Superscalar Microprocessor
MPC755 and MPC745 processors are superscalar, capable of issuing three instructions
per clock cycle (two instructions + branch) into six independent execution
units:
- Two integer units
- Load/store unit
- Double-precision floating-point unit
- System register unit
- Branch processing unit
The ability to execute multiple instructions in parallel, to pipeline instructions,
and the use of simple instructions with rapid execution times yields maximum
efficiency and throughput for MPC755 and MPC745 systems.
Power Management
The MPC755 and MPC745 microprocessors feature a low-power 2V design with three
power-saving user-programmable modes—doze, nap and sleep—which progressively
reduce the power drawn by the processor.
These low-power microprocessors offer dynamic power management to selectively
activate functional units as they are needed by the executing instructions.
Both microprocessors also provide a thermal assist unit and instruction cache
throttling for software-controllable thermal management.
Cache and MMU Support
The MPC755/745 microprocessors have separate 32-Kbyte, physically-addressed
instruction and data caches. Both caches can be locked in part or whole to
provide storage of critical data, key performance algorithms, or code loops
for fast response time.
MPC755/745 microprocessors contain separate memory management units (MMUs)
for instructions and data, supporting 4 Petabytes (252) of virtual memory and
4 Gigabytes (232) of physical memory. Both feature eight instruction block
address translation (iBAT) and eight data block address translation (dBAT)
registers. Access privileges and memory protection are controlled on block
or page granularities. Large, 128-entry translation lookaside buffers (TLBs)
provide efficient physical address translation and support for virtual-memory
management on both page- and variable-sized blocks. Both hardware and software
tablewalks are provided for the TLBs.
Flexible Bus Interface
MPC755/745 microprocessors have a 64-bit data bus with 32-bit mode and a 32-bit
address bus. Support is included for burst, split and pipelined transactions.
The interface provides snooping for data cache coherency. Both microprocessors
maintain MEI coherency protocol in hardware, allowing access to system memory
for additional caching bus leaders, such as DMA devices.
