IBM RS/6000 - Manual (Page 43)

2.3 Electronics Partitioning
The system electronics are divided into two cards for packaging. One card
contains the processor and its support electronics, master clock generation,
memory controller and IPL microcode. The other card is considerably larger and
contains the native I/O subsystems, memory modules and I/O expansion slots.
Figure 7 shows how the components are split across the two boards in the system.
Figure 7. PCI-Based RS/6000 Server's Electronics Partitioning
The division of logic across the two cards was carefully chosen so that it would split
the electronics along industry standard open interfaces only. Furthermore,
packaging the processor and memory controller on the same card allows much
greater flexibility in the design for future upgrade cards. As new processor and
memory controllers are developed, they may be offered to customers as
performance enhancements upgrades while protecting the customer investment in
memory, I/O and packaging.
The electronics on the processor card comprise the core of the PowerPC
Reference Platform architecture. The processor card's interface boundary
separates the PReP architecture from the mostly PC architecture of the I/O
Motherboard. So this “processor card agility” allows other processor cards to be
designed and used with the same I/O Motherboard and mechanical package.
Four industry standard interfaces are used to attach the processor to the I/O
subsystem:
A generic 72-bit, non-interleaved memory DIMM interface to connect the
memory controller on the processor card to the DRAM modules
A 32-bit PCI bus running at up to 33 MHz for I/O subsystem attachment
Chapter 2. PCI-Based RS/6000 Server Hardware 19