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POData segment issue with x86 Assembler (nasm) for print function
 

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  1. POData segment issue with x86 Assembler (nasm) for print function
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    <p>I am currently playing around with x86 assembler since I wanted to refresh my skills for low level programming :-). For testing purposes I tried to write a function that just prints out a given string. The printing function itself works fine. In a further step I wanted to load a second assembler program from disk jump to it and just print out a text. Loading from disk at jump to the address works fine.</p> <p>Here is the given scenario:</p> <pre><code>[... loading from disk etc ... program is loaded to 0x7e0:0001] jmp 0x7e0:0001 [... context of other asm ...] jmp Start ;data fields msg db "Hello World!",0 Start: xor si, si ; clear SI register mov si, msg ; load message to SI register call Print cli hlt ; halt the system Print: .PrintLoop: lodsb ; load byte from SI register or al, al ; check if 0 byte jz short .PrintDone ; if so - stop mov ah, 0Ah ; function - print text to cursor int 0x10 ; BIOS interrupt jmp .PrintLoop ; continue with next char .PrintDone: ret </code></pre> <p>All of this program is working fine. The only problem that I face is, that no text is printed. During debugging I saw that the print function immediately jumps to the .PrintDone label since there seems to be no data in SI and therefore lodsb loads nothing to al (besides null byte).</p> <p>I was thinking about the fact, that there might be something wrong with the data segment.</p> <p>Thus, I added the following line at the beginning of the Start-Routine:</p> <pre><code>xor ax, ax ; clear ax register mov ax, cs mov ds, ax ; set data segment pointer </code></pre> <p>But this changed nothing regarding the programs behaviour. Nothing is printed.</p> <p>Inspecting the CPU registers when execution reaches halt instruction, gives the following:</p> <pre><code>EAX=00000a00 EBX=00000000 ECX=00000002 EDX=00000000 ESI=00000026 EDI=00000000 EBP=00000000 ESP=0000ffff EIP=00000036 EFL=00000046 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=1 ES =07e0 00007e00 0000ffff 00009300 CS =07e0 00007e00 0000ffff 00009b00 SS =9000 00090000 0000ffff 00009300 DS =07e0 00007e00 0000ffff 00009300 </code></pre> <p>Do you have any clue what's going on here?</p> <p><strong>[EDIT - PROBLEM RESOLVED]</strong></p> <p>Replacing:</p> <pre><code>mov ah, 0Ah -&gt; mov ah, 0xE </code></pre> <p>fixes the problem!</p> <p>Best Sebastian</p>
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    1. CODoes it still go straight to the `.PrintDone` with your addition to set `ds`, or does it just not print anything? (I think you want `ah` just before the interrupt, not `ax`.)
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    2. COHey, classic typo on ax - fixed to ah again. Now the program is printing something but just one weird char. When inspecting the registers when the execution reached system halt they have the following entries: ES =07e0 00007e00 0000ffff 00009300 CS =07e0 00007e00 0000ffff 00009b00 SS =9000 00090000 0000ffff 00009300 DS =07e0 00007e00 0000ffff 00009300
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    3. COI don't think that interrupt moves the cursor, which means if more than one character is printed, you will only see the last. Also, check to make sure `si` is pointing to the right spot at the end. If I counted right, there must be 24 bytes of data at the "data fields" comment for it to be right. The code looks fine.
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