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    <blockquote> <p>01370FFF add byte ptr [ecx+6],bh </p> </blockquote> <p>At least I can explain away one mystery. Note the address, 0x1370fff. The CODE segment never starts at an address like that, segments begin at an address that's a multiple of 0x1000. Which makes the last 3 hex digits of the start address always 0. The debugger got confuzzled and started disassembling the code at the wrong address, off by one. The actual start address is 0x1371000. The disassembly starts off poorly because there's a 0 at 0x1370fff. That's a multi-byte ADD instruction. So it displays garbage for a while until it catches up with real machine code instructions by accident.</p> <p>You need to help it along and give it a command to start disassembling at the proper address. In VS that's the Address box, type "0x1371000".</p> <p>Another notable quirk is the strange value of the start address. A process normally starts at address 0x400000. You have a feature called ASLR turned on, Address Space Layout Randomization. It is an anti-virus feature that makes programs start at an unpredictable start address. Nice feature but it doesn't exactly help debugging programs. It isn't clear how you built this code but you need the /DYNAMICBASE:NO linker option to turn it off.</p> <p>Another important quirk of debuggers you need to keep in mind here is the way they set breakpoints. They do so by <em>patching</em> the code, replacing the start byte of an instruction with an <code>int 3</code> instruction. When the breakpoint hits, it quickly replaces the byte with the original machine code instruction byte. So you never see this. This goes wrong if you pick the wrong address to set the breakpoint, like in the middle of a multi-byte instruction. It now no longer breaks the code, the altered byte messes up the original instruction. You can easily fall into this trap when you started with a bad disassembly.</p> <p>Well, do this the Right Way. Start debugging with the debugger's STEP command instead.</p>
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    1. POVisual Studio only breaks on second line of assembly?
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    1. COInteresting about the starting address of the program. I've tried your suggestions and updated my question. Some progress, but VS will still not hit a breakpoint on the first line.
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    2. COI have still not been able to break on the first line of the program, but your answer gives the best description of what might be causing the problem. The bounty is yours, and I'll accept unless a definitive answer comes along.
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