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CHAPTER 3 CPU AND CONTROL UNITS
■ CPU
The CPU is a compact implementation of the 32-bit RISC FR architecture. Five instruction pipelines are
used to execute one instruction per cycle. A pipeline consists of the following stages:
Figure 3.2-2 shows the structure of the instruction pipeline.
Instruction fetch (IF): Outputs an instruction address to fetch an instruction.
Instruction decode (ID): Decodes a fetched instruction. Also reads a register.
Execution (EX): Executes an arithmetic operation.
Memory access (MA): Performs a load or store access to memory.
Write-back (WB): Writes an operation result (or loaded memory data) to a register.
Instruction 1
Instruction 2
Instruction 3
Instruction 4
Instruction 5
Instruction 6
Instructions are never executed randomly. If Instruction A enters a pipeline before Instruction B, it always
reaches the write-back stage before Instruction B.
In general, one instruction is executed per cycle. However, multiple cycles are required to execute a load/
store instruction with a memory wait, a branch instruction without a delay slot, or a multiple-cycle
instruction. The execution of instructions slows down if the instructions are not supplied fast enough.
■ 32-bit/16-bit Bus Converter
The 32-bit/16-bit bus converter provides an interface between the F-bus accessed at high-speed with a 32-
bit width and the R-bus accessed with a 16-bit width. This converter enables data access to the built-in
peripheral circuits from the CPU.
If the CPU performs a 32-bit width access to the R-bus, this bus converter converts the access into two 16-
bit width accesses. Some of the built-in peripheral circuits have limitations on the access bus width.
■ Harvard/Princeton Bus Converter
The Harvard/Princeton bus converter coordinates CPU instruction access and data access, and provides a
smooth interface with the external buses.
The CPU has a Harvard architecture with separate buses for instructions and data. On the other hand, the
bus controller that performs control of external buses has a Princeton architecture with a single bus. The
Harvard/Princeton bus converter assigns priorities to instruction and data accesses from the CPU to control
accesses to the bus controller. This function allows the order of external bus accesses to be permanently
optimized.
52
Figure 3.2-2 Structure of the Instruction Pipeline
CLK
WB
MA
WB
EX
MA
ID
EX
IF
ID
IF
WB
MA
WB
EX
MA
WB
ID
EX
MA
WB

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