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CHAPTER 5 SAR FUNCTION
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V, D/P, S/M bit
Table 5-2. Function of Packet Descriptor
V
D/P
S/M
Function
0
-
-
Vacant packet descriptor
1
0
-
Link pointer
1
1
0
Packet descriptor in multi-buffer mode
1
1
1
Packet descriptor in single-buffer mode
The V bit is set to "1" if the packet descriptor contains data to be transmitted or if the packet
descriptor is functioning as a link pointer, indicating that the packet descriptor is valid. If the V bit is
cleared to "0," the packet descriptor is handled as a "vacant packet descriptor."
The vacant packet descriptor is required to enable the
μ
PD98405 to recognize the last valid packet
descriptor in a chain of packet descriptors. Therefore, the vacant packet descriptor must be located
immediately after one or more valid packet descriptors. If the V bit is set to "0," indicating a vacant
packet descriptor, areas other than the V bit of that packet descriptor have no meaning, such that the
μ
PD98405 ignores these areas. Because the
μ
PD98405 reads a packet descriptor in 4-word units,
even a vacant packet must occupy a 4-word area.
Caution
The host must not set the V bit to 1 until all packet descriptor areas other than the V
bit have been set. If the V bit in Word 0 is set to 1 before Words 1 to 3 have been
set, the
μ
PD98405 will finish processing of the previous packet descriptor before
the host finishes setting it as a valid packet descriptor. This operation may result in
a malfunction.
Figure 5-9. Location of Vacant Descriptor
V = 1
V = 1
Valid packet descriptor
Valid packet descriptor
Vacant packet descriptor
V = 0
Transmit queue
Movement of
transmit queue
read pointer
The packet descriptor can also function as a "link pointer" depending on the setting of the D/P bit.
This allows the user to handle a transmit queue either as a linear or ring arrangement. When the
packet descriptor is used as a link pointer, the first address of the next valid packet descriptor is
stored into the "ADDRESS" field. In this case, those fields other than the V and D/P bits and
ADDRESS field of the packet descriptor have no meaning, and are ignored by the
μ
PD98405.