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220 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
SRAM Interface
6.7.2 Microcode Restrictions to Maintain Ordering
The microcode programmer must ensure order where the program flow requires order and where
the architecture does not guarantee that order. One mechanism that can be used to do this is
signaling. For example, if the microcode needs to update several locations in a table, a location in
SRAM can be used to lock access to the table. Example 31 is the microcode for this table update.
Table 78. Q_array Entry Reference Order
1
st
ref
2
nd
ref
Read_Q
_Descr
head,
tail
Read_
Q_Des
cr
other
Write_Q
_Descr
Enqueue Dequeue Put Get Journal
Read_Q_Descr
head,tail
Order
1
1. The order of Read_Q_Descr_head/tail after Write_Q_Descr to the same element will be guaranteed only if it is to a different
descriptor SRAM address. The order of Read_Q_Descr_head/tail after Write_Q_Descr to the same element with the same
descriptor SRAM address is not guaranteed and should be handled by the Microengine code.
Read_Q_
Descr other
Order
Write_Q_
Descr
2
2. Write_Q_Descr reference order is not guaranteed after any of the other references. The Queue array hardware assumes
that the Microengine managing the cached entries will flush an element ONLY when it becomes the LRU in the Microengine
CAM. Using this scheme, the time between the last use of this element and the write reference is sufficient to guarantee the
order.
Enqueue Order Order Order Order
3
3. Order between Enqueue references and Dequeue references are guaranteed only when the Queue is empty or near empty.
Dequeue Order Order Order
3
Order
Put Order
Get Order
Journal Order
Example 31. Table Update Microcode
IMMED [$xfer0, 1]
SRAM [write, $xfer0, flag_address, 0, 1, ctx_swap [SIG_DONE_2]
; At this point, the write to flag_address has passed the point of coherency. Do
the table updates.
SRAM [write, $xfer1, table_base, offset1, 2] , sig_done [SIG_DONE_3]
SRAM [write, $xfer3, table_base, offset2, 2] , sig_done [SIG_DONE_4]
CTX_ARB [SIG_DONE_3, SIG_DONE_4]
; At this point, the table writes have passed the point of coherency. Clear the
flag to allow access by other threads.
IMMED [$xfer0, 0]
SRAM [write, $xfer0, flag_address, 0, 1, ctx_swap [SIG_DONE_2]