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Hardware Reference Manual 227
Intel
®
IXP2800 Network Processor
SHaC — Unit Expansion
7.1.2 Scratchpad
7.1.2.1 Scratchpad Description
The SHaC Unit contains a 16-Kbyte Scratchpad memory, organized as 4K 32-bit words, that is
accessible by the Intel XScale
®
core and Microengines. The Scratchpad connects to the internal
Command, S_Push and S_Pull, CSR, and APB buses, as shown in Figure 85.
The Scratchpad memory provides the following operations:
• Normal reads and writes. 1 — 16 longwords (32 bits) can be read/written with a single
command. Note that Scratchpad is not byte-writable. Each write must write all four bytes.
• Atomic read-modify-write operations, bit-set, bit-clear, increment, decrement, add, subtract,
and swap. The Read-Modify-Write (RMW) operations can also optionally return the
premodified data.
• 16 Hardware Assisted Rings for interprocess communication.
1
• Standard support of APB peripherals such as UART, Timers, and GPIOs through the ARM*
Advanced Peripheral Bus (APB).
• Fast write and standard read and write operations to CSRs through the CSR Bus. For a fast
write, the write data is supplied with the command, rather than pulled from the source.
• Push/Pull Reflector Mode that supports reading from a device on the pull bus and writing the
data to a device on the push bus (reflecting the data from one bus to the other). A typical
implementation of this mode is to allow a Microengine to read or write the transfer registers or
CSRs in another Microengine. Note that the Push/Pull Reflector Mode only connects to a
single Push/Pull bus. If a chassis implements more than one Push/Pull bus, it can only connect
one specific bus to the CAP.
Scratchpad memory is provided as a third memory resource (in addition to SRAM and DRAM)
that is shared by the Microengines and Intel XScale
®
core. The Microengines and Intel XScale
®
core can distribute memory accesses between these three types of memory resources to provide a
greater number of memory accesses occurring in parallel.
1. A ring is a FIFO that uses a head and tail pointer to store/read information in Scratchpad memory.