3.2.2 Storage Attributes
The PowerPC Architecture defines storage attributes that control data and instruction accesses.
Storage attributes are provided to
control cache write-through policy (the W storage attribute),
cachability (the I storage attribute), memory coherency
in
multiprocessor environments (the M
storage attribute), and guarding against
speculative memory accesses (the G storage attribute). The
IBM PowerPC Embedded Environment defines additional storage attributes for storage compression
(the
UO
storage attribute) and byte ordering (the E storage attribute).
The
PPC405GP provides two control mechanisms for the
W,
I,
UO,
G,
and E attributes. Because the
PPC405GP does not provide hardware support for multiprocessor environments, the M storage
attribute, when present, has no effect.
When the
PPC405GP operates
in
virtual mode (address translation is enabled), each storage
attribute is
controlled by the
W,
I,
UO,
G, and E fields in the translation lookaside buffer (TLB) entry for
each memory page. The size of memory pages, and hence the size of storage attribute
control
regions, is variable. Multiple sizes can be in effect simultaneously on different pages.
When the
PPC405GP operates
in
real mode (address translation is disabled), storage attribute
control registers control the corresponding storage attributes. These registers are:
• Data Cache Write-through Register (DCWR)
• Data Cache Cachability Register (DCCR)
• Instruction Cache Cachability Register (ICCR)
•
Storage Guarded Register (SGR)
•
Storage Little-Endian Register (SLER)
•
Storage User-defined 0 Register
(SUOR)
Each storage attribute control register contains 32 bits; each bit controls one of
thirty~two
128MB
storage attribute
control regions. Bit 0 of each register controls the lowest-order region, with
ascending bits
controlling ascending regions in memory. The storage attributes
in
each storage
attribute region are set independently of each other and of the storage attributes for other regions.
3.3 Registers
All PPC405GP registers are listed
in
this section. Some of the frequently-used registers are described
in
detail. Other registers are covered
in
their respective topic chapters (for example, the cache
registers are described
in
Chapter
4,
"Cache Operations"). All registers are summarized
in
Chapter 25, "Register Summary:'
The registers are grouped into categories:
General Purpose Registers (GPRs), Special Purpose
Registers (SPRs), Time Base Registers (TBRs), the Machine State Register (MSR), the Condition
Register (CR), Device
Control Registers (DCRs), and memory-mapped I/O registers (MMIO).
Different instructions are used to access each category of registers.
For
all registers with fields marked as reserved, the reserved fields should be written as 0 and read as
undefined. That is, when writing to a register with a reserved field, write a 0 to the reserved field.
When reading from a register with a reserved field, ignore that field. '
Preliminary
Programming
Model
3-3
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