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Discovered RTC bug in PC DOS 2000 / IBM DOS 7

Additionally to V20/V30 CPU you will need to have MS-DOS 4.01 or newer to support 100 MB medias)
Zenith's MS-DOS 3.30 Plus and Compaq's MS-DOS 3.31 will probably work, too. Both support partitions larger than 32 MB.
You can always downgrade to 6.3. Or try to convince pcdosretro to come back to the forum and hope he still has the source :)

I took the easy way out and booted from a DOS 6.22 floppy and set it that way :) I was using David_M's clock.com but that didn't seem to work correctly with GLaTICK though I didn't spend enough time to figure out whether it was a real problem or operator error.
I no longer remember where to find the original source, but here's an old website that cites it.


The author of PC-DOS 7 was also a member here at one point under the handle pcdosretro, and he himself corroborated it

Not that it really matters, but digging through that thread I don't actually see any meaningful corroboration of the idea that IBM somehow "optimized" DOS 7 for the V20. There's some vague statements that DOS 7 "runs faster" on a V20, but that doesn't mean anything without actually pointing to a specific example demonstrating that the software is using an alternate code path vs. just running 8086-compatible code slightly faster, which the NEC V-chips already do, mixed in with a lot of yakking about DOS 7 being able to use UMBs for certain functions that MS-DOS can only use the HMA for which... again, has nothing to do with instruction support.

(An 8088 has no inherent problem using UMBs if the software support to do so is present, and conversely the V20/30/etc's support of real mode '186 extensions certainly doesn't magically gift them the ability to support software targeting the PC AT's HMA.)
Any way to patch PCDOS to address this issue? I have GLaTICK running on my Compaq Portable with an AST MegaPlus II (running PCDOS 2000 with a V20) and was scratching my head on why it didn't quite work!

Finally got around to making a fix for PCDOS to fix the issue! This patches IBMBIO.COM, replacing the `AAD 16` instruction that's incompatible with V20/V30 when using a BIOS supported real-time clock (GLaTICK, Xi 8088, NuXT, etc).

Since IBMBIO.COM is a bit of a pest to just update and replace (due to the system/read-only attributes), here are two bootable images - one for Revision 0 and one for Revision 1. The easiest way to update I've found is to boot from one of these and use SYS to replace the IBMBIO.COM on the target device.


  • PCDOS700_R0_V20.zip
    131.2 KB · Views: 9
  • PCDOS700_R1_V20.zip
    157.8 KB · Views: 10