Apple II emulation must have started around 25 years ago but I can't seem to find a history of emulation of the platform.

There are several disk image file formats but for now I'm interested in the two raw formats that use the .dsk file extension.

There's .do for DOS 3.3 sector ordering and .po for ProDOS block ordering. We've discussed them before.

But I'm looking for which programmer or which emulator or other disk tool was the one responsible for introducing each format. Perhaps both appeared together with the first Apple II emulator, or perhaps .do was first and .po came out later with a different emulator.

Raw formats like these that have no structure or headers tend not to have homepages so it's hard to track down their origins.

  • 1
    The alt.emulators.ibmpc.apple2 FAQ, circa May 1994, mentions apl2em as an emulator using DOS order disk images. (and applemu as an emulator using nibbled disk images). It suggests using ShrinkIt to archive your disk on an Apple II then nulib to extract them to a dos-order disk image. Which is to say, the format was invented in 1980 along with DOS 3.3 Commented Jun 25, 2020 at 5:08
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    Umm not quite, otherwise there would not be all the discussions of ambiguous dsk/do/po sector orderings due to physical sector ordering vs logical sector ordering due to sector interleave/skew. I'm also reading the oldest Usenet FAQs and posts I can find. Some of the oldest emulators used their own image formats which have been long since abandoned. It looks like Apl2Em and "Stop the Madness" both used .do .dsk and both go back to at least 1994. .po .dsk is said to have been used with "some unix emulators". Emulation on the Amiga looks like it might also go back to 94 with other file formats. Commented Jun 25, 2020 at 5:16
  • The ProDOS .dsk ordering might be due to a shareware tool called ProBlock from 1990 by Michael Pender / Nanochron but I'm having trouble confirming this. If so it only used .dsk at not .po at that time. Commented Jun 25, 2020 at 6:44
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    ShrinkIt + NuLib is a way to create ProDOS-order images, starting somewhere around Nov '89, but I can't remember how the (more common) DOS-ordered images were getting made. AFAIK there was no way to convert DDD images to files at the time.
    – fadden
    Commented Jun 25, 2020 at 14:50

1 Answer 1


This answer is still a work in progress...

A Unix emulator named "Apple //e Emulator" by Randy Frank (aka "apple2e" or just "ap2e" in filenames) goes right back to 1990.

It used ProDOS order for both ProDOS and DOS 3.3 disk images!

DOS 3.3:

Note: the UNIX files contain all the data on a DOS 3.3 5.25 disk. However, the files are not in straight t0s0,t0s1,t0s2... order. Instead the DOS 3.3 files are in PRODOS interleave format. So the first 256 bytes are t0s0 but the second are t0se. This is exactly the format you get by reading a DOS 3.3 disk with the PRODOS block scheme.


PRODOS UNIX files are just a block by block binary dump of the data on the 5.25 disk.

BUT It seems this emulator did now allow loading and saving arbitrary disk files yet, so it did not use the .dsk or .po file extensions.

The emulator was distributed with a pair of disk images for each disk operating system named as so:

s6d1                   yes                 DOS 3.3 slot 6 drive 1 disk arch
s6d2                   yes                 DOS 3.3 slot 6 drive 2 disk arch
PRODOS.IMAGE.D1        yes                 PRODOS slot 7 drive 1 disk arch
PRODOS.IMAGE.D2        yes                 PRODOS slot 7 drive 2 disk arch

Now it could be that there was an even earlier Apple 2 emulator and I don't yet know which emulator or tool first used the file extensions .dsk or .po for this format, or which emulator first used the DOS 3.3 sector ordering format.

UPDATE: I found the distro archive with the disk images and added the link. The DOS 3.3 images are in ProDOS format as confirmed by opening them with CiderPress. The ProDOS images both have a bunch of junk at the end. After truncating them to exactly 140K CiderPress confirms they are ProDOS in ProDOS order.

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