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)

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    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
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    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. – snips-n-snails Feb 8 at 21:29
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    @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
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    @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

Whilst this is not a definitive answer, by looking at Apple IIc ROM Version, which more fully lists the features of each ROM version, it seems that the disk support was actually improved, whilst, unfortunately, removing the ability to easily boot from the second drive (although the second drive could still be accessed post-boot).

One can only surmise that the designers assumed that booting from the internal drive only would be sufficient, and that losing the easily access ability to boot from the external drive was a price worth paying for access to the AppleTalk instead (using PR#7) as well as the additional drive support, namely:

  • UniDisk 3.5 drive,
  • the Apple 3.5 drives
  • IIc Memory Expansion Card

In actual fact, from information from a Google forum (see below2), Apple IIc Booting from external drive, it turns out that the code was not actually removed, just the ability to access the code easily by using PR#7 - the engineers presumably thought that the AppleTalk was a better use for the PR#7, although PR#5 would work for an external UniDisk 3.5". The leftover code could still be accessed by machine code routine/call.

More poignantly, from 8-THE APPLE IIC - Firmware

What about the unassigned slot 7? Here they put a small piece of code to allow booting from the external 5.25 drive by typing “PR#7” from Applesoft.

and further down the page, Overcoming limitations

The disk port on the original IIc was only designed to control an external 5.25 disk drive. Apple sold the Disk IIc for $329, and other companies later sold similar drives for less. Despite this firmware limitation, Quark Engineering released a 10 MB Winchester hard drive called the QC10 that would work with this disk port, and was the first hard disk available for the IIc.

and Enhancements

Rudimentary firmware was also included to allowing the IIc to be attached to an AppleTalk network (a message that said “AppleTalk Offline” would appear if you typed “PR#7” from BASIC), but it was never completed, and did not appear in future revisions of the IIc ROMs.

From which, one could infer that Apple dropped the boot support as third party drives were cheaper, and they didn't wish to support these more popular drives.

Wikipedia - UniDisk 3.5 support (ROM version ‘0’), confirms that which snip-n-snails stated in their comment:

The new ROM supported “intelligent” devices such as the Apple UniDisk 3.5-inch (800 KB) floppy drive and Smartport-based hardisks, in addition to an external 5.25-inch floppy drive.


The upgraded ROM added rudimentary support for an external AppleTalk networking device which was yet to be developed. When attempting to boot virtual slot 7, users would encounter the message “APPLETALK OFFLINE.” The IIc, however, had no built-in networking capabilities, and no external device was ever released.


1 ROM feature list, from Apple IIc ROM Version (this list expands slightly upon that given in Apple IIc Technical Note#7 - Existing Versions):

Original IIc ( $FBBF = $FF or 255 in decimal)

  • Can use the IIc external drive only
  • No AppleTalk firmware
  • PR#7 boots the second drive
  • Mouse firmware maps to slot 4
  • Serial firmware does not mask incoming linefeed characters
  • Serial firmware does not support XON/XOFF protocol

3.5 ROM IIc ( $FBBF = $00 )

  • Can use the IIc external drive and the UniDisk 3.5 drive
  • AppleTalk firmware maps to slot 7
  • PR#7 returns the message "AppleTalk Off Line"
  • Mouse firmware maps to slot 4
  • Serial firmware defaults to mask all incoming linefeed characters
  • Serial firmware supports XON/XOFF protocol
  • Adds the Mini-Assembler to the Monitor
  • Adds the Built-in Diagnostics

Original "Memory-Expandable" IIc ( $FBBF = $03 )

  • Can use the IIc external drive, the UniDisk 3.5 drive, and the IIc Memory Expansion Card
  • Mouse firmware maps to slot 7
  • No AppleTalk firmware
  • PR#7 kills the system
  • Serial firmware defaults to mask all incoming linefeed characters
  • Serial firmware supports XON/XOFF protocol

Revised "Memory-Expandable" IIc ( $FBBF = $04 )

  • Same as Original Memory-Expandable, plus:

    • Keyboard buffering firmware bug fixed
    • Firmware returns correct information when the Memory Expansion Card is not present

Apple IIc Plus ( $FBBF = $05 )

  • Can use the external IIc drive, the UniDisk 3.5 drive, the Apple 3.5 drives, but not the original IIc Memory Expansion Card.
  • Contains a Memory Expansion Card connector
  • 3.5" internal drive replaces 5.25" internal drive
  • Mouse maps to slot 7
  • PR#7 kills the system
  • 4 MHz 65C02 microprocessor
  • Accelerator chip and static RAM cache permit operation up to 4 MHz
  • Keyboard replaced with Apple Standard Keyboard (minus numeric keypad)
  • Internal power supply
  • Internal modem connector
  • Serial ports refitted with mini-DIN 8 connectors
  • Headphone jack has been removed
  • Volume control relocated above the keyboard
  • 40/80 column switch replaced by keyboard (Sholes/Dvorak) switch

2 However, it is possible to boot from the external UniDisk 3.5". From Apple IIc Booting from external drive, this post

If my copy of the IIc hardware reference manual is to be believed, you can boot from the first external drive by typing PR#5. I suspect this only applies to a UniDisk 3.5, and not a 5.25" drive.

and this post

Yes, PR#5 will boot the first external UniDisk in the //c's drive chain.

I recall typing in a small segment of assembly code (from inCider/A+, Nibble, or some such magazine) that would boot the external 5.25" drive, even if you had a //c with newer ROMs. I don't think it worked on anything but a //c, but I can't remember for sure. If you'd like me to dig it up and post it to the 'net, email me. I'm sure I can find it...it's only a matter of digging through a few of my BASIC programming disks. :^)

and this post

Actually, you can boot from an external 5.25" driver on the //c with ROM revision 2. The code necessary is in ROM (the ROM listing comments even tell that you can do it), but due to some omission you can't activate it by a single command.

You can boot however from the external driver by hacking in a short machine program. All you have to do is set X to $60, Y to $01, and A to $e0 and jump do some address... was it $c611? Take a look at the listing...

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