ENGR 4862 Microprocessors Lecture 19

1
ENGR 4862 MicroprocessorsLecture 19
2
DOS INT 21H 09H Output String

• INT 21H is provided by DOS after it is loaded
• Option 09H output a string of data to monitor
• Send a set of ASCII data for displaying
• AH 09
• DX offset addr. of the ASCII data to be
  displayed
• This function will display ASCII data string
  pointed by the DX until it encounters the dollar
  sign ()
• What if without a at the end of the string?

3

• data_ASC DB The earth is but one
• MOV AH, 09H
• MOV DX, OFFSET data_ASC
• INT 21H

4
DOS INT 21H 0AH Input String (I)

• AH 0AH
• DX offset address at which the data string is
  stored
• Referred to as a buffer area
• DOS requires
• A buffer area is defined in data segment (DS)
• First byte specifies the size of the buffer
• DOS will put the number of characters that come
  in through the keyboard in the second byte
• The data in the buffer starts at the third byte
• The return key will not be counted as an input
  character

5
DOS INT 21H 0AH Input String (II)

• What if input string is more than the buffer
  size?
• The first characters will be put in the buffer
• The last place in the buffer will be a ltCRgt (0DH)
• The remaining characters are ignored
• If no other characters entered, just ltCRgt
  pressed?
• The fist byte holds the buffer size
• The second byte 00H, since no character inputted
• The third byte ltCRgt (0DH)
• Remaining bytes unchanged
• ltCRgt ASCII 0DH ltLFgt ASCII 0AH

6
Define a Buffer String

• A more systematic way of defining buffer area
• data_buf LABEL BYTE word, dword
• max_size DB 10
• buf_count DB ?
• buf_data DB 10 DUP (?)
• MOV AH, 0AH
• MOV DX, OFFSET data_buf
• INT 21H
• MOV CL, buf_count
• MOV SI, OFFSET buf_data

7
DOS INT 21H Char IN/OUT

• Option 02H output a single character to monitor
• AH 02
• DX the character to be displayed
• Can be used to display since it cant be
  displayed with function 09H
• Option 01H input a single character with echo
• AH 01
• AL the input character in ASCII
• Wait until a character is input from the
  keyboard, then echo it to the monitor

8
DOS INT 21H Char IN/OUT

• Option 07H input a single character without echo
• AH 07
• AL the input character in ASCII
• Wait until a character is input from the
  keyboard, but without echo to the monitor

9
INT 16H Keyboard Programming

• Check a key press
• Function 01 (in AH) of INT 16H
• This function does not wait for the user to press
  a key
• Simply checks if there is a key press
• On return, ZF 0, there is a key press
• ZF 1, there is no key press
• Check which key is pressed
• Function 00 (in AH) of INT 16H
• Can be used immediately after function 01H
• On return, AL contains the ASCII code of the key

10

• display press y to exit
• wait MOV AH, 01H
• INT 16H
• JZ wait
• MOV AH, 00H
• INT 16H
• CMP AL, y
• JNE wait

11
String Instruction and Operation

• Capable of performing operations on a series of
  operands located in consecutive memory locations
• In 8086/88, SI and DI always point to the source
  and destination operands
• Default segment register
• ES must be initialized for the string operation
  to work

12
String Instructions

• Often you will need to perform operations on a
  series of data
• The 8086/8088 provides string instructions to
  operate on a series of operands located in
  consecutive memory locations
• Storing, loading, moving, scanning, and comparing
• In string instructions, operands can be byte or
  word, distinguished by letter B or W in mnemonic

13
String Instructions

• When designing the 8086/8088, the designers set
  aside certain registers for the string
  instructions
• SI (source index) points to the source operand
• DI (destination index) points to the destination
  operand
• Both are offsets the segments are different for
  each
• SI uses DS, DI uses ES

14
String Instructions

• These instructions have their own addressing
  modes
• The only 8086/8088 operations capable of memory
  to memory operations (moving and comparing)
• Wrong mov byte ptr DI, SI
• Right movsb

15
Direction Flag

• Increment or decrement the pointer SI or DI
• The string instructions automatically do this
• The programmer should specify the direction
• By setting the direction flag to high or low
• CLD ? DF0, auto-increment
• STD ? DF1, auto-decrement

16
Some Useful Prefix

• REP
• Allows a string instruction to perform repeatedly
  until CX register is decremented to 0, e.g., REP
  MOVSB
• REPZREPE
• Repeat as long as the source and destination
  operands are equal (ZF1) or until CX becomes
  zero
• REPNZREPNE
• Repeat as long as the source and destination
  operands are not equal (ZF0) or until CX becomes
  zero

17
String Instructions

• MOVSB, MOVSW
• STOSB, STOSW
• Store the byte/word in AL/AX register into the
  memory specified by ESDI and increment or
  decrement DI
• LODSB, LODSW
• Load the byte/word in memory specified by DSSI
  to AL/AX register and increment or decrement SI

18
String Instructions

• CMPSB, CMPSW
• Compare two arrays of data pointed by the SI and
  DI registers
• Can test the equality or inequality of data by
  use of the REPE or REPNE

19
Example1 Copy a String

• MOV SI, OFFSET SOURCE
• MOV DI, OFFSET DESTINATION
• CLD increment
• MOV CX, 8
• REP MOVSB
• MOV CX, 4
• REP MOVSW

20
Example2 Compare Two Strings

• MOV SI, OFFSET string1
• MOV DI, OFFSET string2
• CLD
• MOV CX, 16
• REPZ CMPSB
• JZ correct
• ltDisplay wrong messagegt
• correct
• ltDisplay correct messagegt