Examples Of Queues Status - Motorola MC68824 User Manual

II
NOT SURE QUEUE
A transmission queue for which the host cannot determine which of the two previous cat-
egories the queue belongs to. This occurs when confirmation was given to the frame pre-
ceding the final frame, but not the last frame in the queue, so the user is not sure whether
the last frame in the queue will be determined to be empty before the next frame is linked
onto it.
4.2.3 Examples of TBC Transmission Queues
NPV - Next Pointer Valid
This bit resides in the control word for the next FD.
a
Next pointer is not valid
1 Next pointer is valid
The NPV bit is set and cleared by the host.
CFD - Confirm Frame Descriptor
This bit resides in the confirmation/indication word of the FD
a
TBC did not complete this frame's transmission
1 TBC completed this frame's transmission
EMP- Empty
This bit resides in the confirmation/indication word of the FD
a
This queue is not empty
1 This queue is empty
EMP reflects the value of NPV as read by the TBC. EMP is not valid if CFD
= a
because the host
clears it when preparing the frame.
The CFD and EMP bits are set and cleared by the TBC in a single memory access after sampling
NPV.
Figure 4-5 is an illustration of each of the status of queues as designated in 4.2.2 Management
of Transmission Queues. Each block represents a frame and the arrows indicate the order of
transmission.
MOTOROLA
.II 1n
ACTIVE QUEUE
NPV = 1
cm
=
1
EMP = 0
EMPTY QUEUE
NPV:= 1
cm:= 1
EMP:=
0
'NOT SURE' QUEUE
NPV
=
1
CFD
=
1
EMP
=
0
NPV
=
1
cm
=
0
EMP
=
X
NPV
=
1
cm
=
1
EMP
=
0
NPV
=
1
CFD
=
1
EMP
=
0
NPV = 1
cm
=
0
EMP
=
X
NPV = 1
cm:= 1
EMP:= 0
NPV:= 1
cm:= 1
EMP:= 0
Figure 4-5. Examples of Queues Status
NPV
=
0
cm
=
0
EMP
=
X
NPV
=
0
CFD
=
1
EMp:= 1
NPV:= 0
cm
=
0
EMp:= X
MC68824 USER'S MANUAL