5.0 Memory Organization
CN8478/CN8474A/CN8472A/CN8471A
5.2 Descriptors
Multichannel Synchronous Communications Controller (MUSYCC™)
A message descriptor is designed to be usable by both the transmit and receive
functions in MUSYCC. In providing this symmetry, a mechanism known as
self-servicing buffers is available, which allows the reuse of a single descriptor
for both the transmit and receive portions of a channel, and is designed for
diagnostics and loopback capabilities. Table 5-19 lists the message descriptor
details.
Table 5-19. Message Descriptor
Byte Offset
Field Name
dwords
Bytes
00h
Buffer Descriptor (Host Writes)
Buffer Status Descriptor (MUSYCC Writes)
1
4
04h
08h
Data Pointer
Next Pointer
TOTAL
1
1
3
4
4
12
5.2.4.1 Using Message
Descriptors
MUSYCC checks data from a message descriptor before processing the
associated data buffer. When a data buffer is completely processed (either
transmitted or received), MUSYCC overwrites the buffer descriptor field (the
first dword in a message descriptor) with a buffer status descriptor.
The buffer status descriptor specifies the number of bytes transferred, an end
of message indicator, and a buffer owner-bit indicator that assigns control of
associated buffers back to the host.
The owner bit transfers control of a data buffer between MUSYCC and the
host. The message descriptor can be assigned before an associated data buffer is
allocated in memory. In this case, MUSYCC polls the contents of the buffer
descriptor until the host grants ownership of a data buffer to MUSYCC. After
MUSYCC processes the data buffer, it grants the ownership back to the host.
The owner bit prevents MUSYCC from processing the same buffer twice
without intervention from the host. If MUSYCC detects an opening flag of a
received message, but does not have ownership of the current data buffer (via the
current message descriptor), an interrupt is sent to the host indicating that
MUSYCC needed a data buffer and did not have access to one.
The host can append additional information beyond the end of a data buffer as
long as the longest message length can be fitted first into the data buffer. In the
case of additional information, MUSYCC would not know about the information,
nor would it ever read from or write to that space.
For simplicity, the message level descriptions that follow are made in
reference to one channel. Each channel is serviced independently of other
channels, and separate descriptor lists are maintained for each supported channel.
Similarly, the transmit and receive sections of a channel service that the
descriptor lists identically and separate descriptor lists are maintained for each
section. Also, the size of data fields in the descriptors are identical; however, the
layout of fields between receive and transmit descriptors are different.
5-30
Conexant
100660E
CONEXANT [ CONEXANT SYSTEMS, INC ]
