CODEWITHSUNDEEP
assembly
A.Shah
A.Shah

Reputation: 43

How to read from text file in assembly?

I'm new to assembly. I'm trying to read from a text file, this is my code:

_DATI SEGMENT PARA PUBLIC 'DATA' USE16

 Text_Buffer      dw ?
 filehandle       dw ?
 boolean          db 'boolean.txt',0
_DATI ENDS

_CODE SEGMENT PARA PUBLIC 'CODE' USE16
 ASSUME CS:_CODE, DS:_DATI, ES:_DATI

 ReadFile proc 
   mov ah, 3dh ;open the file
   mov al, 0 ;open for reading
   lea dx, file_name 
   int 21h 
   mov [filehandle], ax 

   mov ah, 3fh  
   lea dx, Text_Buffer
   mov cx, 1 ; Read 1 Byte
   mov bx, [filehandle] 
   int 21h

   mov bx, [filehandle]
   mov ah, 3eh ;close file
   int 21h
   ret
ReadFile endp
START:

call ReadFile

_end:

mov ax, 4c00h
int 21h

_CODE ENDS

END START

Isn't the ASCII from the file supposed to be saved in the allocated buffer (Text_Buffer)?

All I get now is some random byte saved, and for some reason, the program ends after I enter input. Can someone explain what's the problem and how to solve it? Thanks in advance.

Upvotes: 1

Views: 13391

Answers (1)

Ped7g
Ped7g

Reputation: 16586

I had to fix your code in question in these points:

 boolean        db 'boolean.txt',0

changed to (to have file_name symbol defined):

 file_name        db 'boolean.txt',0

And the "main" was not setting up ds:

...
START:
mov ax,SEG _DATI
mov ds,ax
call ReadFile
...

After this I compiled the code with TASM (tasm so_readf.asm tlink /x so_readf.obj, warning about missing stack reported and ignored, then td so_readf.exe to debug it), created file "BOOLEAN.TXT" containing two bytes 0x78 0x0A (letter "x" with newline edited with linux text editor), and used turbo debugger to verify it works (under dosbox emulator).

After the "read" service the ds:0000 memory view shows 78 "x" instead of 00 byte value, as expected.

I had problem to open the file because I created it after I already started the dosbox, so its existence was not known to the dosbox and it did report CF=1, ax=2 after "open file" service first time. After using "rescan" command of dosbox the next debugger run did work as expected. But this problem is related only to my testing environment (dosbox under linux), if you are using different emulator or live DOS system, you may not encounter this. Also if the "boolean.txt" is created as result of other code run inside the same emulator/dosbox, the file should be visible. You can do on DOS command line "type boolean.txt" to see it's current content and verify it's existence.

But otherwise as expected, what you did post is correct, as long as all the prerequisites are, i.e. you have your ds valid, and you are in correct working directory, and you didn't run out of file handles (do you close all the files you open before?), etc.. Also while debugging under debugger, the invalid cases with wrong filename (wrong ds), and dosbox not aware of the file, did end correctly with CF=1 after "open file", so I could terminate the code in debugger early (as there's no error handling code, so it would do incorrect things afterwards).

Try again in debugger to identify what exactly doesn't work, and all return values from every int 21h, for example the "read" ones returns number of bytes read in ax, should be 1 in case of success.

Upvotes: 1

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