Shared Memory; Overview Of Tbc Functional Operation; Command Structure Overview; Frame Reception Overview - Motorola MC68824 User Manual

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1.7 SHARED MEMORY
The TBC and the host microprocessor interface through shared memory which consists of:
Initialization Table
Private Area
Fully Linked Buffer Structure
Frame Descriptors
Buffer Descriptors
Data Buffers
Initialization Table
Contains all of the initial parameters necessary for TBC network operation and is prepared
by the host prior to the initialization of the TBC.
Private Area
An external scratch pad for the TBC containing some TBC parameters.
Fully Linked Buffer Structure
Consists of frame descriptors, buffer descriptors, and data buffers.
Frame descriptors contain control information for each frame and are linked together to form
each priority queue.
Buffer descriptors are pointed to by frame descriptors and contain control information for
the data. Buffer descriptors contain an offset value set by the user which tells how far from
the beginning of the data buffer the actual data begins. The offset feature can be used to
add or subtract upper layer headers making it unnecessary to recopy data.
Data buffers contain only data and are flexible in size.
1.8 OVERVIEW OF TBC FUNCTIONAL OPERATION
The following paragraphs provide a high level summary of the command structure, frame recep-
tion, and frame transmission.
1.8.1 COMMAND STRUCTURE OVERVIEW
The host issues a command to the TBC by writing the 8-bit command into the command register.
The commands belong to one of the following categories:
Initialization
Set Operation Mode
Transmit Data Frames
Set/Read Value
Test
Notify TBC
Modem Control
1.8.2 FRAME RECEPTION OVERVIEW
A frame is received by a station if the address comparison is true. For group addressed frame
reception, the address comparison uses the group address mask, this station's address, and the
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MC68824 USER'S MANUAL