MPC562/MPC564 Compression Features
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
A-7
A.2.7
Class Code Compression Algorithm Rules
•
Compressed instruction length may vary between 6 and 36 bits and is even.
•
A compressed instruction can begin at any even location in a memory word.
•
An instruction source may be compressed as a single 32-bit segment or as two independent 16-bit
segments.
•
Possible partitions of an instruction for compression are:
– One 32-bit bypass segment
– One 32-bit compressed segment
– One 16-bit compressed segment and one 16-bit bypass segment
– Two 16-bit compressed segments
•
A bypass field is always the second field of the two possible. Length of a bypass field can be zero,
10, 15, 16 or 32 bits.
•
The class prefix in a compressed instruction is 4 bits long and covers up to 16 classes.
•
The vocabulary table pointer of each field may be 2 to 9 bits long.
•
Vocabulary table pointers are reversed in the code. This means the pointer’s LSB will be the first
bit.
•
In a class with a single segment of full compression, data is fetched from both memories.
•
Every vocabulary table in the DECRAM is 16 bytes (8 entries) aligned (3 LSBs zeroed).
A.2.8
Bypass Field Compression Rules
The bypass field can be either a full bypass, (i.e., the whole segment from the un-compressed instruction
appears as is in the compressed instruction), or it can be represented in one of several compression
encoding formats. These formats are hard-wired in the decompression module.
A.2.8.1
Branch Right Segment Compression #1
For the MPC562/MPC564, a 15-bit bypass is used to indicate that the AA bit of a branch instruction should
be inserted with a value of zero. The decompression process is performed as shown in
Figure A-4. Branch Right Segment Compression #1
This bypass is coded by a value of “13” (0xD) in the TP2LEN field of the DCCR register.
15-bit Compressed
Decompressed
0
13
14
16
29
30
Right Segment
31
0
LK
Bypass Field
Summary of Contents for MPC561
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