BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
10
Section 3: System Overview
Document
1250_1125-UM100CB-R
This section gives an overview of the whole system. Each of the system elements are described in detail in
later sections.
The ZBbus is the on-chip multiprocessor system bus:
•
Data is 256 bits wide, running at half the CPU frequency. (For an 800 MHz part this gives (800/2)*256 =
102.4 Gbit/s bus bandwidth).
•
The address is 40 bits (matching the CPU physical address and the HyperTransport I/O fabric address).
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The separate address and data sections are arbitrated for independently, and operate as a split
transaction bus.
•
All transactions are tagged, data responses can come back in any order.
•
The cache block ownership protocol (MESI) ensures memory coherence is maintained.
•
Individual buffer-full indications from each bus target allow selective flow-control by requestors.
The processors are Broadcom SB-1 CPUs implementing the MIPS64 architecture. These are described in
detail in the SB-1 User Manual. On the BCM1250 the two CPUs are identical in all respects apart from the
processor number that will be read from the Processor Identification register (CP0 register 15). The reset logic
in the System Control and Debug unit (SCD) is different for the two processors. Following a system reset only
CPU 0 will be brought out of reset, it can then setup the system and release CPU 1 when ready. In the
BCM1125/H parts the processor is CPU 0 and will report that it is a uniprocessor in the Processor Identification
Register.
The Level 2 cache (L2) is organized slightly differently than L2 caches in other systems. It is shared by the
processor(s) and any I/O DMA masters. It is best understood as a cache on the front of the memory (as shown
in
), rather than by using the traditional model where the L2 is associated with a CPU. All memory
accesses are checked in the L2. The bus includes a signal to indicate that the data should be allocated in the
L2 cache on an L2 miss, this signal may be used by DMA masters to write data to the L2 so it can quickly be
accessed by the CPU (this has to be done in a controlled way to avoid disrupting the normal gains of having a
L2 cache).
The I/O bridges isolate the peripherals from the bus, and implement the bus and coherence protocols. They
include buffering to allow multiple outstanding I/O requests, and support for DMA masters. The PCI and
HyperTransport expansion buses share an I/O bridge, so any peer-to-peer traffic (between PCI and
HyperTransport devices) is hidden from the ZBbus.
The part can be run as a big endian system or a little endian system. This is set by an input that is read at reset
time. Internally the few data paths that need to will swap byte lanes, however the system is designed to have
as few changes as possible. The CPU endian mode bit (the BE bit in the CP0 Config register) will always reflect
the endian mode of the system. The interface to the PCI and HyperTransport expansion buses includes byte
lane swappers that can be used if required to interface to these devices.