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be altered or read via the SET/READ VALUE command thereafter. This enables the TBC to add
onto the linked list. Initially these receive queue access class pointers must point to a valid frame
descriptor for the TBC to link from. This frame descriptor, referred to as a 'dummy' frame de-
scriptor, must always be provided. For more details on the management of these pointers refer
to SECTION 4 BUFFER STRUCTURES.
Offset
E
D
C
B
A
48, 4C, 50, 54
MSB
Rx Queue Access Class EOQ Pointer - High Word
~------------------------------------------------------~~~~~~
4A, 4E, 52, 56
Rx Queue Access Class EOQ Pointer - Low Word
LSB
~----------------------------------------------------------~
2.1.18 Free Frame Descriptor Pointer
The free frame descriptor pointer is a 32-bit pointer containing the address of the first free frame
descriptor in the linked list of the free frame descriptor pool. This pointer is initialized by the host
through the initialization table and may be altered or read via the SET/READ VALUE command
thereafter. The TBC uses these free frame descriptors upon reception of a frame. Initially, the free
frame descriptor pointer must point to a valid frame descriptor. For more details on the man-
agement of these pointers refer to SECTION 4 BUFFER STRUCTURES.
Offset
58
5A
D
I
MSB
C
B
A
Head of FD Free Pool Pointer - High Word
Head of FD Free Pool Pointer - Low Word
2.1.19 Free Buffer Descriptor Pointer
LSB
The free buffer descriptor pointer is a 32-bit pointer containing the address of the first free buffer
descriptor in the linked list of the free buffer descriptor pool. The TBC uses these free buffer
descriptors upon reception of a frame. This pointer is initialized by the host through the initiali-
zation table and may be altered or read via the SET/READ VALUE command thereafter. For more
details on the management of these pointers refer to SECTION 4 BUFFER STRUCTURES.
Offset
5C
5E
D
MSB
2.1.20 Group Address Mask
C
B
A
Head of BD Free Pool Pointer - High Word
Head of SO Free Pool Pointer - Low Word
LSB
The group address mask is used to determine if a group addressed frame is to be accepted by
the station. A group addressed frame is denoted by the least significant bit of the destination
address field. This bit, called the I/G bit, must be one for a group addressed frame. A group address
is used to send messages to a group of logically related stations. The group address mask is
either two or six bytes in length, depending on the addressing mode of the TBC, and m",ust have
its least significant bit set to one. The group address mechanism can be disabled by setting the
group address mask to zero. Broadcast frames will always be accepted, regardless of the group
address mask. The mathematical expression for accepting frames using the group address mech-
anism, is:
Destination Address AND GA-MASK= Destination Address
MC68824 USER'S MANUAL
MOTOROLA
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