https://www.os2museum.com/wp/more-than-two-hard-disks-in-dos/ Skip to content OS/2 Museum OS/2, vintage PC computing, and random musings OS/2 Museum * Home * About + Wanted List * OS/2 History + OS/2 Beginnings + OS/2 1.0 + OS/2 1.1 + OS/2 1.2 and 1.3 + OS/2 16-bit Server + OS/2 2.0 + OS/2 2.1 and 2.11 + OS/2 Warp + OS/2 Warp, PowerPC Edition + OS/2 Warp 4 + OS/2 Timeline + OS/2 Library o OS/2 1.x SDK o OS/2 1.x Programming o OS/2 2.0 Technical Library + OS/2 Videos, 1987 * DOS History + DOS Beginnings + DOS 1.0 and 1.1 + DOS 2.0 and 2.1 + DOS 3.0, 3.1, and 3.2 + DOS 3.3 + DOS 4.0 + DOS Library * NetWare History + NetWare Timeline + NetWare Library * Windows History + Windows Library * PC UNIX History + Solaris 2.1 for x86 - Learn Something Old Every Day, Part XVII: DHCP and ARP Don't Mix in WSA SMP More than Two Hard Disks in DOS Posted on July 22, 2025 by Michal Necasek Investigating the rather odd behavior of the Microsoft OS/2 1.21 disk driver led me to Compaq and their EXTDISK.SYS driver. While experimenting with various setups, I realized that DOS versions older than 5.0 do not support more than two hard disks exposed by the system's BIOS, and will in fact quite likely hang early during boot-up if there are "too many" hard disks. This seems to have been one of the many things that "everyone knew" back in the day, similar to the fact that DOS versions older than 3.3 may hang while booting from disks with significantly more than 17 sectors per track. As was the case with the "too many sectors per track" problem, the issue with "too many hard disks" was missed for years simply because no one had a PC with more than two hard disks. This was a technical rather than architectural limitation. While the IBM PC/XT and PC/AT BIOS implementations were limited to two hard disks, the INT 13h interface as such was not. In the days of full-height 51/4" drives, it simply was not feasible to install more than two hard disks into a PC, especially when a 51/4" floppy drive was also required. Even the big IBM PS/2 Model 80 (1987) with a tower case could only house two full-height 51/4" drives. There might also be trouble with the power supply, as the PC hard disks of the time were not designed for staggered spin-up and a standard AT power supply might have trouble spinning up four drives at the same time. Sure, there were half-height hard disks, but who wanted four drives in the first place? People who needed to maximize the storage capacity... and the most obvious way to do that was buying a large capacity drive, which in the 1980s was inevitably a full-height 51/4" monster. Like my 1988-model 650 MB ESDI drive, for example. Yes, there were solutions like the NetWare DCB which supported many drives, but those were only usable by NetWare and did not expose the drives via INT 13h. Two things happened circa 1988. One was Compaq releasing the Deskpro 386/25 with an expansion unit option, a system which supported up to four hard AT-style disks (that is, the expansion unit housed up to two ESDI drives accessible via the PC/AT hard disk style programming interface, which may be called WD1003 or WD1010 or several other things). The other development was Adaptec releasing the AHA-1540/ 1542 SCSI HBA, and there were perhaps other SCSI vendors as well. Compaq supported up to four hard disks, Adaptec in theory up to seven. In any case, it is apparent that both companies ran into the same problem with DOS, and solved it in a very similar manner. Compaq simply did not expose the drives in the expansion unit through the BIOS at all. DOS users needed the EXTDISK.SYS driver, and users of other operating systems (such as OS/2 or NetWare) needed a custom driver. Adaptec was in a more complicated situation. The AHA-154x was an add-on card which could be installed in a PC/AT compatible machine (the AHA-154x did not work in older systems because it was a bus-mastering adapter) that already had one or two AT style drives. The AHA-154x BIOS keeps the total hard disk maximum to two. In practice that means that if there are two SCSI hard disks attached to an AHA-154x (which also includes AHA-154xA and AHA-154xB, but not necessarily newer models), the Adaptec BIOS may add zero, one, or two drives to the system, depending on how many hard disks there are already installed. In any case, the total won't be greater than two. For DOS users, Adaptec offered a combination of ASPI4DOS.SYS (ASPI driver for the AHA-154x) plus ASPIDISK.SYS (DOS hard disk device driver). Adaptec's ASPIDISK.SYS was functionally very similar to Compaq's EXTDISK.SYS and allowed DOS users (especially users of DOS 4.x and older) to utilize more than two hard disks. DOS Bug The bug is quite visible in the MS-DOS 4.0 source code. In MSINIT.ASM (IO.SYS/IBMBIO.COM module), DOS calls INT 13h/08h and stores the number of disks in the HNUM variable. No attempt is made to validate the value returned by INT 13h. Further down in MSINIT.ASM, DOS sets up the hard disks, calling the SETHARD routine for each drive, but it will not set up more than two. Trouble will start near the SETIT label, where the DRVMAX variable may end up with a number much higher than the number of drives that SETHARD was run on. Eventually, disaster strikes in the $SETDPB routine in the DOS kernel. The code near LOG2LOOP label attempts to calculate the cluster shift for the FAT file system, but gets stuck in an endless loop because the BPB for a drive was never initialized and contains zeros. This bug is present in every DOS version with hard disk support before 5.0, that is, in DOS 2.0 up to and including DOS 4. In my experiments, all these DOS versions hang when booting on a machine that exposes four BIOS drives. MS-DOS 4.01 from April 1989 still hangs, and so does Russian MS-DOS 4.01 from February 1990. It is clear that the bug went unnoticed or at least unfixed for a number of years simply because PCs with more than two hard disks were extremely rare to nonexistent. DOS 5.0 It is likely that DOS 4.0 (1988) was released just before PCs with multiple hard disks became a thing. By the time Microsoft started working on DOS 5.0 in earnest in 1990, EXTDISK.SYS and ASPIDISK.SYS were certainly well established, and the problem must have been known. MS-DOS 5.00.224 Beta from June 1990 (the oldest DOS 5.0 beta I could test) does not suffer from the bug described above, and shows four hard disks exposed by the BIOS in FDISK. Further related work was done in August 1990 with the following comment: M011 8/07/90 CAS msinit.asm rewrote lots of spaghetti code msbio1.asm used for initializing hard drive partitions. fixed bugs 2141, 2204, 1866 and 1809 and prepared for zenith/etc. support The above is an excerpt from an MS-DOS 5.0 OAK (OEM Adaptation Kit). The first entry in the relevant file (MSBIO.TAG) is dated 7/17/90 which leaves open the question of who actually fixed the problem with more than two hard disks and when, since it must have been fixed by June 1990. There is another rather curious data point: [multidisk-pcdos405v-fdisk]IBM DOS J4.05/V FDISK with four hard disks The above screenshot shows that Japanese IBM DOS 4.05/V does not hang and FDISK correctly shows four hard disks. Here's the boot screen of said DOS version: [multidisk-pcdos405v-boot]Boot screen of IBM DOS J4.05/V This shows that the fix made it into at least some DOS 4.x code base. However, the system files in IBM DOS J4.05/V are dated October 1990, decidedly newer than the MS-DOS 5.00.224 Beta. SCSI HBAs In any case, the fix was well known to SCSI HBA vendors. Starting with the AHA-154xC, Adaptec offered an option for "BIOS Support for More Than 2 Drives (MS-DOS(R) 5.0 and above)". When this option was disabled, the BIOS keep the total number of hard disks to no more than two, just like AHA-154xB and earlier. When enabled, the Adaptec BIOS would expose all the hard disks it would find as BIOS drives 80h, 81h, 82h, 83h, 84h, etc. [multidisk-aha1542cp-640x356]Adaptec AHA-1542CP BIOS setting for DOS 5.0 BusLogic adapters offered a more or less identical setting to solve the identical problem. [multidisk-bt545c-640x356]BusLogic BT-545C BIOS setting for DOS 5.0 When this setting was enabled, DOS 5.0 and later no longer needed ASPIDISK.SYS or any other vendor specific driver. DOS itself could directly use the BIOS to access all hard disks in the system (limited by the number of available drive letters). I believe clone BIOSes with support for more than two IDE hard disks generally started appearing only since 1994 or so, and assumed (not unreasonably) that the user would be installing DOS 5.0 or later. In the worst case, the BIOS could usually be set up to not detect the 3rd and/or 4th hard disk. It was the SCSI HBAs that were prepared to deal with trouble. APAR IR86346 Completely by accident, the puzzle of the DOS fix was solved while I was looking for something totally unrelated. In an IBM announcement letter from October 1990, the following sentence jumped out at me: DOS 3.3 and 4.0 support up to two fixed disks in a system. DOS 4.0 supports up to seven fixed disks when corrective service diskette (CSD) #UR29015 is installed. I happened to have CSD UR29015 on hand, so I looked at the included documentation. The README file states: APAR IR86346 requires DOS 4.0 to be installed with NO MORE THAN two fixed disk drives before installing corrective service. Once corrective service is installed, you can attach the additional fixed disk drives. In the APARLIST file there's a table which includes the following entry: CSD APAR KEYWORD COMPONENT ABSTRACT ------- ------- -------- --------- ----- ... UR27164 IR86346 ABEND IBMBIO DOS 4.0 hangs with more than 2 hardfiles ... Yep, that's exactly the observed problem! With more than hard disks, DOS 4.0 and older simply hangs. There's also a table of fix releases in the same file (excerpted): ... CSD UR25066 05/10/89 IFD UR25788 06/07/89 IFD UR27164 09/25/89 IFD UR27749 10/11/89 ... CSD UR27164 which (was the first to include the fix for APAR IR86346) was released on September 25, 1989. The previous CSD from June 1989 did not include the fix. The documentation does not lie and with CSD UR29015 applied, IBM DOS 4.0 has no trouble booting up and seeing four hard disks: [multidisk-ur29015-fdisk-640x356]IBM DOS 4.0 with CSD UR29015 (March 1990) That clarifies the timeline a lot. MS-DOS 4.01 from April 1989 could not possibly contain the fix. IBM fixed the bug sometime in Summer 1989, which is why IBM DOS J4.05/V includes the fix. Microsoft's Russian MS-DOS 4.01 was likely branched before mid-1989 and the fix was never applied. And this also explains why the earliest MS-DOS 5.0 betas don't have the problem with more than two hard disks, even though there is no record of Microsoft fixing it. Because Microsoft didn't--IBM did, a few months before the work on MS-DOS 5.0 started. The only minor remaining mystery is who opened APAR IR86346. It could have been an external customer, although both the Adaptec AHA-154x HBA and the Compaq Deskpro/25 were designed to protect against DOS hanging. Then again, perhaps some other SCSI HBA was not quite so careful and could trigger the hang with multiple hard disks. It is also possible that the bug was discovered and internally reported when IBM was working on its own SCSI adapters, released in March 1990 together with the first wave of PS/2 machines with SCSI drives. This entry was posted in Bugs, Compaq, DOS, Microsoft, PC history. Bookmark the permalink. - Learn Something Old Every Day, Part XVII: DHCP and ARP Don't Mix in WSA SMP 24 Responses to More than Two Hard Disks in DOS 1. [92bb] Dan V. says: July 22, 2025 at 5:11 pm Fascinating find! I'm sure IBM discovered the bug when they were in the process of introducing SCSI adapters and hard disks as standard equipment. But that CSD is mentioned even earlier than the October 1990 round of PS/2 Model 80 refreshes. The announcement letter dated March 20, 1990 for the Model 80 -121 and -321 which used the Type 2 planar also announced an increase of available storage bays. Your corrective service diskette statement is also part of this letter! It can be found at https:/ /ardent-tool.com/8580/190-054.txt The letter states: "The fixed disk controller is the newly introduced Small Computer System Interface (SCSI), the PS/2 Micro Channel SCSI Adapter. This bus master adapter provides additional expansion capability while providing an interface for the new 3.5-inch, half-high SCSI fixed disk drives of either 120MB (8580-121) or 320MB (8580-321)." Further, "Design enhancements to these systems offer significant advantages in configuration flexibility and expansion. In addition to providing seven available adapter slots, the standard configuration will support up to five internal Direct Access Storage Devices (DASD) and removable media, such as diskette drives, fixed disk drives, tape backup and CD-ROM. An optional feature allows support of up to six internal DASD devices. The standard fixed disk drive configuration of the Model 80 121 and 321 contains one SCSI fixed disk drive located in one of the two non-accessible bays. The other non-accessible bay contains the necessary hardware to install a second IBM SCSI fixed disk drive option. The 5.25-inch, full-high bay can be converted into two 3.5-inch, half-high bays through the use of the optional Fixed Disk Drive Kit A (#1053) (6451053). This conversion allows the installation of a third and fourth IBM SCSI fixed disk drive option." The five bays come from the two externally accessible 3.5 inch bays, the externally accessible 5 1/4 bay (position D), and one dual 3.5 bracket installed in the rearward drive bay slot (position C). Installing a second dual hard drive bracket in Position D brings it up to six bays. As you pointed out earlier, the towers previously used full height ESDI or MFM drives, and those controllers only supported one or two attached disks. The Type 1 planar (the 1987 models) only mention accommodating a second fixed disk in their announcement letter: "Additional positions for a second fixed disk drive and for a second 3.5-inch diskette drive." So this March letter announcing the PS/2 Model 80's new drive capabilities and the corrective service diskette is probably the earliest mention of IBM supporting more than two hard drives. The advent of SCSI supporting many devices on one adapter along with denser 3.5 drives allowed IBM to stuff more disks into the tower and then bam, they hit the bug. At least, that's a logical guess to me. 2. [fd4f] Michal Necasek says: July 22, 2025 at 7:17 pm Yes. The October 1990 announcement was simply the one I found first. Logically the systems (and SCSI adapters) announced in March 1990 needed the same fix. It is possible that someone external reported the problem to IBM, but it's more likely that IBM ran into the problem internally when developing their own SCSI adapters. 3. [92bb] Dan V. says: July 22, 2025 at 8:26 pm I agree. I love whenever you post these deep dives into specific bugs. As a QA person it scratches every one of my itches. IBM would've been developing the Tribble adapter in the spring-summer of 1989 since the earliest adapter BIOS dates from then. That lines up with the Japanese DOS release date and September release of the CSD, so I think your working theory of IBM discovering the bug internally has the most supporting evidence. I'm surprised Compaq didn't bring the issue up with Microsoft, but you know how things get prioritized or how lack of source visibility can make an OEM misjudge a cause. 4. [550a] Zir Blazer says: July 22, 2025 at 10:23 pm > In the days of full-height 51/4" drives, it simply was not feasible to install more than two hard disks into a PC, especially when a 51/4" floppy drive was also required. I'm certainly sure than this was possible with the IBM 5161 Expansion Unit that was released before the PC/XT, so 1982? With PC or PC/XT + 5161 setup you had available four full height 5 1/ 4", and while four HDs is unlikely because the reliance on disquettes, three seems perfectly feasible. 5. [6b39] TRX says: July 23, 2025 at 12:06 am I had four hard drives. IBM DOS 3.3 would check each drive in turn, looking for a valid boot sector, and if it found one, it would boot from that drive. I regularly booted from drive E: MS-DOS 5 - at least early versions - would only boot from C:. I mentioned it on the Microsoft DOS 5 CompuServe group, and the reply was, "Why would you want to boot from any other drive?" It turned out a lot of people did, but Microsoft's official reply was 'that's the way it is now.' I reconfigured my system to boot from C:, then switched to DR-DOS 5.0, which I ran (with DESQview and QEMM) until the late 90s when I got paid to write software to run under Windows 95. 6. [2777] Rich Shealer says: July 23, 2025 at 4:02 am I love these history posts. I worked for small PC clone builder in the late 80' and the first half of the 90's, and I can't remember ever having more than two MFM or RLL hard drives in a system other than a Novell Advanced Netware 386 server. That used an Adaptec 1542C. I know we updated at least one normal PC with a third IDE drive but that would have been a 486 and MS-DOS 5.0 so well supported. I never gave it much thought, but the MFM and RLL controllers could be set to a primary address (1F0 h) and secondary (170 h). I just never though about how it would be used. Maybe something like Ontrack Disk Manager would help, the driver being loaded after the standard drives by MS-DOS. 7. [fd4f] Michal Necasek says: July 23, 2025 at 12:04 pm Are you saying that you had 4 physical hard disks visible to the BIOS but only some of them had DOS-compatible file systems on them? 8. [fd4f] Michal Necasek says: July 23, 2025 at 12:05 pm Yes the 5161 could hold up to two hard disks... but then what? What controller(s) would you use for more than two to be visible to the BIOS? I'm genuinely curious. 9. [fd4f] Michal Necasek says: July 23, 2025 at 12:13 pm Yes, Compaq is a bit of a mystery. They clearly had a very good relationship with Microsoft. And even though DOS was managed by IBM at the time, I'm sure Microsoft could have done something. One possibility is that Compaq found out too late to get it fixed in DOS 4.0, and at any rate they would have needed a fix in DOS 3.3 as well. It is conceivable that Compaq didn't want to bother with that or Microsoft told them "we're not doing that" (another DOS 3.x minor version), and/or "just use OS/2" (which did have the support built in!). And once Compaq had a solution for DOS 3.x, it worked in DOS 4.0 just as well. It is also conceivable that they very much did not want to break booting from existing DOS floppies (which would happen if they exposed the extra drives to the BIOS). 10. [fd4f] Michal Necasek says: July 23, 2025 at 12:27 pm About the only thing I can think of which could use hard disks on the secondary adapter was NetWare. In fact NetWare was the company which introduced the concept of tertiary and quaternary adapters. Technically I am pretty sure you're right and DM could have supported those, but I don't know if it actually did. 11. [82d8] Richard Wells says: July 24, 2025 at 12:34 am DOS could use a hardcard assigned as a secondary controller if the Quantum documentation reflected reality. Needed the supplied hardcard driver to do it. 12. [c2fd] John Elliott says: July 24, 2025 at 11:43 pm In the Digital Research world, DOS Plus 1.2 supports only one or two BIOS hard drives, with a single partition on each one, and doesn't seem to probe for any others. Looking at the code DRDOS 3.40-5 seem to have a maximum of 16 mounted volumes but there's no reason why each volume couldn't be on its own BIOS hard drive. I haven't put this to the test though. 13. [5d67] MiaM says: July 25, 2025 at 12:46 am Nice write up! I think I wrote more or less this as a comment to the previous post, but: IBM releasing the AT with a combined fixed+floppy disk inteface that could be configured as primary/secondary, but no software/ firmware/drivers to support the secondary setting, is surprising. Did WD just add that setting without IBM requesting it? But also, these bugs (both the amount of disks, and problems with more than 17 sectors) really shows that testing was bad at the time. It would probably not had been that hard to modify an AT bios to have the second hard disk show up multiple times, but write protected to ensure it won't be corrupted, just to be able to test this. Also it would had been fairly easy to create a BIOS that would simulate more sectors, more heads or more tracks by some simple hard coded geometry translation. 14. [fd4f] Michal Necasek says: July 25, 2025 at 12:07 pm No, this is not the fault of testing. QA tested products that existed, not products that didn't. Sure they could have discovered these bugs, but at the costs of also spending a lot of time and resources varying all kinds of parameters only to find that there's no problem there. With the number of hard disks, it may be fair to blame the developers, they should have realized that more than two drives would be desirable. With the sectors per track... the code was written in what, 1982? At that point, hard disks with more than 17 spt were years off, and nobody could predict with any kind of certainty how hard disks would evolve. 15. [5d67] MiaM says: July 25, 2025 at 6:57 pm It's at least reasonable to test that code ignores unexpected input. In this case, if the number of hard disks were more than two, it would just round down to two. While it would had been highly unusual, the large CDC drives had more than 17 tracks. The terrible Google AI summary says that a Hawk had 32 sectors per track while a Phoenix had 64 sectors per track. What was predictable was that the BIOS could tell running code that there are a certain number of floppies, hard disks and that they have certain geometries, and the space for storing this information had a fixed size and thus it's reasonable to test between zero and max of each of these. My gut feeling is that Microsoft didn't have any test setup, they just assembled and ran it on a few different PCs they had. On one hand I get why this would be the case, but on the other hand they had grown big enough to be able to afford to do full tests. Or maybe it was an actual policy, to make sure that people had to upgrade in the future? 16. [82d8] Richard Wells says: July 25, 2025 at 9:50 pm In the late 80s, IBM was obsessed with limiting the capability of the Personal Systems to protect the AS/400 market. Comparatively small hard drives with limited connections all played into it and not testing products not shipped from IBM made sure it was harder to go past the IBM designated limits. Remember DOS 4 was conspicuously not tested on software from outside of IBM which had a similar effect. 17. [5d67] MiaM says: July 26, 2025 at 12:27 am Yeah, but IBM wasn't the only customer, and Microsoft wasn't interested in protecting IBMs or other manufacturers larger systems. I.E. Microsoft had incentives to actually make their products work on more expanded systems. 18. [82d8] Richard Wells says: July 26, 2025 at 9:03 am MS had reduced DOS development staff during the PC-DOS 4 era. Most OEMs were wanting an MS-DOS that was as close to IBM's as possible to go with the nearly identical clones being produced. Compaq and a few other OEMs were willing to go beyond what IBM chose to limit themselves to. The specialized drivers were the responsibility of those OEMs as was any support burden caused by those drivers. MS changed things with the run up to DOS 5 as following IBM's lead became less important. 19. [725b] Yuhong Bao says: July 26, 2025 at 1:09 pm (note that early Compaq systems however did not have BIOS support for more than two drives) 20. [725b] Yuhong Bao says: July 26, 2025 at 1:25 pm (though note that it was IBM that decides to support more than two drives and not MS) 21. [7cfb] Abrahan says: July 29, 2025 at 3:03 am i hope that this fix is pushed on to the DOS 4.00 source code availible on github. 22. [cd33] ecm says: July 30, 2025 at 4:24 pm @Abrahan: I fixed this partially in lMS-DOS / lDOS, my fork of the 2024 April release, which is available from my website at pushbx.org In particular, multiple hard disks are supported, but the kernel probably will only log in the first primary partition and a single chain of logical partitions in the first extended partition (type 5 or type 0Fh) per hard disk, where "first" relates to the order of the partition table entries. I do plan to eventually support a wider array of configurations. 23. [725b] Yuhong Bao says: August 5, 2025 at 2:13 pm Don't forget search engines and SSDs as well. 24. [725b] Yuhong Bao says: August 5, 2025 at 2:24 pm (Google was started in 1996 and DOS 5.0 was released in 1991) Leave a Reply Your email address will not be published. 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