Table of Contents
Advertisement
Class-Based Compression Model Main Principles
routines (or branches to them) should start (reside) at the same location in memory as
noncompressed ones. The BBC ICDU passes the vectors unchanged to the MCU internal
bus and provides corresponding compressed address to the RCPU together with the first
exception handler instruction opcode.
This scheme allows use of the BBC exception relocation feature regardless of the MCU
operational mode.
G.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).
G.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.
G-8
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
MPC533 Reference Manual
MOTOROLA
Advertisement
Table of Contents
