Leading Causes for Macintosh IIci Incompatibilities (Con't)
2) Utilization of the PMMU - No invalid addresses
Another change related to the utilization of the PMMU in the Macintosh
IIci memory addressing architecture is the rejection of invalid addresses.
This is perhaps the most prevalent cause for software compatibility
problems on the Macintosh IIci. As described above the PMMU plays a
critical role in interfacing between logical and physical memory. As part
of the process of translating logical to physical, the PMMU also evaluates
the "validity" of the logical address requests it receives. An address is
considered valid if it falls within the valid address range and the valid
address range is determined by how much physical memory is available. A
2MB configuration has a 2MB valid address range and a 5Mb configuration
has a 5MB valid address range. This is determined at startup, and the PMMU
tables are set accordingly. If an application requests access to an
address that lies outside of this range, the PMMU generates a bus error.
That all sounds logical. So why should this cause a problem?
Because on previous systems invalid addresses were not identified. When
an application requested access to an address it would go straight to RAM,
which would decode whatever bits within the address it could and ignore
the rest. For instance if you had 1MB of RAM installed, your valid address
range would theoretically include all addresses up to 20 bits wide (2 to
the 20th is 1024K), which means that memory would only be capable of
decoding up to 20 bits of address information. Now let's say an
application requests access to a 21 bit address that is beyond your 1 MB
address range. On all systems up to the Macintosh IIci, RAM decodes the
lowest 20 bits (ignoring the 21st bit) and grants access to an address
within the valid address range. The fact that a bad address had been
written was concealed. There is the potential for problems with this model
if an application overwrites data stored in a particular address or it
reads in bad data, but often the problem is never revealed.
In the case of a Macintosh IIci this errant behavior is always revealed
and therefore a number of applications, CDEVs, INITs, and drivers that
worked fine up until the Macintosh IIci suddenly broke. Embarrassingly
enough, this change revealed problems with two internally developed Apple
products, version 3.0 of the CD-ROM driver and version 2.3 of MacTerminal.
The majority of developers who have encountered this problem are aware of
it and have either already made the corrections or are in the process of
making corrections necessary to make their applications Macintosh IIci
compatible. However this problem, which is difficult to diagnose, took
many developers by surprise.
IMPORTANT NOTE: It is important to understand that both the move to
non-contiguous memory and the utilization of the PMMU to capture invalid
addresses presaged changes that were required for compatibility with
System 7.0 and virtual memory.