Making DOS Calls in Visual Studio .NET

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Making DOS Calls
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What EATASM really does, as I mentioned previously, is call DOS and instruct DOS to display a string located at a particular place in memory The string itself doesn't go anywhere; EATASM tells DOS where the string is located, and then DOS reaches up into your data section and does what it must with the string data Calling DOS is done with something called a software interrupt I explain these in detail later in this
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chapter But if you look at the code you can get a sense for what's going on: mov mov int dx, eatmsg ah,9 21H ; Mem data ref without [] loads the ADDRESS! ; Function 9 displays text to standard output ; INT 21H makes the call into DOS
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Here, the first line loads the address of the string into register DX The second line simply loads the constant value 9 into register AH The third line makes the interrupt call, to interrupt 21H The DOS call has certain requirements that must be set up before the call is made It must know what particular call you want to make, and each call has a number This number must be placed in AH and, in this case, is call 09H (Display String) For this particular DOS call, DOS expects the address of the string to be displayed to be in register DX If you satisfy those two conditions, you can make the DOS software interrupt call INT 21H-and there's your string on the screen!
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Exiting the Program and Setting ERRORLEVEL
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Finally, the job is done, Joe's has been properly advertised, and it's time to let DOS have the machine back Another DOS service, 4CH (Terminate Process), handles the mechanics of courteously disentangling the machine from EATASM's clutches Terminate Process doesn't need the address of anything, but it will take whatever value it finds in the AL register and place it in the ERRORLEVEL DOS variable DOS batch programs can test the value of ERRORLEVEL and branch on it EATASM doesn't do anything worth testing in a batch program, but if ERRORLEVEL will be set anyway, it's a good idea to provide some reliable and harmless value for ERRORLEVEL to take This is why 0 is loaded into AL prior to ending it all by the final INT 21 instruction If you were to test ERRORLEVEL after running EATEXE, you would find it set to 0 in every case That's really all there is to EATASM Now let's see what it takes to run it, and then let's look more closely at its innards in memory
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Assembling and Running EATASM
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To assemble and run EATASM, we can load it into NASM-IDE, and then let NASM-IDE invoke NASM That's how we're going to do it here You should understand, however, that NASM-IDE is simply a "place to stand" NASM is what actually does the work of assembling the file Here's the sequence: 1 Run NASM-IDE 2 Select the Open item from the File menu (We would say this, in shorthand form, "Select File|Open") 3 Highlight the name of file EATASM, and click on the OK button EATASM will load and be displayed in a window If EATASM isn't in the same directory as NASM-IDE, you may have to navigate to the directory where EATASM lives by clicking on directory names in the dialog box 4 Select Assemble|Assemble The Error window will appear in the lower half of the display, even if only to tell you, "No errors occurred" 5 Assuming no errors occurred, select Assemble|Run The display will clear, and EAT's message will be displayed in the upper-left corner of the display Beneath it you'll see DOS's message, "Press any key to continue " Press any key, and the display will return to NASM-IDE, showing EATASM
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