Intel NETWORK PROCESSOR IXP2800 - Manual (Page 127)

Hardware Reference Manual 127
Intel
®
IXP2800 Network Processor
Intel XScale ® Core
3.11.2 Intel XScale
®
Core Gasket Functional Description
3.11.2.1 Command Memory Bus to Command Push/Pull Conversion
The primary function of the Intel XScale ® core gasket unit is to translate commands initiated from
the Intel XScale ® core in the Intel XScale ® core command bus format, into the IXP2800 internal
command format (Command Push/Pull format).
Table 49 shows how many CPP commands are generated by the gasket from each CMB command.
Write data is guaranteed to be 32-bit (longword) aligned. Table 49 shows only the Store command.
In the Load case, the gasket simply converts it to the CPP format. No command splitting is
required. A Load can only be a byte (8 bits), a word (16 bits), longword (32 bits), or eight
longwords (8x32).
3.11.3 CAM Operation
In the SRAM controller, access ordering is guaranteed only for a read coming after a write. The
gasket enforces order rules in the following two cases.
1. Write coming after a read.
2. Read-Modify-Write coming after read.
The address CAMing is on 8-word boundaries. The SRAM effective address is 28 bits. Deduct
five bits (two bits for the word address and three bits for eight words), and the tag width for the
CAM is 23 bits wide. The CAM only operates on SRAM accesses.
Table 49. CMB Write Command to CPP Command Conversion
Store Length
CPP SRAM
Cmd Count
CPP DRAM
Cmd Count
Remark
Byte, word,
longword
1 1 SRAM uses 4-bit mask, and DRAM uses an 8-bit mask.
2 longwords 1 or 2 1 or 2
SRAM: If there is any mask bit detected as ‘0’,two
commands will be generated.
DRAM: If it starts with odd word address, two commands
will be generated.
3 longwords 1 or 3 2
SRAM: If there is a mask bit of ‘0’ detected, Three SRAM
commands will be generated.
DRAM: always two DRAM commands.
4 longwords 1 or 4 1 or 2
SRAM: If there is a mask bit of ‘0’ detected, four
commands will be generated.
DRAM: If there is a mask bit of ‘0’ detected, two
commands will be generated.
8 longwords Not allowed in a write.