Assembly Language Lab # 12 Part A: Strings and Arrays Part B: Drawing Graphics Using INT 10h

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1 Faculty of Engineering Computer Engineering Department Islamic University of Gaza 2011 Assembly Language Lab # 12 Part A: Strings and Arrays Part B: Drawing Graphics Using INT 10h Eng. Doaa Abu Jabal

2 Objective: Assembly Language Lab # 12 Part A: Strings and Arrays To learn more about strings and arrays in assembly language. Drawing Graphics Using INT 10h. String Primitive Instructions: MOVSB, MOVSW, and MOVSD: MOVSD instructions copy data from the memory location pointed to by ESI to the memory location pointed to by EDI. ESI and EDI are automatically incremented or decremented: MOVSB increments/decrements by 1 MOVSW increments/decrements by 2 MOVSD increments/decrements by 4 Direction Flag: The Direction flag controls the incrementing or decrementing of ESI and EDI. DF = clear (0): increment ESI and EDI DF = set (1): decrement ESI and EDI Using a Repeat Prefix: REP (a repeat prefix) can be inserted just before MOVSB, MOVSW, or MOVSD. ECX controls the number of repetitions CMPSB, CMPSW, and CMPSD: The CMPSB, CMPSW, and CMPSD instructions each compare a memory operand pointed to by ESI to a memory operand pointed to by EDI. CMPSB compares bytes CMPSW compares words CMPSD compares doublewords 2

3 Repeat prefix often used REPE (REPZ) REPNE (REPNZ) SCASB, SCASW, and SCASD: The SCASB, SCASW, and SCASD instructions compare a value in AL/AX/EAX to a byte, word, or doubleword, respectively, addressed by EDI. Useful types of searches: Search for a specific element in a long string or array. Search for the first element that does not match a given value. STOSB, STOSW, and STOSD: The STOSB, STOSW, and STOSD instructions store the contents of AL/AX/EAX, respectively, in memory at the offset pointed to by EDI. LODSB, LODSW, and LODSD: LODSB, LODSW, and LODSD load a byte or word from memory at ESI into AL/AX/EAX, respectively. 3

4 Lab work: Example 1: Write a procedure named Str_substring that take some characters from a string and moves them to other String.There are two input parameters: a start address and end address of characters to be moved. 4

5 Example 2: Write a procedure named Str-Remove that removes n characters from a string. Pass a pointer to the position in the string where the characters are to be removed. Pass an integer specifying the number of characters to remove. 5

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7 Part B: Drawing Graphics Using INT 10h It's easy to draw graphics points and lines using INT 10h Function 0Ch. Before drawing pixels, you have to put the video adapter into one of the standard graphics modes. Each mode can be set using INT 10h function 0 (set video mode). INT 10h Pixel-Related Functions: Slow performance, Easy to program 0Ch: Write graphics pixel 0Dh: Read graphics pixel 7

8 Lab work: Example 1: This program draws a straight line, using INT 10h function calls..model small.386.data savemode BYTE? currentx WORD 100 currenty WORD 100 color BYTE 1 ; default color.code main : mov ax,@data mov ds,ax 8 ; Save the current video mode mov ah,0fh ; save the current video mode ; column number (X-coordinate) ; row number (Y-coordinate)

9 mov savemode,al ; Switch to a graphics mode mov ah,0 ; set video mode mov al,13h ; Draw a straight line LineLength = 100 mov dx,currenty mov cx,linelength ; loop counter L1: push cx mov ah,0ch ; write pixel mov al,color ; pixel color mov bh,0 ; video page 0 mov cx,currentx inc currentx ;inc color ; try this for multi-color modes pop cx Loop L1 ; Wait for a keystroke mov ah,0 int 16h ; Restore the starting video mode mov ah,0 ; set video mode mov al,savemode ; saved video mode mov ah,4ch int 21h END main 9

10 Memory-Mapped Graphics: Binary values are written to video RAM video adapter must use standard address It has Very fast performance no BIOS or DOS routines to get in the way Mode 13h: 320 X 200, 256 Colors Mode 13h graphics (320 X 200, 256 colors) Fairly easy to program read and write video adapter via IN and OUT instructions pixel-mapping scheme (1 byte per pixel) Lab work: Example 1: This program Draw 10 Pixels on the Screen.model small.386.data savemode BYTE?; saved video mode xval WORD? ; x-coordinate yval WORD? ; y-coordinate.code Main: 10

11 mov mov ds,ax call SetVideoMode call Draw_Some_Pixels call RestoreVideoMode mov ah,4ch int 21h SetVideoMode PROC ; This procedure saves the current video mode, switches to ; a new mode, and points ES to the video segment. ; mov ah,0fh ; get current video mode mov savemode,al ; save it mov ah,0 mov al,13h push 0A000h pop es ; set new video mode ; to mode 13h ; video segment address ; ES = A000h (video segment). ret SetVideoMode ENDP ; RestoreVideoMode PROC ; This procedure waits for a key to be pressed and ; restores the video mode to its original value. ; mov ah,10h ; wait for keystroke int 16h mov ah,0 ; reset video mode mov al,savemode ; to saved mode ret RestoreVideoMode ENDP ;

12 Draw_Some_Pixels PROC ; Calculate the video buffer offset of the first pixel. ; Specific to mode 13h, which is 320 X 200. mov xval,160 ; middle of screen mov yval,100 mov ax,320 ; 320 for video mode 13h mul yval ; y-coordinate add ax,xval ; x-coordinate ; Place the color index into the video buffer. mov cx,10 ; draw 10 pixels mov di,ax ; AX contains buffer offset ; Draw the 10 pixels now. DP1: mov BYTE PTR es:[di],1 ; store color index add di,5 ; move 5 pixels to the right Loop DP1 ret Draw_Some_Pixels ENDP END main 12