UM012811-0904
Architectural
Overview
eZ8 CPU
User Manual
2
Figure 1. eZ8 CPU Block Diagram
FETCH UNIT
The Fetch Unit controls the memory interface. Its primary function is to fetch opcodes and
operands from memory. The Fetch Unit also fetches interrupt vectors or reads and writes
memory in the Program or Data Memory.
The Fetch Unit performs a partial decoding of the opcode to determine the number of
bytes to fetch for the operation. The Fetch Unit operation sequence follows:
1. Fetch the opcode
2. Determine the operand size (number of bytes)
3. Fetch the operands
4. Present the opcode and operands to the Instruction State Machine.
The Fetch Unit is pipelined and operates semi-independently from the rest of the eZ8
CPU.
INSTRUCTION STATE MACHINE
The Instruction State Machine is the controller for the eZ8 CPU Execution Unit. After the
initial operation decode by the Fetch Unit, the Instruction State Machine takes over and
completes the instruction. The Instruction State Machine performs register read and write
operations and generates addresses.
Fetch Unit
Instruction
State Machine
CPU Control
Registers
Program
Counter
Arithmetic Logic Unit
