Chapter 12: 10/100 IEEE802.3 Media Access Controller
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
12-37
12.4 Memory Organization (Frame Descriptor)
This section describes the organization of the MAC Frame Descriptor in memory.
12.4.1 Descriptor Lists and Data Buffers
The MAC transfers frame data between the C790 memory and the FIFO using a Descriptor
list. The Descriptors that reside in the host memory act as pointers to locate memory buffers.
The Descriptor may be implemented in two different ways: ring or chain.
Figure 12-6 Descriptor Ring and Chain Structure
There are two Descriptor lists, one for receive and one for transmit. The base address of
each list is written to two registers: the Transmit Descriptor Pointer Register and the Receive
Descriptor Pointer Register. The Descriptor lists reside in C790 memory. Each Descriptor
can point to a maximum of two buffers. A data buffer consists of either an entire frame or
part of a frame, but it cannot exceed one frame. Buffers contain only data. Buffer status is
maintained in the Descriptor.
The last Descriptor can point back to the first entry when creating a ring structure.
Descriptor 0
Buffer 1
Buffer 2
Descriptor 1
Buffer 1
Buffer 2
Descriptor n
Buffer 1
Buffer 2
.
..
Ring Structure
Descriptor 0
Buffer 1
Descriptor 1
Buffer 1
Descriptor m
Buffer 1
.
..
Chain Structure
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