
MOTOROLA
Chapter 1. Overview
1-11
Overview
register changes caused by that instruction. In-order completion ensures the correct
architectural state when the core must recover from a mispredicted branch or any exception.
Instruction state and other information required for completion is kept in a five-entry FIFO
completion queue. A single completion queue entry is allocated for each instruction once it
enters the execution unit from the dispatch unit. An available completion queue entry is a
required resource for dispatch; if no completion entry is available, dispatch stalls. A
maximum of two instructions per cycle are completed in order from the queue.
1.1.6
Memory Subsystem Support
The G2 core provides separate instruction and data caches and MMUs. The core also
provides an efficient processor bus interface to facilitate access to main memory and other
bus subsystems. The memory subsystem support functions are described in the following
sections.
1.1.6.1
Memory Management Units (MMUs)
The G2 core MMUs support up to 4 Petabytes (2
52
) of virtual memory and 4 Gigabytes
(2
32
) of physical memory (referred to as real memory in the architecture specification) for
instruction and data. The MMUs also control access privileges for these spaces on block
and page granularities. Referenced and changed status is maintained by the processor for
each page to assist implementation of a demand-paged virtual memory system. Note that
software assistant is required for the G2 core to maintain reference and changed status. A
key bit is implemented to provide information about memory protection violations prior to
page table search operations.
The LSU calculates effective addresses (EAs) for data loads and stores, performs data
alignment to and from cache memory, and provides the sequencing for load and store string
and multiple word instructions. The instruction unit calculates effective addresses for
instruction fetching.
After an EA is generated, its higher-order bits are translated by the appropriate MMU into
physical address bits. The lower-order EA bits are the same on the physical address which
are directed to the on-chip cache and formed the index into a four-way set-associative tag
array. After translating the address, the MMU passes the higher-order physical address bits
to the cache and the cache lookup completes. For caching-inhibited accesses or accesses
that miss in the cache, the untranslated lower-order address bits are concatenated with the
translated higher-order address bits; the resulting 32-bit physical address is then used by the
memory unit and the system interface to access external memory.
The MMU also directs the address translation and enforces the protection hierarchy
programmed by the operating system in relation to the supervisor/user privilege level of the
access and in relation to whether the access is a load or store.
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n
.