MPC555
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MPC556
BURST BUFFER
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
4-4
• Slight changes in the core and existing RISC development tools — compilers,
simulators, manually coded libraries.
• Compressed address space is up to four Megabytes (4 Mbytes).
• Branch displacement from its target:
— Conditional branch displacement is up to two Kbytes (2 Kbytes).
— Unconditional branch displacement is up to two Mbytes (2 Mbytes).
NOTE
Branch displacement is hardware limited. The compiler can enlarge
the branch scope by creating branch chains.
4.3.2 Model Limitations
No address arithmetic is allowed, because the address map changes during compres-
sion and no software tool can identify address arithmetic structures in the code.
4.3.3 Vocabulary Based Instruction Compression Algorithm
The code compression algorithm is based on creating vocabularies of frequently
appearing PowerPC RISC instructions or instruction halves and replacing these
instructions with pointers to the vocabularies.
Compressed and bypass field lengths may vary. An example of compressed code is
shown in
.
Compression of the instructions in a vocabulary may be in one of the following modes.
1. Compression of the whole instruction into four vocabulary byte pointers. The
four compacted bytes may start on any bit location. Four of the decoded bits
and another bit for starting from the left or right side of the address location de-
termine the bit location for the byte start
2. Compression of a combination of the instruction’s bytes into vocabulary point-
ers and bypass of the other byte(s). Bypass is the placing of the field’s uncom-
pressed instruction information into the compressed code.
3. Bypass of the whole instruction. No compaction permitted.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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