or memory cell • Function: • IP <= (register or memory cell)

12. Indirect transfer in segment • Example: • JMP BX • Function: IP <=（BX） • JMP WORD PTR [SI] • Function: IP <= （DS：SI）

13. Difference between direct and indirect transfer in segment • Direct transfer in segment: • IP <= (IP)+DISP • Indirect transfer in segment: • IP <= (register or memory cell)

14. 1) Program Transfer without Condition • Direct transfer between segment: • JMP label • ;Label is not in the same segment with ‘JMP’. • JMP FAR PTR label

15. Direct transfer between segment • Function: • IP <= EA of lable • CS <= Segment base value of label

16. 1) Program Transfer without Condition • Indirect transfer between segment: • JMP DWORD PTR memory cell • Function: • IP <=（EA） • CS <=（EA+2）

17. Indirect transfer between segment • Example: • … • ADR1 DD L1 • … • JMP DWORD PTR ADR1 • Logic address of ‘L1’ is preserved in ‘ADR1’.

18. 2) Program Transfer Instruction with Condition • These instructions perform transfer only if given conditions are satisfied. • If given conditions are not satisfied, CPU execute the following instruction. • Two Possible Conditions: • Status Flags • Register Status

19. 2) Program Transfer Instruction with Condition • JXX label (8-bit DISP) • ‘JXX’ instructions always composed of two bytes. One byte is for operation code, and another is for DISP. • ‘JXX’ can only execute near transfer. • If far transfer is needed, ‘JMP’ must also be used.

20. Transfer with Single Flag • Correspondent flags: • CF、ZF、SF、OF、PF • Correspondent instructions: • JC, JNC • JZ, JNZ • JS, JNS • JO, JNO • JP, JNP

21. Transfer with Single Flag • Example: Design a program clip to compare byte DB1 and DB2. If they are equal, set (AL) to 0; or else, set (AL) to 0FFH. • MOV AL，DB1 • CMP AL，DB2 • JZ L1 ; Near transfer has a future problem. • MOV AL，0FFH • JMP L2 ; Can this ‘JMP’ be removed? • L1： MOV AL，00H • L2： MOV AH，4CH • INT 21H

22. AL<= DB1 AL=DB2？ AL<= 0 AL<= 0FFH Good Design for Branch Structure MOV AL,DB1 Y：L1 N：L2 CMP AL,DB2 JZ L1 JMP L2 L1: MOV AL,0 JMP L3 L3 L2: MOV AL,0FFH L3: MOV AH,4CH INT 21H

23. Transfer with Register Status • JCXZ label • This instruction perform transfer if (CX) is zero. • Note: (CX)=0 is the condition, not ZF=1.

24. Transfer with Register Status • MOV AX，34H • MOV CX，34H • CMP CX，AX • JCXZ L1 • … • L1: • … • Does this ‘JCXZ’ perform transfer?

25. Transfer with Register Status • General usage of ‘JCXZ’ • mov cx, count • jcxz next • Lop: …… • loop lop • next: ……

26. Unsigned Condition Transfer • These instructions can perform program transfer based on unsigned comparison. • Application Conditions: • 1) If one uses unsigned condition transfer instruction, ‘CMP’ must be used first. • 2) And in ‘CMP’, one must consider operation data as unsigned data.

27. Unsigned Condition Transfer • In ‘CMP’: • CMP DEST, SRC • In unsigned condition transfer instructions, DEST is considered as data A, and SRC is considered as data B. • CF and ZF are combined for unsigned conditions.

28. Unsigned Condition Transfer • JA: if A>B, transfer is performed. • JNBE: if A≤B is not satisfied, transfer is performed. • Flag Condition: CF=0 AND ZF=0 • CF=0: No borrow at highest bit, A≥B • ZF=0: A≠B • Combined Condition: A>B

29. Unsigned Condition Transfer • JAE：A≥B • JNB：not A<B • Flag Condition: CF=0 OR ZF=1 • CF=0: A≥B • ZF=1: A=B • Combined Condition: A≥B

30. Unsigned Condition Transfer • JB: A<B • JNAE: not A≥B • Flag Condition: CF=1 AND ZF=0 • CF=1: A<B • ZF=0: A≠B • Combined Condition: A<B

31. Unsigned Condition Transfer • JBE: A≤B • JNA: not A>B • Flag Condition: CF=1 OR ZF=1 • CF=1: A<B • ZF=1: A=B • Combined Condition: A≤B

32. Unsigned Condition Transfer • Example: Unsigned byte array ARY is preserved in data segment. Find the maximum in ARY, and preserve it in cell MAX. • Data Analysis: • 1) ARY • 2) Max

33. Unsigned Condition Transfer • Design: • 1) Put byte 0 of ARY in AL. • 2) Use a cycle structure, query every byte in ARY orderly. • 3) Use a branch in this cycle. If the current byte of ARY is greater than (AL), put the current byte in AL.

34. Unsigned Condition Transfer • SI is used to locate byte in ARY. • CX is used for counting cycle.

35. Flow Chart and Program Design 开始 MOV SI, OFFSET ARY MOV CX, 9 MOV AL, [SI] 初始化SI、 CX、AL LOP1: 进入循环，修改SI 使它指向下一个数据 LOP1: INC SI CMP AL, [SI] JB L1 JMP L3 ；L2=L3 Y: L1 N: L2=L3 (AL)<((SI)) AL<=((SI)) L1: MOV AL, [SI] JMP L3（可省略） L3

36. Flow Chart and Program Design N: LOP1 L3 L3: DEC CX CX<=(CX)-1 JNZ LOP1 ；条件转移实现循环 (CX)=0? MAX<=(AL) MOV MAX， AL MOV AH， 4CH INT 21H 结束

37. Signed Condition Transfer • These instructions can perform program transfer based on signed comparison. • Application Conditions: • 1) If one uses signed condition transfer instruction, ‘CMP’ must be used first. • 2) And in ‘CMP’, one must consider operation data as signed data.

38. Signed Condition Transfer • In ‘CMP’: • CMP DEST, SRC • In signed condition transfer instructions, DEST is considered as data A, and SRC is considered as data B. • SF, OF and ZF are combined for signed conditions.

39. Signed Condition Transfer • JG: if A>B, transfer is performed. • JNLE: if A≤B is not satisfied, program transfer is performed. • Flag Condition: SF=OF AND ZF=0 • SF=OF : Correct SF is 0, A≥B • ZF=0: A≠B • Combined Condition: A>B

40. Signed Condition Transfer • JGE: A≥B • JNL: not A<B • Flag Condition: SF=OF OR ZF=1 • SF=OF: A≥B • ZF=1: A=B • Combined Condition: A≥B

41. Signed Condition Transfer • JL: A<B • JNGE: not A≥B • Flag Condition: SF≠OF AND ZF=0 • SF≠OF: Correct SF is 1, A<B • ZF=0: A≠B • Combined Condition: A<B

42. Signed Condition Transfer • JLE: A≤B • JNG: not A>B • Flag Condition: SF≠OF OR ZF=1 • SF≠OF: A<B • ZF=1: A=B • Combined Condition: A≤B

43. Signed Condition Transfer • Consider the last unsigned operation example. • If the byte of ARY is signed, the ‘JB’ instruction in program must be replaced with ‘JL’.

44. LOOP Instruction and Cycle Structure • LOOP instruction is designed for cycle structure in program. • LOOP label (symbol DISP) • LOOP instruction composed of two bytes. One byte is for operation code, and another is for DISP.

45. LOOP Instruction and Cycle Structure • Execution of LOOP: • 1) CX<=（CX）-1 • 2) If (CX) ≠0, CPU perform program transfer. Or else, the following instruction will be executed. • Application condition of LOOP: • Initial count must be preserved in CX before cycle structure is executed.

46. LOOP Instruction and Cycle Structure • Remember the cycle structure in last unsigned operation example? • dec cx • jnz lop • These instructions can be simplified as: • loop lop

47. LOOP Instruction and Cycle Structure • LOOPZ / LOOPE label • 1）CX<=（CX）-1 • 2）If（CX）≠0 AND ZF=1, program transfer is performed. Or else, the following instruction will be executed.

48. LOOP Instruction and Cycle Structure • LOOPNE / LOOPNZ label • 1）CX<=（CX）-1 • 2）If（CX）≠0 AND ZF=0, program transfer is performed. Or else, the following instruction will be executed.

49. LOOP Instruction and Cycle Structure • Example: Search the first non-space character in a string. If searching is successful, index (1~n) of the character is preserved in cell index. Or else, 0FFH is sent to cell index.

50. LOOP Instruction and Cycle Structure • data segment • strg db ‘CHECK NO_SPACE’ • leng db $-strg • index db ? • data ends • stack1 segment stack • dw 20h dup(0) • stack1 ends