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Если почитать описание функции буфферизованного ввода символов с эхом (Int 21 Function 0A) то ответ очевиден:

7.1.11. Int 21H Function 0AH
Description:
Buffered Keyboard Input
Reads a string of bytes from the standard input device, up
to and including an ASCII carriage return (0DH), and places
them in a user-designated buffer. The characters are echoed to
the standard output device. Input may be redirected (if input
has been redirected, there is no way to detect EOF).
Input Output
------------------------------ ------
AH=0AH Nothing
DS : DX=segment : offset of buffer
- The buffer used by this function has the following format:
Byte
Contents
0
maximum number of characters to read, set by
program
1
number of characters actually read (excluding
carriage return), set by MS-DOS
2+
string read from keyboard or standard input,
terminated by a carriage return (0DH)