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Chapter 8
Input/Output Overview
Conceptually, Alpha systems can consist of processors, memory, a processor-memory inter-
connect (PMI), I/O buses, bridges, and I/O devices.
Figure 8–1 shows the Alpha system overview.
Figure 8–1: Alpha System Overview
As shown in Figure 8–1, processors, memory, and possibly I/O devices, are connected by a
PMI.
A bridge connects an I/O bus to the system, either directly to the PMI or through another I/O
bus. The I/O bus address space is available to the processor either directly or indirectly. Indi-
rect access is provided through either an I/O mailbox or an I/O mapping mechanism. The I/O
mapping mechanism includes provisions for mapping between PMI and I/O bus addresses and
access to I/O bus operations.
Alpha I/O operations can include:
• Accesses between the processor and an I/O device across the PMI
• Accesses between the processor and an I/O device across an I/O bus
• DMA accesses — I/O devices initiating reads and writes to memory
• Processor interrupts requested by devices
• Bus-specific I/O accesses
Processor-Memory Interconnect
I/O Device Processor Memory
I/O Bus
I/O Device I/O Device
Bridge