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36 Hardware Reference Manual
Intel
®
IXP2800 Network Processor
Technical Description
Each of the eight Contexts is in one of four states.
1. Inactive — Some applications may not require all eight contexts. A Context is in the Inactive
state when its CTX_ENABLE CSR enable bit is a 0.
2. Executing — A Context is in Executing state when its context number is in
ACTIVE_CTX_STS CSR. The executing Context’s PC is used to fetch instructions from the
Control Store. A Context will stay in this state until it executes an instruction that causes it to
go to Sleep state (there is no hardware interrupt or preemption; Context swapping is
completely under software control). At most one Context can be in Executing state at any time.
3. Ready — In this state, a Context is ready to execute, but is not because a different Context is
executing. When the Executing Context goes to the Sleep state, the Microengine’s context
arbiter selects the next Context to go to the Executing state from among all the Contexts in the
Ready state. The arbitration is round robin.
4. Sleep — Context is waiting for external event(s) specified in the
INDIRECT_WAKEUP_EVENTS CSR to occur (typically, but not limited to, an I/O access).
In this state the Context does not arbitrate to enter the Executing state.
The state diagram in Figure 5 illustrates the Context state transitions. Each of the eight Contexts
will be in one of these states. At most one Context can be in Executing state at a time; any number
of Contexts can be in any of the other states.
The Microengine is in Idle state whenever no Context is running (all Contexts are in either Inactive
or Sleep states). This state is entered:
1. After reset (CTX_ENABLE Local CSR is clear, putting all Contexts into Inactive states).
2. When a context swap is executed, but no context is ready to wake up.
3. When a
ctx_arb[bpt] instruction is executed by the Microengine (this is a special case of
condition 2 above, since the
ctx_arb[bpt] clears CTX_ENABLE, putting all Contexts into
Inactive states).
Figure 5. Context State Transition Diagram
A9352-03
Inactive Ready
CTX_ENABLE bit is set by
Intel XScale
®
Core
CTX_ENABLE bit is cleared
CTX_ENABLE
bit is cleared
Reset
Sleep Executing
Context executes
CTX Arbitration instruction
Executing Context goes
to Sleep state, and this
Context is the highest
round-robin priority.
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Note:
After reset, the Intel XScale
®
Core processor must load the starting address of the CTX_PC, load the
CTX_WAKEUP_EVENTS to 0x1 (voluntary), and then set the appropriate CTX_ENABLE bits to begin
executing Context(s).