Pipelined Virtual-Channel-Based Routers - Microarchitecture of Network-on-Chip Routers - page 152

SU

outVCAvailable

VA SA

SA req RC VA req

"0...00"

ready VC

candidateOutVC

en

outPort[i]

outVC[i]

RC

deMUX

en

CC

dst

credit
update outVCLock[i]

head

SU

ready
valid
data

replace VC id field

ST

Input VC #i

from other VCs
of the same input

from other inputs

Output #j

Fig. 9.8 The organization of a 2-stage pipelined VC-based router that pipelines VA from switch
allocation and traversal. RC and VA are executed in series in one pipeline stage and SA, ST in
the second one. The outVC Œi and outVCLock Œi state variables per input VC play the role of the
pipeline registers in this configuration

0 1 2 3 4 5 6 7

RC - V SU

CC

H

LT - BW LT - BW

SA-DQ-ST

B

SA-DQ-S CC

LT- BW LT - BW

CC
SU

T

LT - BW SA-DQ-ST LT - BW

RC - V SU

SA-DQ-S CC

H

LT - BW LT - BW

Fig. 9.9 An example of the operation of a 2-stage pipelined router that executes RC and VA in the
first pipeline stage and SA-ST in the next, for the flits of two packets that arrive at the same input
VC

9.3.1 Example 1: Two Packets Arriving at the Same Input VC

Figure 9.9 depicts the effect of this pipeline configuration on the operation of the
router. In cycle 0, a head flit is written at the input VC buffer and is able to perform
RC in cycle 1, since it is at the frontmost position of the corresponding buffer. In the
same cycle, a set of candidate output VCs, along with the computed output port are
sent to the VC allocator, which replies, by asserting the VCgranted Œi signal, with
an allocated output VC, indexed by the selOutVC Œi . The state of the output VC is
updated, i.e., outVCAvailable Œj D 0, and so does the input VC's state variables,
outVC Œi and outVCLock Œi . Due to the pipelined configuration, the update of the
input VC state variables will be visible in the SA request generation logic in the
next cycle. In the same cycle, a body flit arrives at the same input VC buffer. In cycle
2, the assertion of the outVCLock Œi variable passes to the next pipeline stage and
the new values of outVC Œi and outPort Œi values are used to setup a request to SA.