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					<h1 class="entry-title">Hang with early DOS boot sector</h1>

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						<span class="meta-prep meta-prep-author">Posted on</span> <a href="http://www.os2museum.com/wp/?p=685" title="9:19 pm" rel="bookmark"><span class="entry-date">September 3, 2011</span></a> <span class="meta-sep">by</span> <span class="author vcard"><a class="url fn n" href="http://www.os2museum.com/wp/?author=4" title="View all posts by Michal Necasek">Michal Necasek</a></span>					</div><!-- .entry-meta -->

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						<p>While installing various versions of DOS for the DOS history 
series of articles, I was faced with a mysterious problem: Some versions
 of DOS would hang right away when booting from fixed disk, but not from
 floppy. I already knew that DOS 4.x is very sensitive to BIOS stack 
usage; if a BIOS needs more than 100 bytes or so of stack to process a 
disk read request, it will fail to boot DOS 4.x from fixed disk, even 
though the same DOS 4.x can access the same disk just fine when booted 
from floppy.</p>
<p>However, the hangs I was observing were happening with DOS 2.x and 
3.x, and those do not have such tight stack usage requirements. I 
quickly realized that the problem is caused by a bug in the DOS boot 
sector: the boot sector code tries to optimize the loading of IBMBIO.COM
 and attempts to read a whole disk track at a time. That sounds like a 
good idea, but it’s not.<span id="more-685"></span></p>
<p>The boot sector is loaded at address 0:7c00h, or just under 32KB. The
 BIOS component of DOS (IBMBIO.COM) is loaded at address 70h:0 (in other
 words, 0:700h). The boot sector also sets up the top of stack at 
0:7c00h, just below the boot sector code. There is therefore slightly 
under 30KB of room for IBMBIO.COM, which in the 2.x and 3.x versions of 
DOS is well under 20KB size (just 4.5KB in DOS 2.0 in fact). In theory 
there should be no problem.</p>
<p>Unfortunately, the author of the DOS 2.0 boot sector was too eager to
 optimize the loading and not thinking far enough ahead. Loading a track
 at a time sounds clever, except when it’s not… On a modern disk, there 
are usually 63 sectors per track, or 31.5KB of data. If the boot sector 
reads that whole track, it <em>will</em> destroy the stack and overwrite
 itself. But back when the boot sector code was written, almost all 
fixed disks had 17 sectors per track, which meant that no amount of 
testing would have caught the bug. Floppy booting is no problem either, 
since even 1.44MB diskettes only have 18 sectors per track.</p>
<p>Now, the above explains why an old version of DOS might hang when 
booted from a modern fixed disk. But it does not explain why some of my 
DOS 2.x and 3.x installs worked just fine, including a case with one 
install of DOS 2.0 booting fine and another persistently hanging.</p>
<p>After some head scratching, it turned out that the DOS partition size
 is key. Depending on how big the partition is, the FATs will have 
varying size, and IBMBIO.COM will start on a different sector relative 
to the start of the track. If IBMBIO.COM starts on, say, sector 30, the 
boot sector will load 33 sectors and DOS will boot fine. If IBMBIO.COM 
starts on sector 1, DOS will hang. Similarly if IBMBIO.COM starts on 
sector 60, the first read of four sectors will be fine, but the next 
read of 63 sectors will crash the system.</p>
<p>For reference, a 30-cylinder partition on a typical disk with 63 
sectors per track and 16 heads tends to cause problems. A 60-cylinder 
partition (close to the 32MB partition size limit) tends to work, but 
the exact behavior depends on the DOS version.</p>
<p>The bug most likely affects all 2.x and 3.x versions of DOS prior to 
3.3. Version 3.3 relaxed the restriction that IBMBIO.COM/IO.SYS must be 
contiguous (which enabled the track-at-once loading); this is documented
 by Microsoft in <a title="Microsoft KB 66530" href="http://support.microsoft.com/kb/66530" target="_blank">KB 66530</a>. The removed restriction required changes to the boot sector and IBMBIO.COM/IO.SYS had to be loaded cluster by cluster.</p>
<p>In DOS 4.0, IBMBIO.COM/IO.SYS grew beyond 30KB and could no longer be
 loaded in one go at all, even if it were contiguous (that would in 
essence always cause the hang problem described above). There just 
wasn’t enough space between 70h:0 and 0:7c00h anymore. The BIOS 
component was therefore loaded in two stages, which avoided the problem.
 However, the staged loading caused the previously mentioned issue with 
tight stack space—but that’s a different story.</p>
<p>This bug does not appear to be well known. The most likely reason is 
that extremely few people attempt installing DOS 3.2 or older on a 
computer with a multi-gigabyte disk. DOS 2.x is simply not useful for 
running any “modern” DOS software, and DOS 3.0/3.1/3.2 has a serious 
drawback in that it does not support 1.44MB floppy drives.</p>
											</div><!-- .entry-content -->


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						This entry was posted in <a href="http://www.os2museum.com/wp/?cat=5" title="View all posts in DOS" rel="category">DOS</a>. Bookmark the <a href="http://www.os2museum.com/wp/?p=685" title="Permalink to Hang with early DOS boot sector" rel="bookmark">permalink</a>.											</div><!-- .entry-utility -->
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			<h3 id="comments-title">15 Responses to <em>Hang with early DOS boot sector</em></h3>


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					<li class="comment even thread-even depth-1" id="li-comment-2617">
		<div id="comment-2617">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/704a1e4bb9b4982564141ff4c15b7eda" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn"><a href="http://yuhongbao.blogspot.com/" rel="external nofollow" class="url">Yuhong Bao</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-2617">
				November 6, 2011 at 9:12 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Not to mention the 32MB partition limit turns the 58 sectors per track limit into a non-issue anyway.</p>
</div>

			<div class="reply">
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	</li>
	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-2619">
		<div id="comment-2619">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/ad516503a11cd5ca435acc9bb6523536" class="avatar avatar-40 photo avatar-default" height="40" width="40">				<cite class="fn"><a href="http://www.os2museum.com/" rel="external nofollow" class="url">michaln</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-2619">
				November 6, 2011 at 12:19 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Sorry, that makes no sense. Can you please elaborate? What 58 SPT limit are you talking about?</p>
</div>

			<div class="reply">
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	<li class="comment even thread-even depth-1" id="li-comment-2631">
		<div id="comment-2631">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/704a1e4bb9b4982564141ff4c15b7eda" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn"><a href="http://yuhongbao.blogspot.com/" rel="external nofollow" class="url">Yuhong Bao</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-2631">
				November 6, 2011 at 10:08 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>7c00h – 700h / 512 bytes = 58</p>
</div>

			<div class="reply">
							</div><!-- .reply -->
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	</li>
	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-2632">
		<div id="comment-2632">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/704a1e4bb9b4982564141ff4c15b7eda" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn"><a href="http://yuhongbao.blogspot.com/" rel="external nofollow" class="url">Yuhong Bao</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-2632">
				November 6, 2011 at 10:14 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Actually, it would be 57 sectors per track to allow for the stack itself.</p>
</div>

			<div class="reply">
							</div><!-- .reply -->
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	</li>
	<li class="comment even thread-even depth-1" id="li-comment-2665">
		<div id="comment-2665">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/ad516503a11cd5ca435acc9bb6523536" class="avatar avatar-40 photo avatar-default" height="40" width="40">				<cite class="fn"><a href="http://www.os2museum.com/" rel="external nofollow" class="url">michaln</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-2665">
				November 7, 2011 at 6:23 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>There’s room for 58.5 sectors between 
700h and 7C00h. The stack is extremely unlikely to need more than 256 
bytes, so 58 sectors should be fine.</p>
<p>But what does that have to do with the 32MB partition size limit?</p>
</div>

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	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-3472">
		<div id="comment-3472">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/704a1e4bb9b4982564141ff4c15b7eda" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn"><a href="http://yuhongbao.blogspot.com/" rel="external nofollow" class="url">Yuhong Bao</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-3472">
				December 7, 2011 at 8:50 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Well, DOS 3.2 and earlier did not support partitioning at all without third-party drivers.</p>
</div>

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							</div><!-- .reply -->
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	<li class="comment even thread-even depth-1" id="li-comment-3476">
		<div id="comment-3476">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/ad516503a11cd5ca435acc9bb6523536" class="avatar avatar-40 photo avatar-default" height="40" width="40">				<cite class="fn"><a href="http://www.os2museum.com/" rel="external nofollow" class="url">michaln</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-3476">
				December 7, 2011 at 11:00 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>That’s a rather misleading statement. 
DOS always supported partitioning (ever since hard disk support was 
added in DOS 2.0), but prior to 3.3 DOS could not access multiple DOS 
partitions at the same time. It was always possible to have multiple 
partitions present on a fixed disk, even multiple DOS partitions (but 
only one active/accessible).</p>
</div>

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	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-4012">
		<div id="comment-4012">
			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/704a1e4bb9b4982564141ff4c15b7eda" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn"><a href="http://yuhongbao.blogspot.com/" rel="external nofollow" class="url">Yuhong Bao</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-4012">
				January 2, 2012 at 4:08 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Yea, I think it was primarily for booting other OSes like Xenix.</p>
</div>

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	<li class="comment even thread-even depth-1" id="li-comment-5484">
		<div id="comment-5484">
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				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/1951012db8638980296d4a790aaf4610" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn">rauli</cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5484">
				March 22, 2012 at 1:03 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>Let’s suppose:<br>
1 – We have a hard disk partition [A] with MS DOS 3.0/3.1/3.2 installed 
on it, and crashing at boot (because of the issue described in this 
article).<br>
2 – We have MS DOS 3.30 installed on another disk [B] (another hard disk partition or a diskette).</p>
<p>Replacing [A] boot sector with [B] boot sector… would make [A] boot?<br>
(replacing the boot sector, but maintaining the BPB part, of course)</p>
</div>

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	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-5487">
		<div id="comment-5487">
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				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/ad516503a11cd5ca435acc9bb6523536" class="avatar avatar-40 photo avatar-default" height="40" width="40">				<cite class="fn"><a href="http://www.os2museum.com/" rel="external nofollow" class="url">michaln</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5487">
				March 22, 2012 at 1:35 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>It might help, but I can’t guarantee that it will work. I’d give it about 80% chance of success. If you do try, be very careful.</p>
</div>

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	<li class="comment even thread-even depth-1" id="li-comment-5500">
		<div id="comment-5500">
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				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/4fb67488b5ddd81a1206953bab2ff0aa" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn">rauli</cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5500">
				March 22, 2012 at 7:25 pm</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>It works!<br>
But, if you take DOS 3.3 boot sector from a diskette, you have to 
maintain also byte at 01FD from the [A] boot sector (3rd from the end) 
which is 80h for a hard disk boot sector, and 00h for a diskette boot 
sector.<br>
I didn’t notice that byte, and it almost makes me quit…<br>
As I’ve said before, you also have to maintain the original BPB from [A] (for DOS 3.x it’s at bytes 0Bh to 1Dh, I think).<br>
After some rest I will try to boot DOS 2.x and 4 with this same method 
(using 3.3 boot sector). Just to try, but I think they will not boot.</p>
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	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-5785">
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				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/fb60a38329dabcf2e8af6f2ae9ca19db" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn">alex peter</cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5785">
				April 7, 2012 at 5:23 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>anyone know how to get more than 4 
partitions from dos 3.2? assuming the drive can handle more than 4 32 
meg partitions.  I heard ast and nec came up with 8 32 meg partitions I 
think, but im looking for software |i can use with dos 3.2 that will 
give me more partitions.  any takers?</p>
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	<li class="comment even thread-even depth-1" id="li-comment-5786">
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			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/fb60a38329dabcf2e8af6f2ae9ca19db" class="avatar avatar-40 photo" height="40" width="40">				<cite class="fn">alex peter</cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5786">
				April 7, 2012 at 5:28 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>I want to run my old games on my Tandy 
1k.  3.3 wont run them due to too much conventional mem ue but 3.2 does.
  Drawback is that I have an 8 bit ide card that can take 2 gig 
partitions but the dos it uses (6.22) definitely wont run my games.  
tandy had a very weird video set up that used conventional mem to run 
its video ram.  (sucks) but If I can circumvent the problem by using 3.2
 then I got it licked.  Assuming that I can get some sort of 
partitioning sorftware or a version of fdisk that will give me more than
 the said 4 partition limitations in 3.2.</p>
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	<li class="comment odd alt thread-odd thread-alt depth-1" id="li-comment-5791">
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			<div class="comment-author vcard">
				<img alt="" src="Hang%20with%20early%20DOS%20boot%20sector,%20OS2%20Museum_files/ad516503a11cd5ca435acc9bb6523536" class="avatar avatar-40 photo avatar-default" height="40" width="40">				<cite class="fn"><a href="http://www.os2museum.com/" rel="external nofollow" class="url">michaln</a></cite> <span class="says">says:</span>			</div><!-- .comment-author .vcard -->
			
			<div class="comment-meta commentmetadata"><a href="http://www.os2museum.com/wp/?p=685&amp;cpage=1#comment-5791">
				April 7, 2012 at 10:15 am</a>			</div><!-- .comment-meta .commentmetadata -->

			<div class="comment-body"><p>If they really supported more than 4 partitions, they probably had OEM DOS versions with adapted IO.SYS.</p>
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		<p>Pingback: <a href="http://www.os2museum.com/wp/?p=1132" rel="external nofollow" class="url">DOS boot hang update | OS/2 Museum</a></p>
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