Reading a String:
Reading a string is accomplished by Function 0AH INT 21H. DOS function 0AH
will accept a string of text entered at the keyboard and copy that string
into a memory buffer.
DOS 0AH is invoked with DS:DX pointing to an input
buffer, whose size should be at least three bytes longer than the
largest input string anticipated.
Before invoking DOS function 0AH, you must set the
first byte of the buffer with the number of character spaces in the buffer.
After returning from DOS function 0AH, the second byte of the buffer will
contain a value giving the number of characters actually read form the
keyboard (see table).
where BL = Buffer Length and AL = Actual Length
Figure: Keyboard buffer structure
The following table summarizes string input and output functions:
|Function 0AH ||Read from Keyboard
|Entry ||AH = 0AH ; DX = address of keyboard input buffer|
First byte of buffer contains the size of the buffer
(up to 255).
|Exit ||Second byte of buffer contains the number of characters|
The string reading operation continues until the buffer is
full, or a carriage return (CR = 0DH) is hit.
Table: Functions 0AH of DOS interrupt
Here is an example that shows the use of function 0AH. The first part shows how to
declare the buffer that will hold the string to be input throught the keyboard,
in this case the string "hello".
buffer db 8
db 9 dup(?)
After this declaration, the buffer would look like:
If after the following code:
|Read from keyboard the string “hello”|
MOV AH, 0AH
MOV DX, offset buffer
the user enters the string "hello", the output will be: