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G2 PowerPC Core Reference Manual
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MOTOROLA
Implementation-Specific Information
Computational instructions do not modify memory. To use a memory operand in a
computation and then modify the same or another memory location, the memory contents
must be loaded into a register, modified, and then written back to the target location with
distinct instructions.
The G2 core follows the program flow when it is in the normal execution state. However,
the flow of instructions can be interrupted directly by the execution of an instruction or by
an asynchronous event. Either kind of exception may cause one of several components of
the system software to be invoked.
1.3.2.2
Implementation-Specific Instruction Set
The G2 core instruction set is defined as follows:
The core provides hardware support for all 32-bit PowerPC instructions.
The core provides two implementation-specific instructions used for software table
search operations following TLB misses:
— Load Data TLB Entry (
tlbld
)
— Load Instruction TLB Entry (
tlbli
)
The G2_LE implements the following instruction which is added to support critical
interrupts. This is a supervisor-level, context synchronizing instruction.
— Return from Critical Interrupt (
rfci
)
1.3.3
Cache Implementation
The following sections describe the general cache characteristics as implemented in the
PowerPC architecture and the core implementation, specifically. G2_LE-specific
information is noted where applicable.
1.3.3.1
PowerPC Cache Characteristics
The PowerPC architecture does not define hardware aspects of cache implementations. The
G2 core controls the following memory access modes on a page or block basis:
Write-back/write-through mode
Caching-inhibited mode
Memory coherency
Note that in the core, a cache block is defined as eight words. The VEA defines cache
management instructions that provide a means by which the application programmer can
affect the cache contents.
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