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The PowerPC Core
6-2
MPC801 USER’S MANUAL
MOTOROLA
6
6.1.1 Basic Structure of the Core
To accomplish its tasks, the core is divided into the following subunits:
Sequencer Unit
and the interrupt handling mechanism. It controls some data structures within the
register unit.
Register Unit
—Consists of all the user-visible registers, the register’s scoreboard
mechanism, and a history of previous operations to allow for a precise interrupt model.
This module is physically split so that each data structure is implemented near the area
where it is used.
Fixed-Point Unit
—Consists of all fixed-point instruction implementations, except load/
store. This module is subdivided into the following two execution units:
— IMUL/IDIV—Fixed-point multiply and divide instruction implementations
— ALU/BFU—Fixed-point logic, add, and subtract instruction implementations, as well
as the bit field instructions.
Load/Store Unit
—Consists of all load and store instructions, except floating-point
processor load and store.
—Consists of the branch processor, the instruction prefetch queue,
6.1.2 Instruction Flow Within the Core
When fetched, instructions enter the instruction queue, thus enabling branch folding by
allowing out-of-order branch execution. Nonbranch instructions reaching the top of the
instruction queue are issued to the execution units. Instructions can be flushed from the
instruction queue when an exception on a previous instruction, interrupt, or miss-predicted
fetch occurs.
All instructions, including branches, enter the history buffer along with processor state
information that can be affected by the instruction’s execution. This information is used to
enable out-of-order completion of instructions together with precise exception handling.
When exceptions or interrupts occur, instructions can be flushed or recovered from the
machine. The instruction queue is always flushed when the history buffer is recovered. An
instruction retires from the machine after it finishes executing without exception and all
preceding instructions are retired from the machine. Figure 6-1 illustrates the core’s
microarchitecture.