Classic RISC pipeline: Difference between revisions

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{{Short description|Instruction pipeline}}

{{Use American English|date = March 2019}}

{{NoMore footnotes|date=December 2012}}

In the [[history of computing hardware|history of computer hardware]], some early [[reduced instruction set computer]] [[central processing unit]]s (RISC CPUs) used a very similar architectural solution, now called a '''classic RISC pipeline'''. Those CPUs were: [[MIPS architecture|MIPS]], [[SPARC]], Motorola [[Motorola 88000|88000]], and later the notional CPU [[DLX]] invented for education.

Each of these classic scalar RISC designs fetches and tries to execute one [[Instructions per cycle|one instruction per cycle]]. The main common concept of each design is a five-stage execution [[instruction pipeline]]. During operation, each pipeline stage works on one instruction at a time. Each of these stages consists of a set of [[flip-flop (electronics)|flip-flops]] to hold state, and [[combinational logic]] that operates on the outputs of those flip-flops.

==The classic five stage RISC pipeline==

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===Instruction fetch===

The instructions reside in memory that takes one cycle to read. This memory can be dedicated to SRAM, or an Instruction [[Cache (computing)|Cache]]. The term "latency" is used in computer science often, and means the time from when an operation starts until it completes. Thus, instruction fetch has a latency of one [[clock cycle]] (if using single -cycle SRAM or if the instruction was in the cache). Thus, during the [[Instruction fetch|Instruction Fetch]] stage, a 32-bit instruction is fetched from the instruction memory.

The [[Programprogram Countercounter]], or (PC,) is a register that holds the address that is presented to the instruction memory. The Ataddress theis startpresented ofto ainstruction cycle,memory at the addressstart isof presenteda to instruction memorycycle. Then during the cycle, the instruction is being read out of instruction memory, and at the same time, a calculation is done to determine the next PC. The calculation of the next PC is donecalculated by incrementing the PC by 4, and by choosing whether to take that as the next PC or alternatively to take the result of a branch / jump calculation as the next PC. Note that in classic RISC, all instructions have the same length. (This is one thing that separates RISC from CISC <ref>{{cite paperweb |first=David |last=Patterson| title=RISC I: A Reduced Instruction Set VLSI Computer |series=Isca '81|date=12 May 1981|pages=443–457|url=https://dl.acm.org/doi/10.5555/800052.801895}}</ref>). In the original RISC designs, the size of an instruction is 4 bytes, so always add 4 to the instruction address, but don't use PC + 4 for the case of a taken branch, jump, or exception (see '''delayed branches''', below). (Note that some modern machines use more complicated algorithms ([[branch prediction]] and [[branch target predictor|branch target prediction]]) to guess the next instruction address.)

===Instruction decode===

Another thing that separates the first RISC machines from earlier CISC machines, is that RISC has no [[microcode]] .<ref>{{cite paperweb |first=David |last=Patterson| title=RISC I: A Reduced Instruction Set VLSI Computer |series=Isca '81|date=12 May 1981|pages=443–457|url=https://dl.acm.org/doi/10.5555/800052.801895}}</ref>. In the case of CISC micro-coded instructions, once fetched from the instruction cache, the instruction bits are shifted down the pipeline, where simple combinational logic in each pipeline stage produces control signals for the datapath directly from the instruction bits. In those CISC designs, very little decoding is done in the stage traditionally called the decode stage. A consequence of this lack of decoding is that more instruction bits have to be used to specifying what the instruction does. That leaves fewer bits for things like register indices.

All MIPS, SPARC, and DLX instructions have at most two register inputs. During the decode stage, the indexes of these two registers are identified within the instruction, and the indexes are presented to the register memory, as the address. Thus the two registers named are read from the [[register file]]. In the MIPS design, the register file had 32 entries.

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At the same time the register file is read, instruction issue logic in this stage determines if the pipeline is ready to execute the instruction in this stage. If not, the issue logic causes both the Instruction Fetch stage and the Decode stage to stall. On a stall cycle, the input flip flops do not accept new bits, thus no new calculations take place during that cycle.

If the instruction decoded is a branch or jump, the target address of the branch or jump is computed in parallel with reading the register file. The branch condition is computed in the following cycle (after the register file is read), and if the branch is taken or if the instruction is a jump, the PC in the first stage is assigned the branch target, rather than the incremented PC that has been computed. Some architectures made use of the [[Arithmetic logic unit|ALU]] (ALU) in the Execute stage, at the cost of slightly decreased instruction throughput.

The decode stage ended up with quite a lot of hardware: MIPS has the possibility of branching if two registers are equal, so a 32-bit-wide AND tree runs in series after the register file read, making a very long critical path through this stage (which means fewer cycles per second). Also, the branch target computation generally required a 16 bit add and a 14 bit incrementer. Resolving the branch in the decode stage made it possible to have just a single-cycle branch mis-predict penalty. Since branches were very often taken (and thus mis-predicted), it was very important to keep this penalty low.

===Execute===

The Execute stage is where the actual computation occurs. Typically this stage consists of an Arithmetic and Logic UnitALU, and also a bit shifter. It may also include a multiple cycle multiplier and divider.

The Arithmetic and Logic UnitALU is responsible for performing booleanBoolean operations (and, or, not, nand, nor, xor, xnor) and also for performing integer addition and subtraction. Besides the result, the ALU typically provides status bits such as whether or not the result was 0, or if an overflow occurred.

The bit shifter is responsible for shift and rotations.

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Instructions on these simple RISC machines can be divided into three latency classes according to the type of the operation:

* Register-Register Operation (Single-cycle latency): Add, subtract, compare, and logical operations. During the execute stage, the two arguments were fed to a simple [[Arithmetic logic unit|ALU]], which generated the result by the end of the execute stage.

* Memory Reference (Two-cycle latency). All loads from memory. During the execute stage, the ALU added the two arguments (a register and a constant offset) to produce a virtual address by the end of the cycle.

*[[Cycles per instruction|Multi-cycle Instructions]] (Many cycle latency). Integer multiply and divide and all [[floating-point]] operations. During the execute stage, the operands to these operations were fed to the multi-cycle multiply/divide unit. The rest of the pipeline was free to continue execution while the multiply/divide unit did its work. To avoid complicating the writeback stage and issue logic, multicycle instruction wrote their results to a separate set of registers.

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===Memory access===

If data memory needs to be accessed, it is done so in this stage.

During this stage, single cycle latency instructions simply have their results forwarded to the next stage. This forwarding ensures that both one and two cycle instructions always write their results in the same stage of the pipeline so that just one write port to the register file can be used, and it is always available.

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During this stage, both single cycle and two cycle instructions write their results into the register file.

Note that two different stages are accessing the register file at the same time -- thetime—the decode stage is reading two source registers, at the same time that the writeback stage is writing a previous instruction's destination register.

On real silicon, this can be a hazard (see below for more on hazards). That is because one of the source registers being read in decode might be the same as the destination register being written in writeback. When that happens, then the same memory cells in the register file are being both read and written the same time. On silicon, many implementations of memory cells will not operate correctly when read and written at the same time.

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Suppose the CPU is executing the following piece of code:

<syntaxhighlight lang="nasm">

SUB r3,r4 -> r10 ; Writes r3 - r4 to r10

AND r10,r3 -> r11 ; Writes r10 & r3 to r11

</syntaxhighlight>

The instruction fetch and decode stages send the second instruction one cycle after the first. They flow down the pipeline as shown in this diagram:

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However, consider the following instructions:

<syntaxhighlight lang="nasm">

LD adr -> r10

ANDLD adr r10,r3 -> r11r10

AND r10,r3 -> r11

</syntaxhighlight>

The data read from the address <code>adr</code> is not present in the data cache until after the Memory Access stage of the <code>LD</code> instruction. By this time, the <code>AND</code> instruction is already through the ALU. To resolve this would require the data from memory to be passed backwards in time to the input to the ALU. This is not possible. The solution is to delay the <code>AND</code> instruction by one cycle. The data hazard is detected in the decode stage, and the fetch and decode stages are '''stalled''' - they are prevented from flopping their inputs and so stay in the same state for a cycle. The execute, access, and write-back stages downstream see an extra no-operation instruction (NOP) inserted between the <code>LD</code> and <code>AND</code> instructions.

This NOP is termed a pipeline ''[[bubble (computing)|bubble]]'' since it floats in the pipeline, like an air bubble in a water pipe, occupying resources but not producing useful results. The hardware to detect a data hazard and stall the pipeline until the hazard is cleared is called a '''pipeline interlock'''.

{| align=center style="text-align:center"

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* Predict Not Taken: Always fetch the instruction after the branch from the instruction cache, but only execute it if the branch is not taken. If the branch is not taken, the pipeline stays full. If the branch is taken, the instruction is flushed (marked as if it were a NOP), and one cycle's opportunity to finish an instruction is lost.

* Branch Likely: Always fetch the instruction after the branch from the instruction cache, but only execute it if the branch was taken. The compiler can always fill the branch delay slot on such a branch, and since branches are more often taken than not, such branches have a smaller IPC penalty than the previous kind.

* [[Branch delay slot|Branch Delay Slot]]: AlwaysDepending fetchon the instructiondesign afterof the delayed branch fromand the instructionbranch cacheconditions, andit alwaysis executedetermined it,whether the instruction immediately following the branch instruction is executed even if the branch is taken. Instead of taking an IPC penalty for some fraction of branches either taken (perhaps 60%) or not taken (perhaps 40%), branch delay slots take an IPC penalty for those branches into which the compiler could not schedule the branch delay slot. The SPARC, MIPS, and MC88K designers designed a branch delay slot into their ISAs.

* [[Branch Prediction]]: In parallel with fetching each instruction, guess if the instruction is a branch or jump, and if so, guess the target. On the cycle after a branch or jump, fetch the instruction at the guessed target. When the guess is wrong, flush the incorrectly fetched target.

Delayed branches were controversial, first, because their semantics isare complicated. A delayed branch specifies that the jump to a new ___location happens ''after'' the next instruction. That next instruction is the one unavoidably loaded by the instruction cache after the branch.

Delayed branches have been criticized{{By whom|date=May 2012}} as a poor short-term choice in ISA design:

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==Exceptions==

Suppose a 32-bit RISC processes an ADD instruction that adds two large numbers, and the result does not fit in 32 bits. What happens?

The simplest solution, provided by most architectures, is wrapping arithmetic. Numbers greater than the maximum possible encoded value have their most significant bits chopped off until they fit. In the usual integer number system, 3000000000+3000000000=6000000000. With unsigned 32 bit wrapping arithmetic, 3000000000+3000000000=1705032704 (6000000000 mod 2^32). This may not seem terribly useful. The largest benefit of wrapping arithmetic is that every operation has a well defined result.

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But the programmer, especially if programming in a language supporting [[large numbers|large integers]] (e.g. [[Lisp (programming language)|Lisp]] or [[Scheme (programming language)|Scheme]]), may not want wrapping arithmetic. Some architectures (e.g. MIPS), define special addition operations that branch to special locations on overflow, rather than wrapping the result. Software at the target ___location is responsible for fixing the problem. This special branch is called an exception. Exceptions differ from regular branches in that the target address is not specified by the instruction itself, and the branch decision is dependent on the outcome of the instruction.

The most common kind of software-visible exception on one of the classic RISC machines is a [[Translation_lookaside_bufferTranslation lookaside buffer#TLB-miss_handlingmiss handling|''TLB miss'']].

Exceptions are different from branches and jumps, because those other control flow changes are resolved in the decode stage. Exceptions are resolved in the writeback stage. When an exception is detected, the following instructions (earlier in the pipeline) are marked as invalid, and as they flow to the end of the pipe their results are discarded. The program counter is set to the address of a special exception handler, and special registers are written with the exception ___location and cause.

To make it easy (and fast) for the software to fix the problem and restart the program, the CPU must take a precise exception. A precise exception means that all instructions up to the excepting instruction have been executed, and the excepting instruction and everything afterwards have not been executed.

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Another strategy to handle suspend/resume is to reuse the exception logic. The machine takes an exception on the offending instruction, and all further instructions are invalidated. When the cache has been filled with the necessary data, the instruction that caused the cache miss restarts. To expedite data cache miss handling, the instruction can be restarted so that its access cycle happens one cycle after the data cache is filled.

== See also ==

* [[Iron law of processor performance]]

== References ==

{{Reflist}}

{{refbegin}}

* {{cite book |firstfirst1=John L. |lastlast1=Hennessy |first2=David A. |last2=Patterson |title=Computer Architecture, A Quantitative Approach |publisher=Morgan Kaufmann |edition=5th |year=2011 |isbn=978-0123838728 |pages= |url=https://books.google.com/books?id=v3-1hVwHnHwC&printsec=frontcover}}

{{refend}}