
MOTOROLA
Chapter 9. Core Interface Operation
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9-1
Chapter 9
Core Interface Operation
This chapter describes the 60x bus interface of the G2 core and its operation. It shows how
the core signals, defined in Chapter 8, “Signal Descriptions,” interact to perform address
and data transfers. For a detailed discussion about the 60x bus interface, multiple bus
masters, and memory coherency, refer to the
PowerPC Microprocessor Family: The Bus
Interface for 32-Bit Microprocessors.
9.1
Overview
The core interface prioritizes requests for bus operations from the instruction and data
caches and performs bus operations following the 60x bus protocol. It includes address
register queues, prioritization logic, and the bus control unit. The core interface latches
snoop addresses for snooping in the data cache and address register queues, snoops for
direct-store reply operations and reservations controlled by the Load Word and Reserve
Indexed (
lwarx
) and Store Word Conditional Indexed (
stwcx.
) instructions, and maintains
the touch load address for the data cache. The interface allows one level of pipelining; that
is, with certain restrictions described in subsequent sections, there can be as many as two
outstanding transactions at any given time. Accesses are prioritized with load operations
preceding store operations.
Instructions are automatically fetched from the memory system into the instruction unit
where they are dispatched to the execution units or forwarded to the branch processing unit
at a peak rate of three instructions per clock (see Section 7.3, “Timing Considerations”).
Conversely, load and store instructions explicitly specify the movement of operands to and
from the general-purpose and floating-point registers (GPRs and FPRs) and the memory
system.
When the G2 core encounters an instruction or data access, it calculates the logical address
(effective address) and uses the low-order address bits to check for a hit in the on-chip,
16-Kbyte instruction or data caches. During cache lookup, the instruction and data memory
management units (MMUs) use the higher-order address bits to calculate the virtual
address, allowing them to calculate the physical address (real address). The physical
address bits are then compared with the corresponding cache tag bits to determine if a cache
hit occurred. If the access misses in the corresponding cache, the physical address is used
to access system memory.
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