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Apple II computers don't come with DOS in ROM, but rather need to boot from a floppy disk to make the disk drives usable. The IIc was the first Apple II that came with a built-in disk drive, so booting it was a must (it also omitted the cassette ports earlier Apple IIs had). You could extend to further drives by connecting to a floppy expansion port.

The original IIc ROM version (version 255) supported booting from external drives by simply entering

PR #7

(Apparently, this was also documented in the manuals, and apparently, it only ever worked for ProDOS, the standard that came with the computer, but still better than nothing)

The follow-up ROM version (version 0, came in a ROM double the size of the original, so any omissions can't have been for space reasons), removed this feature, severely hampering usability of externally connected drives. This feature was never restored in any of the later ROM revisions (3, 4) until just recently in ROM version 4X, a hobbyist's project, which nicely presents a boot menu including an option to boot from external drives on CTRL-[closed Apple]-Reset.

As a work-around for the missing feature in later ROM revisions, Apple proposed (in the IIc Technical Reference Manual) using the built-in monitor and entering and calling a 10-bytes code sequence to boot from external drives (not exactly an improvement over PR #7)

Why did Apple remove such an important feature? How was this accepted by customers? Why did they never restore that feature by adding the 10 missing bytes to a ROM that had by far enough space?

EDIT: The 3.5 ROM update manual bluntly claims

PR #7 (from BASIC) or 7<CONTROL>P (from Monitor) no longer causes the system to boot from the external Disk II drive because that ROM space is now used for new features.

The removal I can confirm but the reasoning seems untrue to me (considering the size of the new ROM).

The IIc Technical Notes blatantly state

ROM Version 255 (the "correct" behavior)

PR #7 boots the second drive

ROM Version 0

PR #7 returns the message "Apple Talk Off Line"

(With no Apple Talk support on the IIc ever this does not look like a very useful message, but rather like a bug)

ROM Version 3

PR #7 kills the system

(Similarly unhelpful, but rather looks like another bug)

  • 3
    This is a very nicely asked question. You explained well enough that I both understand the question, and learned something about the subject matter besides. – Wayne Conrad Jan 24 at 18:54
  • Is it just the ROM that changed? Or is the Disk II no longer exposed in a slot? – Tommy Feb 8 at 19:31
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    The Internet thinks (ref1,ref2) that Apple tried, and failed, to add support for AppleTalk networking to the IIc through an external serial adapter. Maybe it ran out of space in the ROM and decided booting from external drives was an expendable feature, then neglected to put it back in when the experiment failed. – traal Feb 8 at 21:29
  • @Tommy The IIc doesn't have slots, but "emulates" them. I guess Disk II still occupies a slot with the new ROM, because it still works - you simply can't boot from it. So, IMHO, it's just the ROM that was changed. – tofro Feb 11 at 8:49
  • @traal Replacing working code with a broken implementation of AppleTalk and just leaving the remnants in a production machine sounds even worse than I thought. – tofro Feb 11 at 8:51

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