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Branch Target Buffer
access rights to the DECRAM array may be defined by programming the R, D, and S bits
of the BBCMCR register:
• Read/write or read only
• Instruction/data or data only
• Supervisor/user or supervisor only
External access mode of the RAM is activated by the BBCMCR[DCAE] bit setting (see
Section 4.6.2.1, "BBC Module Configuration Register (BBCMCR))." In this mode the
DECRAM can be accessed from the U-bus and cannot be accessed by the ICDU logic.
In this mode:
• The DECRAM supports word, half-word and byte operations.
• The DECRAM is emulated to be 32 bits wide. For example: a load access from
offset 0 in the DECRAM will deliver the concatenation of the first word in each of
the DECRAM banks when RAM 1 contains the 16 LSB of the word and RAM 2
contains the 16 MSB.
• Load accesses at any width are supplied with 32 bits of valid data.
• The DECRAM communicates with the U-bus pipeline but does not support
pipelined accesses to itself. If a store operation is second in the U-bus pipe, the store
is carried out immediately and the U-bus acknowledgment is performed when the
previous transaction in the pipe completes.
• Burst access is not supported.
4.4.1.1
Memory Protection Violations
The DECRAM module does not acknowledge U-bus accesses that violate the configuration
defined in the BBCMCR. This causes the machine check exception for the internal RCPU
or an error condition for the MPC533 external master.
4.4.1.2
DECRAM Standby Operation Mode
The bus interface and DECRAM control logic are powered by V
array is supplied by a separate power pin (). If main power is shut off, may subsequently
be lowered for purposes of low voltage data retention.
When the DECRAM array is powered only by the pin, access to the RAM array is blocked.
4.5
Branch Target Buffer
The burst buffer controller contains a branch target buffer (BTB) to reduce the impact of
branches on performance.
Following is a summary of the BTB features:
4-12
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
MPC533 Reference Manual
supply. The memory
DD
MOTOROLA
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