Fast system bus speeds may cause a reduction in both performance
and stability when upgrading a PowerPC 601, 603 or 604 based
Mac to a G3 card based on the PowerPC 750 processor. In order
to understand this confusing issue a basic understanding of
the dual bus G3 architecture is necessary.
The PowerPC 601, 603 and 604 processors have one bus. The
I/O, Cache memory and system memory (SIMMs and DIMMs) all
share this bus. This is to say these devices share the same
physical connections for communication with the processor.
By far the fastest device on the bus is the L2 cache. L2 cache
is relatively small high speed static memory. The cache controller
predicts what data is likely to be needed from the slower
DRAM based system memory and preloads this data to the faster
A "cache hit" occurs when data requested by the
processor is found in the cache. In the case of a cache hit
the data will be accessed at full bus speed, usually between,
25ns (40 MHz bus) and 17ns (60 MHz bus). For a single bus
system increasing the bus speed gives better performance.
A "cache miss" occurs if the requested data is
not found in the cache memory. In the case of a cache miss
the data must be fetched from the slower system memory. In
a single bus system the fast cache memory along with the slower
memory and other slower I/O devices are connected to a common
bus. This mismatch in device speed and bus speed is handled
by adding "wait states" when accesses are made to
slower devices. Once a bus cycle has started, wait states
are added by holding the state of the bus for one or more
additional bus clock cycles.
The likelihood of any particular access being a cache hit
or miss will vary depending on the cache load algorithm and
on the application itself. A typical system will usually average
a 70% to 80% cache hit rate making fast cache accesses the
majority of bus activity.
The MAXpowr G3 cards from Newer Technology, based on the
Power PC 750 processor, have two buses. One for the traditional
I/O and system memory and a second bus dedicated only for
the L2 cache. Because the second bus is dedicated to high
speed memory it allows for much faster L2 cache accesses than
a single bus system could have. This high speed memory is
soldered directly to the MAXpowr G3 card so access times of
3ns to 8ns are possible. This eliminates the need for a fast
system bus clock because the L2 cache is no longer on the
System DRAM memory have access times of either 60ns or 70ns
so wait states are required to access this memory. A faster
system bus clock may actually reduce performance due to the
required additional wait states. For best performance with
a dual bus system such as MAXpowr G3 it is more important
to match, or sync,the system bus frequency with the speed
of the system memory. Wait states can only be added in increments
equal to the period of the system bus clock frequency. There
is also some fixed overhead time due to system components
of about 5ns which must be accounted for.
The idea is to pick a system bus clock where a given number
of wait states minus 5ns will access system memory closest
to the access time rating of the installed memory modules.
This must be done without violating the memory timing margins.
For 60ns memory the best bus clock is around 45 MHz with two
wait states. Increasing the bus much beyond 46 MHz will require
an additional wait state reducing overall performance.
Because wait states can be confusing for many users to understand,
the MAXpowr G3 comes with a control panel which allows the
user to specify the slowest speed of the memory installed.
The hardware and software automatically configure the card
and mother board system memory for best performance. Setting
the system bus to something other than the mid forties MHz
factory default may compromise the ability of the control
panel to optimize for best performance.
Operating the bus at higher clock rates not only reduces
memory performance but may causes other problems. The Power
PC 750 G3 processor has narrow timing margins as compared
to the 601, 603 and 604 processors which these systems were
designed for. There is much less timing margin to push the
system bus faster, and as mentioned above, there is no advantage.
As a side note, future machines will use a new kind of DRAM
called Synchronous DRAM (SDRAM). These modules are optimized
for moving blocks of data at full bus speed taking full advantage
of the faster clock. They do however require several wait
states for random data access. The MAXpowr G3 upgrade cards
are designed for the existing Macintosh and Macintosh clones
which use either Fast Page Mode (FPM) or Extended Data Out
(EDO) which require waits states as explained above.
Some upgrade card manufactures may not have the expertise
or the technology to optimize the bus speed and system memory
timing for best performance. Worse yet, they may inadvertently
be pushing the system bus without making the necessary corrections
to the system memory timing. This will over clock the memory
which could result in unreliable operation or data corruption.
Not all bench mark tools will indicate a performance increase
from faster memory access due to the high cache hit rate with
these applications. However, real world applications will
benefit from optimized system memory timing.
V.P. of Engineering
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