Command Parameter Area - Motorola MC68824 User Manual

status of the queue to enabled and not empty and writes the pointer to the appropriate head of
transmit queue pointer entry in the private area.
This command should not be used to enable a disabled low priority (4, 2, 0) queue. The token
rotation timers of disabled priority queues are not maintained. Therefore, a lower priority queue
must be enabled at least one token rotation before queueing a frame for transmission on that
queue. Lower priority queues are preferably enabled in the OFFLINE state by the SET ONE WORD
command (see 3.5.3 SET VALUE Command). Note that adding a frame to a transmit queue which
is not empty and is enabled is an implicit request for transmission, therefore the START command
II
is not needed in this case.
The coding of this command is 84 and its format is shown below.
1
I
0
o
I
0
I
1
o
I
0
3.5 SET/READ VALUE
Commands in this category are used by the host processor to read or set TBC parameters residing
in the private area or on chip. These commands use the command parameter area in the initial-
ization table to transfer parameters between the host and the TBC.
3.5.1 Command Parameter Area
The command parameter area (CPA) is a seven word memory area within the initialization table
(offset 90 to 9C) used to set and read internal variables of the TBC which reside in the private
area or on chip via the SET/READ VALUE commands. The CPA consists of the command area
and the command result area.
The format of the command parameter area is shown in Figure 3-1. The command parameter
area is a three word area located at offset 90 through 94 of the initialization table. The first word
contains the opcode for the parameter to be set or read, see Tables 3-6 and 3-7 for the opcodes
list, while the other two words contain the parameters to be passed from the host processor to
the TBC.
INITIALIZATION TABLE
+ 90 CPA VALO (CODE)
+92 CPA VAll
+94 CPA VAL2
+96 CPA RElO
+98 CPA RET!
+9A CPA REl2
+9C CPA RESULT (STATUS WORD)
CODE - DEFINES THE CODE OF THE OPERATION IN READ/SET VALUE. THIS CODE IS ALSO USED FOR
THE START COMMAND (SEE 3.4.3 START).
VAL - DEFINES THE VALUE OF THE PARAMETER TO BE WRITTEN
RET - CONTAINS THE RETURNED PARAMETER FROM THE TBC
RESULT -CONTAINS THE STATUS
Figure 3-1. Command Parameter Area
MC68824 USER'S MANUAL
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