
MOTOROLA
Chapter 1. Overview
1-13
Overview
The load/store and instruction fetch units provide the caches with the address of the data or
instruction to be fetched. In the case of a cache hit, the cache returns two words to the
requesting unit.
Because the data cache tags are single-ported, simultaneous load or store and snoop
accesses cause resource contention. Snoop accesses have the highest priority and are given
first access to the tags, unless the snoop access coincides with a tag write; in this case the
snoop is retried and must rearbitrate for cache access. Loads or stores deferred due to snoop
accesses are performed on the clock cycle following the snoop.
1.1.7
Core Interface
Because the caches are on-chip, write-back caches, the most common transactions are
burst-read memory operations, burst-write memory operations, and single-beat
(noncacheable or write-through) memory read and write operations. There can also be
address-only operations, variants of the burst and single-beat operations, (for example,
global memory operations that are snooped and atomic memory operations), and address
retry activity (for example, when a snooped read access hits a modified cache block).
Memory accesses can occur in single-beat (1–8 bytes) and four-beat burst (32 bytes) data
transfers when the 60x bus is configured as 64 bits, and in single-beat (1–4 bytes), two-beat
(8 bytes), and eight-beat (32 bytes) data transfers when the bus is configured as 32 bits. The
60x address and data buses operate independently to support pipelining and split
transactions during memory accesses. The core can pipeline its own transactions to a depth
of one level.
Access to the system interface is granted through an external arbitration mechanism that
allows devices to compete for bus mastership. This arbitration is flexible, allowing the core
to be integrated into systems that implement various fairness and bus parking procedures
to avoid arbitration overhead.
Typically, memory accesses are weakly ordered—sequences of operations, including
load/store string and multiple instructions, do not necessarily complete in the order they
begin—maximizing the efficiency of the bus without sacrificing coherency of the data. The
core allows read operations to precede store operations (except when a dependency exists,
or in cases where a noncacheable access is performed), and provides support for a write
operation to proceed a previously queued read data tenure (for example, allowing a snoop
push to be enveloped by the address and data tenures of a read operation). Because the
processor can dynamically optimize run-time ordering of load/store traffic, overall
performance is improved.
1.1.8
System Support Functions
The G2 core implements several support functions that include power management, time
base/decrementer registers for system timing tasks, an IEEE 1149.1 (JTAG)/common
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