
MOTOROLA
Chapter 4. Burst Buffer Controller 2 Module
4-1
Chapter 4
Burst Buffer Controller 2 Module
The burst buffer controller module (BBC) consists of four main functional parts: the bus
interface unit (BIU), the instruction memory protection unit (IMPU), branch target buffer
(BTB) and the instruction code decompressor unit (ICDU). See
Figure 4-1. Information
The BBC master BIU interfaces between the RCPU instruction port and the internal U-bus
and can support burstable and non-burstable U-bus accesses.
The IMPU allows the instruction memory to be divided into four regions with different
protection attributes. The IMPU compares the attributes of the RCPU memory access
request with the attributes of the appropriate region. If the access is allowed, the proper
signals are sent to the BIU. If access to the memory region is disallowed because the region
is protected, an interrupt is sent to the RCPU and the master BIU cancels U-bus access.
The IMPU is able to relocate the RCPU exception vectors. The IMPU always maps the
exception vectors into the internal memory space of the MPC561/MPC563. This feature is
important for a multi-MPC561/MPC563 system, where, although the internal memories of
some controllers are not shifted to the lower 4 Mbytes, they can still have their own internal
exception vector tables with the same exception addresses issued by their RCPU cores.
The IMPU also supports an MPC561/MPC563-enhanced interrupt controller by extending
an exception vector’s relocation mechanism to translate the RCPU external interrupt
exception vector separately and splitting it into 48 different vectors, corresponding to the
The branch target buffer (BTB) improves the performance of the MPC561/MPC563 by
holding and supplying previously accessed or decompressed instructions to the RCPU core.
The BTB can be enabled in either decompression on or off mode.
The ICDU provides decompressed instructions to RCPU in the decompression ON mode
and contains a 2 Kbyte RAM (DECRAM) to hold decompression vocabularies. The
DECRAM can serve as a general purpose RAM memory on the U-bus if code compression
is not used.