Assembler Programming. Lecture 8

Transcription

1 Assembler Programming Lecture 8

2 Lecture 8 Decision directives. Loops. Text macros. Macro procedures. String directives. Macro functions.

3 Decision directives You can use the HLL structures for decision making:.if.elseif.else These directives generate conditional jumps..if.if condition1 statements [[.ELSEIF condition2 statements]] [[.ELSE statements]].endif

4 Decision directives example.if.if cx cx == == mov mov dx, dx, ELSE mov mov dx, dx, ENDIF.IF.IF cx cx == == cmp cmp cx, cx, 014h jne mov mov dx, dx, mov mov dx, dx, 30

5 Loops LOOP ends when CX = 0. LOOPE / LOOPZ checks CX and the zero flag, ends when CX = 0 or ZF = 0. LOOPNE / LOOPNZ checks CX and the zero flag, ends when CX = 0 or ZF = 1. All loop instructions automatically decrement CX. JCXZ jumps to a label if CX = 0. JECXZ jumps to a label if ECX = 0.

6 Loop example ; The The LOOP instruction: For For to to 0 do do task mov mov cx, cx, ; Set Set counter next:. ; Do Do the the task here.. loop next ; Do Do again ; Continue after loop ; The The LOOPNE instruction: While AX AX is is not not 'Y', do do task mov mov cx, cx, ; Set Set count too too high to to interfere wend:. ; But But don't do do more than times. ; Some statements that change AX AX. cmp cmp al, al, 'Y' 'Y' ; Is Is it it Y or or too too many times? loopne wend ; No? No? Repeat ; Yes? Continue

7 Loop example ; Decrements CX CX on on the the first pass even if if it s zero mov mov cx, cx, LoopCount ; Load loop counter next:. ; Iterate loop CX CX times.. loop next ; Do Do again ; If If CX CX is is zero skips the the loop mov mov cx, cx, LoopCount ; Load loop counter jcxz done ; Skip loop if if count is is 0 next:. ; Else iterate loop CX CX times.. loop next ; Do Do again done: ; Continue after loop

8 Loop generating directives.while....endw Execute statements while the condition is true..repeat....until Loop executes at least once and continues until the condition given after.until is true. Uses jumps..repeat....untilcxz Same as previous but generates loop instructions..break Ends loop unconditionally.continue Jump unconditionally past any remaining code to bottom of loop

9 Loop directives example.data buf1 BYTE "This is is a string",'$' buf2 BYTE DUP DUP (?) (?).CODE sub sub bx, bx, bx bx ; Zero out out bx bx.while (buf1[bx]!=!= '$') mov mov al, al, buf1[bx] ; Get Get a character mov mov buf2[bx], al al ; Move it it to to buffer 2 inc inc bx bx ; Count forward.endw

10 Loop directives example ASSUME bx:ptr inc inc ax ax.until ax==6 cmp cmp ax, ax, 006h mov mov ax, ax, [bx].field!=!= 6 cmp cmp [bx].field, 006h

11 Loop directives example.while 1 ; Loop forever mov mov ah, ah, 08h 08h ; Get Get key key without echo int int 21h 21h ; DOS DOS function.break.if.if al al == == ; If If ENTER, break out out of of the the loop.continue.if.if (al (al < '0') (al (al > '9') ; If If not not a digit, continue loop mov mov dl, dl, al al ; Save the the char for for processing mov mov ah, ah, 02h 02h ; Output the the character int int 21h 21h ; DOS DOS function.endw

12 Loop directives example.while mov mov ah, ah, 08h 08h int int 21h 21h.BREAK.IF.IF al al == == cmp cmp al, al, 00Dh je (al (al '0') (al (al '9') cmp cmp al, al, '0' '0' jb cmp cmp al, al, '9' '9' ja mov mov dl, dl, al al mov mov ah, ah, 02h 02h int int 21h 21h.ENDW

13 Loop conditions == Equal!= Not equal > Greater than >= Greater than or equal to < Less than <= Less than or equal to & Bit test! Logical NOT && Logical AND Logical OR.

14 ZERO? CARRY? OVERFLOW? SIGN? PARITY? Loop conditions.while (CARRY?) ; Loop when CF CF not not zero.. ; Statements here..endw

15 Macros Text macros. Macro procedures. Repeat blocks. Macro functions. Predefined macro functions and string directives.

16 Text Macros name TEXTEQU <text> name TEXTEQU macroid textmacro name TEXTEQU %constexpr

17 Text Macros - Examples msg msg TEXTEQU <Hello> ; Text assigned to to symbol string TEXTEQU msg msg ; Text macro assigned to to symbol msg msg TEXTEQU <Welcome> ; New New text assigned to to symbol value TEXTEQU %(3 %(3 + num); Text representation of of ; resolved expression assigned ; to to symbol pi pi TEXTEQU <3.1416> ; Floating point constant WPT WPT TEXTEQU <WORD PTR>; Sequence of of key key words arg1 TEXTEQU <[bp+4]> ; Bracketed expression

18 Macro procedures Macro procedure definition without the parameters: name MACRO statements Macro procedure with the parameters: name MACRO parameterlist statements

19 Macro procedure - example beep MACRO mov mov ah, ah, 2 ;; ;; Select DOS DOS Print Char function mov mov dl, dl, 7 ;; ;; Select ASCII 7 (bell) int int 21h 21h ;; ;; Call DOS DOS.IF.IF error beep beep.endif ; If If error flag is is true ; execute macro two two times

20 Macro procedure - example.if.if error cmp cmp error, 000h je beep mov mov ah, ah, 2 mov mov dl, dl, 7 int int 21h 21h beep mov mov ah, ah, 2 mov mov dl, dl, 7 int int 21h

21 Macro procedure with parameters writechar MACRO char mov mov ah, ah, 2 ;; ;; Select DOS DOS Print Char function mov mov dl, dl, char ;; ;; Select char int int 21h 21h ;; ;; Call DOS DOS writechar 'A' 'A' ; Writes A on on the the screen writechar 7 ; Causes computer to to beep writechar ; Causes error: ; expands to to the the line ; mov mov dl, dl,

22 Macro parameter attributes Required parameters. Parameter:REQ Default value. Parameter:=textvalue Variable number of parameters. Parameter:VARARG

23 Required parameters writechar MACRO char:req mov mov ah, ah, 2 ;; ;; Select DOS DOS Print Char function mov mov dl, dl, char ;; ;; Select char int int 21h 21h ;; ;; Call DOS DOS writechar ; Causes error: ; expands to to the the line ; mov mov dl, dl,

24 Default parameters writechar MACRO char:=<7> mov mov ah, ah, 2 ;; ;; Select DOS DOS Print Char function mov mov dl, dl, char ;; ;; Select char int int 21h 21h ;; ;; Call DOS DOS writechar 'A' 'A' ; Writes A on on the the screen writechar 7 ; Causes computer to to beep writechar ; Causes computer to to beep

25 Local symbols LOCAL directive. Any identifier may be declared as local. Local symbol will be visible only inside the macro. Any label within the macro must be local. LOCAL directive must appear immediately after the MACRO statement.

26 Local symbols power MACRO factor:req, exponent:req LOCAL again, gotzero ;; ;; Local symbols sub sub dx, dx, dx dx ;; ;; Clear top top mov mov ax, ax, 1 ;; ;; Multiply by by one one on on first loop mov mov cx, cx, exponent ;; ;; Load count jcxz gotzero ;; ;; Done if if zero exponent mov mov bx, bx, factor ;; ;; Load factor again: mul mul bx bx ;; ;; Multiply factor times exponent loop again ;; ;; Result in in AX AX gotzero:

27 Repeat blocks Unnamed macro defined with loop directive: REPEAT (REPT) WHILE FOR (IRP) FORC (IRPC)

28 REPEAT REPEAT constexpr Statements alpha LABEL BYTE ; Name the the data generated letter = A A ; Initialize counter REPEAT ;; ;; Repeat for for each letter BYTE letter ;; ;; Allocate ASCII code for for letter letter = letter + 1 ;; ;; Increment counter

29 WHILE WHILE expression Statements cubes cubes LABEL LABEL BYTE BYTE ;; ;; Name Name the the data data generated root root = 1 ;; ;; Initialize root root cube cube = root root * root root * root root ;; ;; Calculate first first cube cube WHILE WHILE cube cube LE LE ;; ;; Repeat until until result too too large large WORD WORD cube cube ;; ;; Allocate cube cube root root = root root + 1 ;; ;; Calculate next next root root and and cube cube cube cube = root root * root root * root root

30 FOR FOR parameter, <argumentlist> Statements series LABEL BYTE FOR FOR arg, <1,2,3,4,5,6,7,8,9,10> BYTE arg arg DUP DUP (arg)

31 FOR variable number of parameters show MACRO chr:vararg mov mov ah, ah, 02h 02h FOR FOR arg, <chr> mov mov dl, dl, arg arg int int 21h 21h show O, K, 13, 13, 10 10

32 FOR variable number of parameters show MACRO chr:vararg mov mov ah, ah, 02h 02h FOR FOR arg:=< >, >, <chr> mov mov dl, dl, arg arg int int 21h 21h show O,, K, 13, 13, 10 10

33 FORC FORC parameter, <text> Statements FORC arg, <ABCDEFGHIJKLMNOPQRSTUVWXYZ> BYTE &arg ;; ;; Allocate uppercase letter BYTE &arg + 20h 20h ;; ;; Allocate lowercase letter BYTE &arg - 40h 40h ;; ;; Allocate ordinal of of letter

34 String directives SUBSTR - Assigns part of string to a new symbol. INSTR - Searches for one string within another. SIZESTR - Determines the size of a string. CATSTR - Concatenates one or more strings to a single string.

35 String directives example ;assign "3 "3 + 7 = 10" 10" to to the the newstr num num = 7 newstr CATSTR <3 <3 + >, >, %num, < = >, >, %3 %3 + num num num num = 7 newstr <3 <3 + >, >, %num, < = >, %3 %3 + num num ) ; expansion operator a TEXTEQU <3 <3 + 4> 4> ; a = "3 "3 + 4" 4" b TEXTEQU %3 %3 + 4 ; b = "7" "7" c TEXTEQU %(3 %(3 + 4) 4) ; c = "7" "7"

36 String directives example regpushed TEXTEQU <> <> ;; ;; Initialize empty empty string SaveRegs MACRO MACRO regs:vararg LOCAL LOCAL reg reg FOR FOR reg, reg, <regs> ;; ;; Push Push each each register push push reg reg ;; ;; and and add add it it to to the the list list regpushed CATSTR <reg>, <,>, <,>, regpushed ;; ;; Strip Strip off off last last comma comma regpushed CATSTR <!<>, <!<>, regpushed ;; ;; Mark Mark start start of of list list with with < regpushed SUBSTR regpushed, 1, regpushed ) regpushed CATSTR regpushed, <!>> <!>> ;; ;; Mark Mark end end with with > RestoreRegs MACRO MACRO LOCAL LOCAL reg reg %FOR %FOR reg, reg, regpushed ;; ;; % expands regpushed pop pop reg reg ;; ;; Pop Pop each each register

37 Returning Values with Macro Functions Macro functions always return text. Macro functions always return a value thrugh the EXITM directive. Return value must be text, or result of another macro function. Numeric values must be converted to the text with expansion operator %.

38 Returning values example factorial MACRO num:req LOCAL i, i, factor factor = num num i = 1 WHILE factor GT GT 1 i = i * factor factor = factor 1 EXITM %i %i var var WORD factorial( 4 )