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Hardware Reference Manual 233
Intel
®
IXP2800 Network Processor
SHaC — Unit Expansion
The ring commands operate as outlined in the pseudo-code in Example 32. The operations are
atomic, meaning that multi-word “Gets” and “Puts” do all the reads and writes, with no other
intervening Scratchpad accesses.
Prior to using the Scratchpad rings, software must initialize the Ring registers (by CSR writes). The
Base address of the ring must be written, and also the size field that determines the number of
32-bit words for the Ring.
Note: Detailed information about CSRs is provided in the Intel
®
IXP2400 and IXP2800 Network
Processor Programmer’s Reference Manual.
Writes
For an APB or CAP CSR write, the Scratchpad arbitrates for the S_Pull_Bus, pulls the write data
from the source identified in the instruction (either a Microengine transfer register or an Intel
XScale
®
core write buffer), and puts it into one of the Pull Data FIFOs. It then drives the address
and writes data onto the appropriate bus. CAP CSRs locally decode the address to match their own.
The Scratchpad generates a separate APB device select signal for each peripheral device
(up to 15 devices). If the write is to an APB CSR, the control logic maintains valid signaling until
the APB_RDY_H signal is returned (the APB RDY signal is an extension to the APB bus
specification, specifically added for the Network Processor). Upon receiving the APB_RDY_H
signal, the APB select signal is deasserted and the state machine returns to the idle state between
commands. The CAP CSR bus does not support a similar acknowledge signal on writes since the
Fast Write functionality requires that a write operation be retired on each cycle.
Example 32. Ring Command Pseudo-Code
GET Command
Get(ring, length)
If count[ring] >= length //enough data in the ring?
ME <-- Scratchpad[head[ring]] // each data phase
head[ring]+= length % ringSize
count[ring] -= length
else ME <--nil // 1 data phase signals read off empty list
NOTE: The Microengine signal is delivered with last data. In the case of nil, the signal is delivered with the 1
data phase.
PUT Command
Before issuing a PUT command, it is the responsibility of the Microengine thread issuing the command to make
sure the Ring has enough room.
Put(ring, length)
SRAM[tail[ring]] <-- ME pull data // each data phase
tail[ring]+= length % ringSize
Count[ring] += length
Table 80. Head/Tail, Base, and Full Threshold – by Ring Size
Size
(Number of 32-Bit Words)
Base Address Head/Tail Offset Full Threshold (Entries)
128 13:9 8:2 32
256 13:10 9:2 64
512 13:11 10:2 128
1024 13:12 11:2 256
NOTE: Note that bits [1:0] of the address are assumed to be 00.