UM0434 datasheet(209/390 Pages) STMICROELECTRONICS

209 / 390 page

Instruction pipeline and execution timing

UM0434

210/391

instruction, the data is fed directly into the ALU for execution. No pipeline bubble is incurred

in this case.

Multiply instructions require one clock to execute. All condition-setting instructions complete

in the execute stage of the pipeline.

Feed-forwarding allows the result of one instruction to be made available as the source

operand(s) of a subsequent instruction so that data-dependent instructions can execute

without waiting for previous instructions to write back their results.

8.3.2

Instruction buffers

The e200z3 contains a set of instruction buffers that supply instructions into the instruction

Instruction prefetches request a 64-bit double word and the buffer is filled with a pair of

instructions at a time, except for the case of a change of flow fetch where the target is to the

second (odd) word. In that case, only a 32-bit prefetch is performed to load the instruction

buffer. This 32-bit fetch may be immediately followed by a 64-bit prefetch to fill slots 0 and 1

in the event that the branch is resolved to be taken.

In normal sequential execution, instructions are loaded into the IR from slot 0, and as a pair

of slots are emptied, they are refilled. Whenever a pair of slots is empty, a 64-bit prefetch is

initiated that fills the earliest empty slot pairs beginning with slot 0.

If the instruction buffer empties, instruction issue stalls, and the buffer is refilled. The first

returned instruction is forwarded directly to the IR.

Figure 14.

Instruction buffers

Slot0

Decode

MUX

DATA 0:63

Slot1

Slot2

Slot3

UM0434 Datasheet(PDF) 209 Page - STMicroelectronics