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PCI Bus Characteristics

The PCI bus on Power Macintosh follows the requirements of the PCI specification described in PCI Local Bus Specification, Revision 2.0. However, the PCI specification allows certain options. Table 1-2 shows the specification options chosen for the first implementation of the PCI bus in Power Macintosh computers.

Table 1-2 PCI options chosen for Power Macintosh

Option	Power Macintosh implementation
PCI clock rate	33 MHz (30 ns cycle time) and 66 MHz
Address/data bus width	32 bits and 64 bits
Signal voltage	5 V, 3.3 V
PCI address spaces supported	Memory,¹ I/O, and configuration
Minimum power supplied	5 V rail: 3 A (15 W) per slot² 3.3 V rail: 2 A (6.6 W) per slot²
PCI bus arbitration	Fair, round-robin, all slots master-capable
Mechanical bracket	ISA style
Plug-in card expansion ROM	Highly recommended³
IDSEL signals	Provided by resistive connections to AD lines
Interrupt routing	INTA#, INTB#, INTC#, INTD# wires combined by OR per slot to provide a unique slot interrupt for each card
LOCK#	Not used by the Macintosh system ⁴
PERR#, SERR#	Not used by the Macintosh system
SBO#, SDONE	Not used by the Macintosh system. No cache coherency (snooping) across the PCI bus
JTAG	Not used by the Macintosh system

Notes

¹ The Power Macintosh implementation does not support devices that address memory space below 1 MB.

² The PCI specification allocates power per slot, but the Macintosh implementation contains one power allocation for all slots. For example, a three-slot Power Macintosh computer has 9 A of 5 V power or 6 A of 3.3 V power available for PCI cards, which can be installed in any combination among the slots. Apple recommends that cards stay within the proportional allotment: 3 A for 5 V and 2 A for 3.3 V cards. However, configurations with fewer cards or lower-power cards can support other cards that need more power. These figures are minimum power allocations; some Power Macintosh models may provide more power for PCI cards.

³ While expansion ROMs are optional in the PCI specification, Apple strongly recommends their inclusion on plug-in cards. True "plug-and-play" operation (plug it in, turn it on, it is available during system boot) can be provided only when an expansion ROM contains both startup firmware and run-time driver code that supports the Open Firmware model for PCI cards. See Startup and System Configuration for more information on expansion ROM benefits, contents, and data formats.

⁴ LOCK# is an optional pin in the PCI specification.

Semaphores must be maintained in main system memory through processor control, using the routines described in Atomic Memory Operations. Power Macintosh does not support the use of semaphores in PCI memory space.