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User’s Manual
Preliminary PPC440x5 CPU Core
regsummIntro.fm.
September 12, 2002
Page 457 of 589
10.2 Reserved Fields
For all registers with fields marked as reserved, the reserved fields should be written as zero and read as
undefined. That is, when writing to a reserved field, write a zero to that field. When reading from a reserved
field, ignore that field.
The recommended coding practice is to perform the initial write to a register with reserved fields as described
in the preceding paragraph, and to perform all subsequent writes to the register using a read-modify-write
strategy: read the register, alter desired fields with logical instructions, and then write the register.
10.3 Device Control Registers
Device Control Registers (DCRs), which are architecturally outside of the processor core, are accessed using
the mfdcr and mtdcr instructions. DCRs are used to control, configure, and hold status for various functional
units that are not part of the RISC processor core. Although the PPC440x5 core does not contain DCRs, the
mfdcr and mtdcr instructions are provided.
The mfdcr and mtdcr instructions are privileged, for all DCR numbers. Therefore, all DCR accesses are
privileged. All DCR numbers are reserved, and should be neither read nor written, unless they are part of a
Core+ASIC implementation.