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G.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
• Decompression flow is as follows: (See Figure G-12)
— RCPU provides to the BBC a 2-bit aligned COF
— The ICDU:
– Converts the COF address to a word-aligned physical address to access the
memory
– Fetches the compressed instruction code from the memory, decompresses it
and delivers non-compressed instruction code, together with the bit-aligned
next instruction address, to the RCPU.
Compressed
Instructions
Memory
COF Word Aligned
Physical Address
Compressed
G.2.13 Compression Environment Initialization
In order to commence the execution of the compressed code, the DECRAM and the class
information (in the DCCR registers) must be programmed. The data to be programmed is
1
COF = Change of Flow
MOTOROLA
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
De
compressor
Vocabulary
Instruction
Code
Classes (DCCR)
Registers
ICDU
Figure G-12. Code Decompression Process
Appendix G. MPC534 Compression Features
Class-Based Compression Model Main Principles
1
address
Bit-Aligned COF
Address
Noncompressed
Instruction Code
Compressed Space
"Next Instruction"
Address
MPC500
Embedded
CPU
G-15
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