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Operational Registers
6.1.4 Message Buffer (MB0—MB15)
The MDEU hashes a message contained in the 16-word Message Buffer. The message
should be processed such that a single-character message would be written to MB0. MB15
should only be programmed if the message block uses at least 481 bits.
The Message Buffer is not cleared upon completion of a computation process. Therefore,
when programming the final block of a multi-block message, all locations should be
appropriately written using the padding required by the selected Message Digest algorithm.
The message is double-buffered; once hashing begins the MDEU does not depend on the
value stored in the Message Buffer. Therefore, the next block of a multi-block message may
be written as soon as MSR:BE is asserted.
If IPAD or OPAD are asserted while the Message Buffer is written, then the value stored
will be the value applied to the data bus exclusive-ORed with the appropriate pad value. In
addition, assertion of OPAD causes the contents of the Message Digest Buffer to be copied
into the first four or five words of the Message Buffer, with all other words set appropriately
for a two-block message.
6.1.5 Message Digest Buffer (MA–ME)
When DONE and IRQ are asserted, the current hash value for all message blocks processed
since the last reset are available in Message Digest Buffer locations MA–ME. For MD4 and
MD5, which produce a 128 bit hash, ME is to be ignored.
Chapter 6. Message Digest Execution Unit
6-5
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
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