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    <p>Actually you don't <strong>put</strong> the character in <code>ds:dx</code>, but rather you use <code>ds:dx</code> to refer to the memory address where the character or whatever data is stored, in Intel syntax it would be <code>[ds:dx]</code> (but <code>dx</code> cannot be used for memory addressing).</p> <p>So, what you need to do: store the value read from keyboard with DOS interrupt <code>21h</code> / <code>ah = 01h</code>, returned in <code>al</code> to some memory address and then refer to that memory address with <code>ds:dx</code>.</p> <p>In your code you don't store the file handle at all, and load 0 from the variable <code>file_handle</code> before write to file call, as that's how you have it initialized. And then you try to read file handle from <code>ds:0</code> (as <code>si</code> equals 0), and that doesn't make sense at all. All you need to do with the file handle is to store it (return value in <code>ax</code> after file creation/trunction) and always load it to relevant register in subsequent <code>int 21h</code> referring to that same file (write to file, read from file, close file, etc.).</p> <p>So, with the fixes below it should work (didn't test). I also organized the function call parameters to the order used by Ralph Brown's interrupt list to make it easier to understand.</p> <pre><code>.section data file db 'file.txt',0 character_read db 0 </code></pre> <p>...</p> <pre><code>% create file: mov ah,3Ch % create or truncate a file mov dx,file % ds:dx points to ASCIIZ filename xor cx,cx % file attributes. int 21h mov [file_handle], ax % file handle must be stored in memory or in a register! INPUTSTART: mov ah,1 % read character from STDIN, with echo. int 21h mov [character_read], al % store the ASCII code to memory. % unless you want to loop eternally, you can check the ASCII code and exit. cmp al,20h % space. je EXIT mov ah,40h % write to file or device mov bx,[file_handle] % file handle is just a number. mov cx,1 % number of bytes to write. mov dx,character_read int 21h jmp INPUTSTART EXIT: % here you can add some clean-up code, if needed. mov ah,3Eh % close file. mov bx,[file_handle] % here you need the file handle again. int 21h mov ah,4Ch int 21h </code></pre>
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    1. POHow to read a character from the keyboard and save it to a file in DOS assembly?
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    1. COThanks, after some modifications code works. I used Alexey's code too. I use NASM.
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