System Overview
1-9
The AlphaServer DS20 is a switch-based interconnect system; it uses a cross-bar
switch chipset that allows data to move directly from place to place in the system. The
CPU, memory, and I/O devices physically connect to the system board and each has
one or two logical connections to the switch. The arrows on the block diagram shown
in Figure 1-4 indicate the flow of data, command/address, and control signals.
On the system board is:
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A system switch consisting of a control chip (C-chip) and 8 data chips (D-chips)
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Three buses to the D-chips: the memory data bus, the CPU data bus, and the
P-chip address and data bus (PAD bus)
•
One bus from the C-chip to the P-chips (CAP bus)
•
Two 60 command/address and control connections between each CPU and the
C-chip: the command/address line from the CPUs to the C-chip and the probe
address lines from the C-chip to the CPUs
•
A TIG bus connected to the C-chip
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Control lines from the C-chip to the D-chips
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Control, timing, and address lines from the C-chip to each memory array
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Two 64-bit PCI buses with three PCI option slots each
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One ISA bus bridged on PCI0 and two SCSI ports (unused at FRS) also on PCI0
(If an ISA option is used, PCI 1 slot 9 cannot be used for a PCI option.)
•
One ISA to XBUS bridge to the built-in XBUS options
A fully configured pedestal system can have two CPUs, sixteen DIMMs, a total of six
I/O options, and seven 18 Gbyte UltraSCSI disks. Maximum memory is 4 Gbytes.
The I/O options can be all PCI options or five PCI options and a single ISA option.
Other major lines in the system are used for command, addresses, and control.
For information on CPUs, see Section 1.5.
For information on memory, see Sections 1.6 and 1.7.
There are two 64-bit PCI buses connected to the cross-bar switch chipset by two PAD
buses and a single CAP bus. Each bus has three PCI slots for I/O devices. PCI bus 0
has an ISA bridge and a SCSI adapter with two ports (not used) embedded on the bus.
A single ISA slot is available on PCI bus 0 that, when used, eliminates the use of one
PCI slot on PCI bus 1.
Logic and sensors on the system feature module monitor power status and the system
environment (temperature and fan speeds).
