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PCI and NuBus

This section provides some background about the differences between the PCI and NuBus architectures. The PCI bus exhibits a number of fundamental differences from NuBus , the previous Macintosh bus standard. The most important of these differences are listed in Table 1-1.

Table 1-1 Comparison of NuBus and the PCI bus

Feature	NuBus	PCI bus
Bus clock rate	10 MHz	33 MHz, 66 MHz
Addressing	Geographic	Dynamic
Signal loading	No enforced rules	One load per signal
Transaction length determination	Determined at start of transaction	Determined at end of transaction
Bus termination	Resistor network	Not required
Bus control arbitration	Distributed	Centralized
Addressing spaces	Memory only	Memory, I/O, and configuration
Wait-state generators	Slave only	Slave and master
Kinds of expansion	Cards only	Cards and ASIC chips
Timeout	255 bus clocks	5 bus clocks
Burst capability	8, 16, 32, or 64 bytes	Any number of bytes
Power allocation	15 W per card	7.5, 15, or 25 W per card