Chapter 12
364 MIPS R4000 Microprocessor User's Manual
12.9 System Interface Commands and Data Identifiers
System interface commands specify the nature and attributes of any
System interface request; this specification is made during the address
cycle for the request. System interface data identifiers specify the
attributes of data transmitted during a System interface data cycle.
The following sections describe the syntax, that is, the bitwise encoding of
System interface commands and data identifiers.
Reserved bits and reserved fields in the command or data identifier
should be set to 1 for System interface commands and data identifiers
associated with external requests. For System interface commands and
data identifiers associated with processor requests, reserved bits and
reserved fields in the command and data identifier are undefined.
Command and Data Identifier Syntax
System interface commands and data identifiers are encoded in 9 bits and
are transmitted on the SysCmd bus from the processor to an external
agent, or from an external agent to the processor, during address and data
cycles. Bit 8 (the most-significant bit) of the SysCmd bus determines
whether the current content of the SysCmd bus is a command or a data
identifier and, therefore, whether the current cycle is an address cycle or a
data cycle. For System interface commands, SysCmd(8) must be set to 0.
For System interface data identifiers, SysCmd(8) must be set to 1.
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