Service manual

System Overview 1-17
AlphaServer 4000/4100 systems use the Alpha chip for the CPU. The CPU,
memory, and I/O bridge modules, one to PCI/EISA I/O buses and another (4000
only) to another pair of PCIs, are connected to the system bus motherboard. A
fourth type of module, the power control module, also plugs into the system
motherboard. A fully configured 4100 system drawer can have up to four CPUs,
four memory pairs, and a total of eight I/O options. The I/O options can be all PCI
options or a combination of PCI options and EISA options, but there can be no more
than three EISA options. A fully configured 4000 system drawer can have up to two
CPUs, two memory pairs, and a total of sixteen I/O options. The I/O options can be
all PCI options or a combination of PCI options and EISA options, but there can be
no more than three EISA options.
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 which 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 are available with and without 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). Some variants
of the CPU module include an onboard cache. The cache system is write-back. The
system drawer supports up to four CPUs.
The memory modules are placed on the system motherboard in pairs. Each module
drives half of the system bus, along with the associated ECC bits. Memory pairs
consist of two modules that are the same size and type. Two types are available:
synchronous and asynchronous (EDO) memory.
The system bus to PCI bus bridge module 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. Both the 4100 and the 4000
have one PCI/EISA card cage, and the 4000 may contain a second PCI card cage.
The power control module, which is on the system motherboard, monitors power and
the system environment.