Figure 8.4: Packet format of the BlackWidow network.
8.1.6 NETWORK LAYER FLOW CONTROL
The allocation unit for flow control is a 24-bit phit — thus, the phit is really the flit (flow control
unit). The BlackWidow network uses two virtual channels (VCs) [ 21 ], designated request ( v =0) and
response ( v =1) to avoid request-response deadlocks in the network. Therefore, all buffer resources
are allocated according to the virtual channel bit in the head phit. Each input buffer is 256 phits
and is sized to cover the round-trip latency across the network channel. Virtual cut-through flow
control [ 37 ] is used across the network links.
8.1.7 DATA-LINK LAYER PROTOCOL
The YARC data-link layer protocol is implemented by the link control block (LCB). The LCB
receives phits from the router core and injects them into the serializer logic where they are transmitted
over the physical medium. The primary function of the LCB is to reliably transmit packets over the
network links using a sliding window go-back-N protocol. The send buffer storage and retry is on
a packet granularity.
The 24-bit phit uses 2-bits of sideband dedicated as a control channel for the LCB to carry
sequence numbers and status information. The virtual channel acknowledgment status bits travel in