@tman can you give us the exact things you enter at the prompt?

(edit, since the spamm message got delete...)

I just open DosBox by double-clicking on it in the Finder. I also tried editing the preference file in ~/Library/Preferences to add a "boot" command to it and then double-clicked DosBox. I start DosBox with no command line options at all, nor do I know what command line switches are available. Once inside DosBox, I run:

BOOT /Users/me/floppy.img

(no options at all, just BOOT and the filename)

When I do this on OS X (10.6) and dosbox 0.74 it works, meaning it boots the image. So I guess it does the same for you but the image's autoexec.bat probably does something which reboots the "computer".
Doubleclick the image in Finder and OS X *should* mount the image as drive. Then take a look at what autoexec.bat (open in an editor) does. If you can't find the culprit, post the contents of autoexec.bat here.

(btw. command line switches are explained in the readme, but mostly you don't need those on OS X)

There is no autexec.bat on the floppy. I am not even sure there is any version of DOS on it either. I suspect it contains a custom boot block, so no need for an autoexec.bat. It just assumes a standard PC (XT? AT?) hardware platform.

If there is no Dos on it how should it ever work?
What game is it?

Well there does not need to be a DOS on the floppy in order for it to boot ... it can rely purely on BIOS calls for its I/O. However, the floppy is in standard MS-DOS 1.44MB FAT format, so there must be some stripped-down DOS-like functionality provided. There is no COMMAND.COM etc. on it. It is Ultima V.

Suuuuuuurrrrrre.... Just because it is fat formated doesn't mean there is an OS on it...
You need some kind of operating system on the medium you boot from. So if you boot from a floppy disk, there needs to be an OS on it. And on a DOS emulator, it should have DOS on it...

Why do you think it needs to be booted?
Ultima 5 was a Dos game, but not a game that came with an OS on its flopy. So what you need to do is, imgmount the floppy (readme provides you with instructions), then in dosbox go to that drive, and execute the game exe.

dosbox supports those images in general just fine. (If it really is a booter and not just a image of the installation/game floppy)

Think there is in the games section a post with which games work or not
Booter games compatibility overview

Yes, it has a boot block on it. I only ever used the floppy that was imaged to create the floppy image file to boot a 386 PC-compatible. I cannot find Ultima V named on the link you posted but it is listed at http://www.dosbox.com/comp_list.php?showID=769&letter=U as being supported. It does work if I manually run ultima.exe from the command line, so that's great. Interesting though that the emulator is unable to boot the floppy, although the physical PC was able to.

tman wrote:

Once inside DosBox, I run:

BOOT /Users/me/floppy.img)

Usually, the path is not /Users/me/floppy.img,
but ~/floppy.img

The ~ stands for your user name, to be entered with alt-1-2-6 using the numeric keypad of your keyboard.

Usually? Always depends on where such an image is... Could also be /volumes/dosusbstick/images/ultima/u5/floppy.img 😉
That is just path nitpicking and has nothing to do with the problem.

Yes, it has a boot block on it.

Please post it (hex dump).

Dominus wrote:

Suuuuuuurrrrrre.... Just because it is fat formated doesn't mean there is an OS on it...
You need some kind of operating system on the medium you boot from. So if you boot from a floppy disk, there needs to be an OS on it. And on a DOS emulator, it should have DOS on it...

Booters don't generally work that way in my experience. They run the system at a very low level and do not depend on a fully-featured OS. This is possible because the BIOS knows how to run arbitrary boot code on a disk without caring what OS might be there. This is also where boot sector viruses come from.

My old Rasterscan disk looked like a blank FAT-formatted 360KB floppy to DOS, but when I booted it there was a game there that had no dependence on DOS.

wd wrote:

Please post it (hex dump).

100000000  eb 2e 90 52 4d 58 24 20  33 2e 33 00 02 01 01 00  |...RMX$ 3.3.....|
200000010  02 e0 00 40 0b f0 09 00  12 00 02 00 00 00 00 00  |...@............|
300000020  00 00 00 00 00 00 00 00  00 00 00 00 00 00 01 00  |................|
400000030  bc 00 7c 56 1e b8 c0 07  8e d8 b8 40 18 8e c0 33  |..|V.......@...3|
500000040  f6 bf 00 7c b9 00 02 fc  f3 a4 1f 5e 06 68 51 7c  |...|.......^.hQ||
600000050  cb b8 40 18 8e d0 0e 1f  88 16 fd 7d 80 fa 80 73  |..@........}...s|
700000060  2e a1 13 7c 8b 0e 18 7c  f6 f1 8b 1e 1a 7c f6 f3  |...|...|.....|..|
800000070  8a e8 fe cd 80 f9 08 75  02 fe c1 b4 18 cd 13 be  |.......u........|
900000080  78 00 33 c0 8e d8 89 3c  8c 44 02 33 d2 eb 04 8b  |x.3....<.D.3....|
1000000090  16 1e 7c 0e 1f a1 16 7c  f6 26 10 7c 03 06 0e 7c  |..|....|.&.|...||
11000000a0  03 06 1c 7c 83 d2 00 e8  5c 00 b9 01 00 33 db b8  |...|....\....3..|
12000000b0  50 00 8e c0 e8 8a 00 be  c3 7d e8 e3 00 be ce 7d  |P........}.....}|
13000000c0  e8 dd 00 a1 11 7c 8b 1e  0b 7c c1 eb 05 33 d2 f7  |.....|...|...3..|
14000000d0  f3 0b d2 74 01 40 48 e8  2c 00 26 8b 45 1c 33 d2  |...t.@H.,.&.E.3.|
15000000e0  f7 36 0b 7c 0b d2 74 01  40 8b c8 b8 70 00 8e c0  |.6.|..t.@...p...|
16000000f0  33 db e8 4c 00 8a 16 fd  7d ea 00 00 70 00 b8 61  |3..L....}...p..a|
1700000100  46 ba 6c 69 cd 19 f7 36  18 7c 8b ca 8b d8 33 d2  |F.li...6.|....3.|
1800000110  f7 36 1a 7c 8b da 03 0e  2e 7c 3b 0e 18 7c 7e 05  |.6.|.....|;..|~.|
1900000120  43 2b 0e 18 7c 03 1e 2c  7c 3b 1e 1a 7c 7c 05 40  |C+..|..,|;..||.@|
2000000130  2b 1e 1a 7c 01 06 2a 7c  89 1e 2c 7c 89 0e 2e 7c  |+..|..*|..,|...||
2100000140  c3 bf 03 00 8b f7 e3 57  80 3e fd 7d 80 73 0c a1  |.......W.>.}.s..|
2200000150  18 7c 2b 06 2e 7c 40 3b  c1 7e 02 8b c1 51 50 8a  |.|+..|@;.~...QP.|
2300000160  36 2c 7c 8a 16 fd 7d 8b  0e 2a 7c c0 e5 06 0a 2e  |6,|...}..*|.....|
2400000170  2e 7c 86 cd b4 02 cd 13  58 59 73 0c 4e 7c 06 b4  |.|......XYs.N|..|
2500000180  00 cd 13 73 c1 e8 76 ff  8b f7 2b c8 50 f7 26 0b  |...s..v...+.P.&.|
2600000190  7c 03 d8 58 53 51 33 d2  e8 6b ff 59 5b eb a7 c3  ||..XSQ3..k.Y[...|
27000001a0  33 ff b0 18 b9 10 00 26  84 45 0b 75 0d 57 56 51  |3......&.E.u.WVQ|
28000001b0  b9 0b 00 f3 a6 59 5e 5f  74 08 83 c7 20 e2 e8 e8  |.....Y^_t... ...|
29000001c0  3c ff c3 49 42 4d 44 4f  53 20 20 43 4f 4d 49 42  |<..IBMDOS  COMIB|
30000001d0  4d 42 49 4f 20 20 43 4f  4d 00 00 00 00 00 00 00  |MBIO  COM.......|
31000001e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
32000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|

Yes, sorry, forgot about the booters. So I apologize about that part.
On the other hand, afaik u5 was not a booter game. So if that one was supposed to boot, then it got altered somehow.
Maybe the op needs to post the image for the devs...

Are IBMDOS.COM et al on the image? (hidden files)

No ... no IBMDOS.COM nor IBMBIO.COM, not as a hidden file, nor is there even a record which would indicate that a file with that name existed at any point in time and was subsequently deleted.

Didn't trace the bootsector too far, but it seems a standard drdos (or other
ibm-style derived dos) boot block so those files are needed. Maybe try the
image in bochs which could give you some actual error output before the
reboot (check its log window or the window itself) but looks like the
bootability of that disk was lost somehow.

Is it important to be able to use the boot method if it works by running ultima.exe from a DOS prompt?

Strange that it has a boot block that calls out IBMDOS.COM and IBMBIO.COM but that those files are not on the disk.

Is there a config.sys on the disk?