Apple II
Technical Notes
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                                                  Developer Technical Support


ProDOS 8
#15:    How ProDOS 8 Treats Slot 3

Revised by:    Matt Deatherage                                  November 1988
Revised by:    Pete McDonald                                    November 1985

This Technical Note describes how ProDOS 8 reacts to non-Apple 80-column cards 
in slot 3 and how it identifies them.
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The ProDOS 8 Update Manual now documents much of the information which was 
originally covered in this Note about how ProDOS 8 reacts to non-Apple 80-
column cards in slot 3.  However, since there is still some confusion on the 
issue, we summarize it again in this Note.

On an Apple ][+, ProDOS 8 considers the following four Pascal 1.1 protocol ID 
bytes sufficient to identify a card in slot 3 as an 80-column card and mark 
the corresponding bit in the MACHID byte:  $C305 = $38, $C307 = $18, $C30B = 
$01, $C30C = $8x, where x represents the card's own ID value and is not 
checked.  On any other Apple II, the following fifth ID byte must also match:  
$C3FA = $2C.  This fifth ID byte assures ProDOS 8 that the card supports 
interrupts on an Apple IIe.  Unless ProDOS 8 finds all five ID bytes in an 
Apple IIe or later, it will not identify the card as an 80-column card and 
will enable the built-in 80-column firmware instead.  In an Apple IIc or IIGS, 
the internal firmware always matches these five bytes (see below).

If you are designing an 80-column card and wish to meet these requirements, 
you must follow certain other considerations as well as matching the five 
identification bytes; the ProDOS 8 Update Manual enumerates these other 
considerations.

The ProDOS 8 Update Manual notes that an Apple IIGS does not switch in the 80-
column firmware if it is not selected in the Control Panel.  However, due to a 
bug in ProDOS 8 versions 1.6 and earlier, it switches in the 80-column 
firmware unconditionally.  ProDOS 8 cannot respect the Control Panel setting 
for 80-column firmware in certain situations; it cannot operate in a 128K 
machine in a 128K configuration (including /RAM) without the presence of the 
80-column firmware, since it must utilize the extra 64K.  This is just one of 
the reasons ProDOS 8 does not recognize a card in slot 3 if it is not an 80-
column card, as outlined above.

With ProDOS 8 version 1.7 and later, an Apple IIGS behaves exactly like an 
Apple IIe with respect to slot 3.  If a card is slot 3 is selected in the 
Control Panel, ProDOS 8 ignores it in favor of the built-in 80-column 
firmware--unless the card matches the five identification bytes listed above.  
This works the same on a Apple IIe.


Further Reference
o    ProDOS 8 Technical Reference Manual
o    ProDOS 8 Update Manual
o    ProDOS 8 Technical Note #11, The ProDOS 8 MACHID Byte

