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Chapter Five 80x86 Assembly Program Development （ 3 ）

Chapter Five 80x86 Assembly Program Development （ 3 ）. 5.6 汇编语言程序设计的基本方法. 通常 , 编制一个汇编语言源程序应按如下步骤进行 : ⑴ 明确任务 , 确定算法。 ⑵ 绘制流程图。 ⑶ 根据流程图编写汇编语言程序。 ⑷ 上机调试程序。 程序的基本结构有四种：顺序结构、分支结构、循环结构和子程序结构。. 5.6.1 顺序程序设计. 开始. 顺序结构也称线性结构，其特点是其中的语句或结构被连续执行。

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Chapter Five 80x86 Assembly Program Development （ 3 ）

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  1. Chapter Five80x86 Assembly Program Development（3） 2014年10月3日1

  2. 5.6 汇编语言程序设计的基本方法 通常,编制一个汇编语言源程序应按如下步骤进行: ⑴ 明确任务,确定算法。 ⑵ 绘制流程图。 ⑶ 根据流程图编写汇编语言程序。 ⑷ 上机调试程序。 程序的基本结构有四种：顺序结构、分支结构、循环结构和子程序结构。 2014年10月3日2

  3. 5.6.1 顺序程序设计 开始 顺序结构也称线性结构，其特点是其中的语句或结构被连续执行。 顺序程序是最简单的，也是最基本的一种程序结构。这种结构的程序从开始到结尾一直是顺序执行的，中途没有任何分支。从这种结构的流程图来看，除了有一个起始框，一个终止框外，就是若干执行框，没有判断框。 S1 S2 S3 结束 2014年10月3日3

  4. ［例1］求两个数的平均值。这两个数分别存放在X单元和Y单元中，而平均值放在Z单元中。源程序编制如下:［例1］求两个数的平均值。这两个数分别存放在X单元和Y单元中，而平均值放在Z单元中。源程序编制如下: DATA SEGMENT X DB 8CH Y DB 64H Z DB ? DATA ENDS STACK SEGMENT DW 20H DUP(0) TOP LABLE WORD STACK ENDS 2014年10月3日4

  5. CODE SEGMENT MAIN PROC FAR ASSUME CS:CODE ASSUME DS:DATA ASSUME SS:STACK START: PUSH DS MOV AX, 0 PUSH AX MOV AX, DATA MOV DS, AX MOV AX, STACK MOV SS, AX MOV SP, OFFSET TOP 2014年10月3日5

  6. MOV AL, X ADD AL, Y MOV AH, 0 ADC AH, AH SHR AX, 1 MOV Z, AL RET MAIN ENDP CODE ENDS END START 2014年10月3日6

  7. 5.6.2 分支程序设计 分支程序结构也称条件结构，通常是在两种或两个以上的不同的操作中选择其中的一个，如下图所示： 分支是通过条件转移指令来实现的。分支结构有一个共同点：运行方向总是向前的。 Y Y 条件 条件 条件 … N N S S1 S2 Sn S1 S2 2014年10月3日7

  8. [例2] 现有一符号函数: 1 当X>0时 Y= 0 当X=0时 -1 当X<0时 假定X为-25，且存放在VARX单元中，函数值Y存放在VARY单元，试编写程序根据X的值确定函数Y的值。 根据题意画出流程图如下： 2014年10月3日8

  9. 开始 AL←X 实现符号函数程序的流程图 AL≥0? Y N AL＝0 Y Y←-1 N Y←1 Y←0 结束 2014年10月3日9

  10. 编写程序如下: DSEG SEGMENT VARX DB -25 VARY DB ? DSEG ENDS CSEG SEGMENT ASSUME CS:CSEG,DS:DSEG START: MOV AX, DSEG MOV DS, AX MOV AL, VARX CMP AL, 0 JGE NEXT MOV AL, 0FFH JMP HALT 2014年10月3日10

  11. MOV DL, VARY MOV CL, 4 ROR DL, CL AND DL, 0FH CMP DL, 0AH JB NUM ADD DL, 7 NUM: ADD DL, 30H MOV AH, 2 INT 21H MOV DL, VARY AND DL, OFH CMP DL, 0AH JB NUB ADD DL, 7 NUB: ADD DL, 30H MOV AH, 2 INT 21H MOV DL, ‘H’ MOV AH, 2 INT 21H NEXT: JE ZARE MOV AL, 1 JMP HALT ZARE: MOV AL, 0 HALT: MOV VARY, AL MOV AH, 4CH INT 21H CODE ENDS END START 2014年10月3日11

  12. Flow of Control Principles of Microcomputer Computer Science Department, XIPT 2014年10月3日12

  13. Unconditional Jump • JMP [operator] destination • Intrasegment direct E9 disp_16 • Intrasegment direct short EB disp_8 • Intrasegment indirect FF mod 100 r/m ?? • Intersegment direct EA disp_16 seg_16 • Intersegment indirect FF mod 101 r/m ?? • operator : SHORT, NEAR PTR, or FAR PTR • NEAR PTR is the usual default 2014年10月3日13

  14. Specifying the Jump Target • Instruction Label • A symbolic name defined to be an address in the code segment of a program • A label may be attached to any point in the code of a program • a_Label: jmp a_Label • Labels definitions usually have a colon at the end signifying them as NEAR 2014年10月3日14

  15. Executing a Jump • Intrasegment jumps are caused by changing the IP register to a new value • Short jumps add a signed 8-bit displacement to IP • Near jumps add a signed 16-bit displacement to IP • Intersegment jumps change both the CS and IP registers • Far jumps simply assign new values to these registers 2014年10月3日15

  16. Sample Jump Encodings 1106:0100 EB2A JMP 012C • 012C-0102=002A 1106:0102 EBFC JMP 0100 • 0100-0104=FFFC 1106:0104 E97F00 JMP 0186 • 0186-0106=0080 (too far for short!) • 0186-0107=007F 1106:0107 E9F5FE JMP FFFF • FFFF-010A=FEF5 2014年10月3日16

  17. Conditional Jumps • Jxxx destination • There are 30 some variations that interrupt sequential flow based on various flag settings • JNZ - Jump if zero flag is clear (0) meaning the result of a previous operation was non-zero • JC - Jump if a previous operation caused the carry flag to be set (1) 2014年10月3日17

  18. Range of Conditional Jumps • All conditional jumps are SHORT • range is -128 to +127 bytes • 80386+ allow larger distances • Combine a conditional and unconditional jump to overcome this range limitation jz too_far ;ugh jnz is_close ;use code at jmp near ptr too_far ;right! is_close: 2014年10月3日18

  19. Using Conditional Jumps • Conditional jumps typically follow an instruction that alters the flag bits • CMP destination, source • Computes (destination-source) and sets flag bits • result is not stored • flags allow us to decide <, <=, >, >=, ==, <>, etc • we can also interpret the results meaningfully for signed or unsigned data 2014年10月3日19

  20. unsigned int n; if (n>7) do_it(); If n is a signed int, use jng (not greater) unsigned: above, below signed less, greater ;if (n>7) mov ax,n cmp ax,7 jna skip_it ;then-part call do_it ;end if skip_it: Implementing an IF-THEN 2014年10月3日20

  21. char n; if (n=='7') do_it(); else do_that(); Document the control structures and keep the parts in the usual order ;if (n=='7') cmp n,'7' jne else_ ;then-part call do_it jmp short endif else_: call do_that endif: Implementing an IF-ELSE 2014年10月3日21

  22. int n; while (n>0) n-=2; This loop could be optimized by keeping n in a register and storing to memory only at end of loop ;while (n>0) while_: cmp n,0 jle end_while ;loop-body sub n,2 jmp while_ end_while: Implementing a WHILE 2014年10月3日22

  23. char n; int w,x; if (n>='A' && w==x) whatever(); This example uses short-circuit evaluation if the first condition is false it immediately skips past the then-part ;if(n>='A'&&w==x) cmp n,'A' jl no_go mov ax,w cmp ax,x jne no_go ;then-part call whatever no_go: Compound Conditions 2014年10月3日23

  24. char n,k; unsigned int w; if (n<>k || w<=10) whatever(); This example uses short-circuit evaluation if the first condition is true it immediately skips to the then-part ;if(n<>k||w<=10) mov ah,n cmp ah,k jne then_ cmp w,10 ja end_if then_: call whatever end_if: Compound Conditions - OR 2014年10月3日24

  25. LOOP destination decrements CX but does not change any flags if CX is not zero after the decrement, control is transferred to the destination label This is a SHORT jump only for (x=9;x>0;x--) n+=x; ;for(x=9;x>0;x--) mov cx,9 top_loop: add n,cx ;n=n+x loop top_loop LOOP 2014年10月3日25

  26. Directly compares CX to 0 and jumps to the destination if equal This instruction does not affect the flags It is commonly used to bypass the first iteration of a loop if the count is already 0 ;while(x>0)do_it(); mov cx,x jcxz skip_it top_loop: call do_it loop top_loop skip_it: JCXZ destination 2014年10月3日26

  27. Enhancement of the LOOP instruction The state of the ZERO Flag may also cause loop termination Loop while ZF/equal && CX!=0 Loop while (NZ/ not equal) && CX!=0 Remember that LOOP decrements CX, but this does not affect the flags! LOOPZ == LOOPE LOOPNZ==LOOPNE Some action inside the loop should affect the zero flag (cmp ?) LOOPZ/E and LOOPNZ/E 2014年10月3日27

  28. This program accepts at most 9 characters from the keyboard When the 9th character is pressed (or the enter key is used) the number of keypresses is displayed mov ah,1 mov cx,9 next_char: int 21h cmp al,13 loopne next_char ;determine count mov ax, 0239h sub al,cl mov dl,al int 21h LOOPZ Example 2014年10月3日28

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