System Overview 1-9
AlphaServer 1200 systems use the Alpha chip for the CPU. The CPU, memory, and
I/O devices connect to the system motherboard. On the system motherboard is:
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the system bus
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two system bus to PCI bus chip sets that bridge two PCI busses to the system bus
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two 64-bit PCI busses with three PCI options slots each
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one EISA/ISA bus bridged to one of the PCIs (If an EISA/ISA option is used,
one PCI slot cannot be used)
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one CD-ROM controller built-in to the other PCI
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one EISA/ISA to XBUS bridge to the built-in XBUS options
A fully configured 1200 system can have two CPUs, one memory pair, and a total of
six I/O options. The I/O options can be all PCI options or a combination of PCI
options and a single EISA/ISA option.
The system bus has a 144-bit data bus, protected by 16 bits of ECC, and a 40-bit
command/address bus, protected by parity. The bus speed depends on the speed of
the CPU in slot 0 that provides the clock for the buses. The 40-bit address bus can
create one terabyte of addresses (that’s a million billion). The bus connects CPUs,
memory, and the system bus to PCI bus bridge(s).
The CPU modules have an external cache. The Alpha chip has an 8-Kbyte
instruction cache (I-cache), an 8-Kbyte write-through data cache (D-cache), and a
96-Kbyte, write-back secondary data cache (S-cache). The cache system is write-
back. The system supports up to two CPUs.
Memory on 1200 systems is constructed of DIMM memory pairs place onto two
memory modules called riser cards. The riser cards are placed into the two memory
slots on the system motherboard. One member of a DIMM pair is placed onto one
riser card, and the other member is placed onto another riser card. Each riser card
drives half of the system bus, along with the associated ECC bits. Memory pairs
consist of two synchronous DIMMs of the same size and are placed into the same
slot on each riser card.
The system bus to PCI bus bridge chip set translates system bus commands and data
addressed to I/O space to PCI commands and data. It also translates PCI bus
commands and data addressed to system memory or CPUs to system bus commands
and data. The PCI bus is a 64-bit wide bus used for I/O.
Logic and sensors on the system motherboard monitor power status and the system
environment (temperature and fan speeds).
