
MPC562/MPC564 Compression Features
MPC561/MPC563 Reference Manual, Rev. 1.2
A-12
Freescale Semiconductor
The vocabulary and class configurations are generated by profiling the static code, based on the instruction
class algorithm.
The code compression can be created by using either default or specific application vocabularies,
generated at the previous step. In case of default vocabularies, the generation step can be omitted, but
compression efficiency is reduced.
The compression tool replaces regular PowerPC ISA instructions with a compressed representation that
contains fewer bits. The tool also updates offset fields in direct branch instructions to include a compressed
format offset (four bits of IP and word offset). Thus, maximum branch offsets in decompression on mode
are reduced. The RCPU uses the word offset for direct branch target address computation. The RCPU
provides the instruction pointer portion of the branch offset field to the decompression unit as it is
represented in the branch instruction.
Figure A-11. Code Compression Process
A.2.12
Decompression
The instruction code is stored in the memory in the compressed format
The vocabularies are stored in a dedicated ICDU RAM (DECRAM)
The class configuration is stored in a dedicated ICDU register (DCCR)
The decompression is done on-line by the dedicated decompressor unit
— RCPU provides to the BBC a 2-bit aligned change of flow (COF) address
Compiler/
Program
Executable
Compressor
Linker
Tool
Vocabulary
Generator
Classes
Non-compressed
Program
Executable
Compressed
Vocabulary
Generation Tool