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Control Structure vs. Assembly Language

Control Structure vs. Assembly Language. NASM. If-then-else. If conditional then then_actions jump to endif else else_actions endif. Ex. If-Then-Else. If AL < 0xA then DL= AL + ’0’ Else DL = AL - 0xA +’A’ EndIf. MOV DL, AL CMP AL, 0xA JAE TenUp ADD DL,’0’ JMP EndIf

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Control Structure vs. Assembly Language

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  1. Control Structurevs. Assembly Language NASM

  2. If-then-else • If conditional then then_actions jump to endif • else else_actions • endif

  3. Ex. If-Then-Else • If AL < 0xA then DL= AL + ’0’ • Else DL = AL - 0xA +’A’ • EndIf • MOV DL, AL • CMP AL, 0xA • JAE TenUp • ADD DL,’0’ • JMP EndIf • TenUp: • ADD DL,’A’-0xA • EndIf:

  4. Repeat-Until • Loop at lease one time • Repeat Statements … • Until Condition is True

  5. Ex. Repeat-Until • BX <- 1 • AX <- 1 • CX <- 5 • Repeat BX <- BX + 2 AX <- BX * AX CX <- CX – 1 • Until CX=0 • = 10395 • MOV BX, 1 • MOV AX, 1 • MOV CX, 5 • RLOOP: • ADD BX, 2 • MUL BX • DEC CX • JNZ RLOOP • = 0x289B

  6. While loop • Check the condition before the loop • While Condition is true Statements …

  7. Ex. While-loop • BX <- 1 • AX <- 1 • CX <- 5 • While CX > 0 BX <- BX + 2 AX <- BX * AX CX <- CX – 1 • = 10395 • MOV BX, 1 • MOV AX, 1 • MOV CX, 5 • WLOOP: • CMP CX, 0 • JE EndLOOP • ADD BX, 2 • MUL BX • DEC CX • JMP WLOOP • EndLOOP:

  8. For loop • Loop in the specific range with automatic decrease index. • Loop instruction with CX register.

  9. Ex. For loop • “Loop” Instruction will automatically decrease CX and loop if CX is not zero • CX <- number of loop • StartLoop: Instructions … Loop StartLoop • MOV BX, 1 • MOV AX, 1 • MOV CX, 5 • FLOOP: • ADD BX, 2 • MUL BX • LOOP FLOOP • 5 Loop

  10. Frequently used routinesInt 21h DOS Service routines

  11. Int 21h’s format • Usage: Reg AH is used as function number you wish to perform. • Ex. To call function 1 int 21h to get a character from keyboard. • First MOV AH, 01 INT 21H

  12. Keyboard input with echo • Usage : • AH <- 1 • Return: • AL -> character from standard input • device waits for keyboard input from STDIN and echoes to STDOUT • Returns 0 for extended keystroke, then function must be called again to return scan code • if Ctrl-Break is detected, INT 23 is executed

  13. Display a character • Usage: • AH <-02 • DL <-character to output • Return: • Returns nothing • outputs character to STDOUT • backspace is treated as non-destructive cursor left • if Ctrl-Break is detected, INT 23 is executed

  14. Direct console input without echo • Usage: • AH <- 07 • Return: • AL -> character from STDIN • waits for keyboard input until keystroke is ready • character is not echoed to STDOUT • returns 0 for extended keystroke, then function must be called again to return scan code • ignores Ctrl-Break and Ctrl-PrtSc - see INT 21,1

  15. Console input without echo • Usage: • AH <- 08 • Return: • AL -> character from STDIN • returns 0 for extended keystroke, then function must be called again to return scan code • waits for character from STDIN and returns data in AL • if Ctrl-Break is detected, INT 23 is executed

  16. Display a string end with “$” • Usage: • AH <-09 • DS:DX <-pointer to string ending in "$“ • Returns: • Returns nothing • outputs character string to STDOUT up to "$“ • backspace is treated as non-destructive • if Ctrl-Break is detected, INT 23 is executed

  17. Read keyboard into a buffer • Usage • AH <- 0A • DS:DX <- pointer to input buffer of the format: | max | count | BUFFER (N bytes) | | `------ input buffer | `------------ number of characters returned (byte) `-------------- maximum number of characters to read (byte) • e.g. DB 0x5, 6 dup (?)

  18. Read keyboard into a buffer • Returns • since strings can be pre-loaded, it is recommended that the default string be terminated with a CR • N bytes of data are read from STDIN into buffer+2 • max buffer size is 255, minimum buffer size is 1 byte • chars up to and including a CR are placed into the buffer beginning at byte 2; • Byte 1 returns the number of chars placed into the buffer (extended codes take 2 characters) • DOS editing keys are active during this call • INT 23 is called if Ctrl-Break or Ctrl-C detected

  19. Clear Keyboard Buffer & Invoke Function • Usage: • AH <-0C • AL <-01, 06, 07, 08 or 0A (INT 21 input functions) • Return: • See return values from INT 21, AL where AL is 1, 6, 7, 8 or A • Main function is to clear the input buffer and call INT 21h with the specified function (in AL)

  20. Create file using handle • Usage: • AH <-3C • CX <-file attribute (see FILE ATTRIBUTES) • DS:DX <-pointer to ASCIIZ path name • Return: • CF = 0 if successful /=1 if error • AX = files handle if successful = error code if failure(see DOS ERROR CODES) • if file already exists, it is truncated to zero bytes on opening

  21. Open file using handle • Usage: • AH <-3D • AL <- open access mode 00 read only, 01 write only, 02 read/write • DS:DX <-pointer to an ASCIIZ file name • Return: • AX = file handle if CF not set = error code if CF set (see DOS ERROR CODES)

  22. Read File or Device Using Handle • Usage: • AH <-3F • BX <-file handle • CX <-number of bytes to read • DS:DX <-pointer to read buffer • Return: • AX = number of bytes read is CF not set = error code if CF set (see DOS ERROR CODES) • read specified number of bytes from file into buffer DS:DX • when AX is not equal to CX then a partial read occurred due to end of file • if AX is zero, no data was read, and EOF occurred before read

  23. Write To File or Device Using Handle • Usage: • AH <- 40h • BX <- file handle • CX <-number of bytes to write, a zero value truncates/extends the file to the current file position • DS:DX <-pointer to write buffer • Return: • AX = number of bytes written if CF not set = error code if CF set (see DOS ERROR CODES) • if AX is not equal to CX on return, a partial write occurred • this function can be used to truncate a file to the current file position by writing zero bytes

  24. Close File Using Handle • Usage: • AH <-3E • BX <-file handle to close • Return: • AX = error code if CF set (see DOS ERROR CODES) • if file is opened for update, file time and date stamp as well as file size are updated in the directory • handle is freed

  25. Delete File • Usage: • AH = 41h • DS:DX = pointer to an ASCIIZ filename • Return: • AX = error code if CF set (see DOS ERROR CODES) • marks first byte of file directory entry with E5 to indicate the file has been deleted. • The rest of the directory entry stays intact until reused. FAT pointers are returned to DOS • documented as not accepting wildcards in filename but actually does in several DOS versions

  26. Rename File • Usage: • AH <-56h • DS:DX <-pointer to old ASCIIZ path/filename • ES:DI = pointer to new ASCIIZ path/filename • Return: • AX = error code if CF set (see DOS ERROR CODES) • supports full pathnames and allows renaming files across directories and in DOS 3.x allows renaming subdirectories • does not support use of wildcards unless invoked from via INT 21,5D in which case error code 12h is returned • unpredictable result may occur if an opened file is renamed

  27. Move File Pointer Using Handle • Usage: • AH <- 42h • AL <- origin of move: 00 = beginning of file plus offset (SEEK_SET) 01 = current location plus offset (SEEK_CUR) 02 = end of file plus offset (SEEK_END) • BX <-file handle • CX <-high order word of number of bytes to move • DX <-low order word of number of bytes to move • Return: • AX = error code if CF set (see DOS ERROR CODES) • DX:AX = new pointer location if CF not set • seeks to specified location in file

  28. org 100h jmp start fh dw 0 buf db 0 start:mov bl,[0x80] cmp bl,0 jz noparam xor bh,bh add bl,0x81 ;set filename mov byte [bx],00; mov ah,0x3d ;open mov al,0x00 mov dx,0x82 ;param int 21h jc error ;if flag set mov [fh],ax;save file handle next: mov bx,[fh];get file handle mov ah,0x3f;read file mov cx,1 ;1 byte mov dx,buf int 21h jc error ;check error cmp ax,0 ;ax=0 :: eof jz done mov ah,2 ;display a char mov dl,[buf] int 21h jmp next Ex. Show files’ contents

  29. done: mov ah,0x3e;close file int 21h int 20h errorMsg db "There is an error$" error:mov ah,09 mov dx,errorMsg int 21h int 20h noParamMsg db "Please enter a filename$" noparam:mov ah,09 mov dx,noParamMsg int 21h int 20h Save -> show.asm nasm show.asm -oshow.com Usage: Show.com readme.txt Ex. Show files’ contents

  30. Q & A

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