In the design of
pipelined computer processors, a pipeline stall is a delay in execution of an
instruction in order to resolve a
hazard
A hazard is a potential source of harm. Substances, events, or circumstances can constitute hazards when their nature would allow them, even just theoretically, to cause damage to health, life, property, or any other interest of value. The probab ...
.
Details
In a standard
five-stage pipeline, during the
decoding stage, the control unit will determine whether the decoded instruction reads from a register to which the currently executed instruction writes. If this condition holds, the control unit will stall the instruction by one clock cycle. It also stalls the instruction in the fetch stage, to prevent the instruction in that stage from being overwritten by the next instruction in the program.
In a
Von Neumann architecture
The von Neumann architecture — also known as the von Neumann model or Princeton architecture — is a computer architecture based on a 1945 description by John von Neumann, and by others, in the '' First Draft of a Report on the EDVAC''. T ...
which uses the program counter (PC) register to determine the current instruction being fetched in the pipeline, to prevent new instructions from being fetched when an instruction in the decoding stage has been stalled, the value in the
PC register and the instruction in the fetch stage are preserved to prevent changes. The values are preserved until the instruction causing the conflict has passed through the execution stage.
Such an event is often called a bubble, by analogy with an air bubble in a fluid pipe.
In some architectures, the execution stage of the pipeline must always be performing an action at every cycle. In that case, the bubble is implemented by feeding
NOP ("no operation") instructions to the execution stage, until the bubble is flushed past it.
Examples
Timeline
The following is two executions of the same four instructions through a 4-stage pipeline but, for whatever reason, a delay in fetching of the purple instruction in cycle #2 leads to a bubble being created delaying all instructions after it as well.
Classic RISC pipeline
The below example shows a bubble being inserted into a
classic RISC pipeline
In the history of computer hardware, some early reduced instruction set computer central processing units (RISC CPUs) used a very similar architectural solution, now called a classic RISC pipeline. Those CPUs were: MIPS, SPARC, Motorola 88000, ...
, with five stages (IF = Instruction Fetch, ID = Instruction Decode, EX = Execute, MEM = Memory access, WB = Register write back). In this example, data available after the MEM stage (4th stage) of the first instruction is required as input by the EX stage (3rd stage) of the second instruction. Without a bubble, the EX stage (3rd stage) only has access to the output of the previous EX stage. Thus adding a bubble resolves the time dependence without needing to propagate data backwards in time (which is impossible).
See also
*
Branch predication
In computer science, predication is an architectural feature that provides an alternative to conditional transfer of control, as implemented by conditional branch machine instructions. Predication works by having conditional (''predicated'') no ...
*
Delay slot
In computer architecture, a delay slot is an instruction slot being executed without the effects of a preceding instruction. The most common form is a single arbitrary instruction located immediately after a branch instruction on a RISC or DSP ...
*
Pipeline flush
In the domain of central processing unit (CPU) design, hazards are problems with the instruction pipeline in CPU microarchitectures when the next instruction cannot execute in the following clock cycle, and can potentially lead to incorrect comput ...
*
Wait state
A wait state is a delay experienced by a computer processor when accessing external memory or another device that is slow to respond.
Computer microprocessors generally run much faster than the computer's other subsystems, which hold the data the ...
References
{{CPU technologies
Instruction processing