Editing Out-of-order execution (section)

=== Decoupling ===
Smith also researched how to make different execution units operate more independently of each other and of the memory, front-end, and branching.<ref>{{cite journal |last1=Smith |first1=James E. |author1-link=James E. Smith (engineer) |title=Decoupled Access/Execute Computer Architectures |journal=ACM Transactions on Computer Systems |date=November 1984 |volume=2 |issue=4 |pages=289–308 |doi=10.1145/357401.357403 |s2cid=13903321 |url=https://course.ece.cmu.edu/~ece447/s15/lib/exe/fetch.php?media=p289-smith.pdf}}</ref> He implemented those ideas in the [[Astronautics Corporation of America|Astronautics]] ZS-1 (1988), featuring a decoupling of the integer/load/store [[Instruction pipelining|pipeline]] from the floating-point pipeline, allowing inter-pipeline reordering. The ZS-1 was also capable of executing loads ahead of preceding stores. In his 1984 paper, he opined that enforcing the precise exceptions only on the integer/memory pipeline should be sufficient for many use cases, as it even permits [[virtual memory]]. Each pipeline had an instruction buffer to decouple it from the instruction decoder, to prevent the stalling of the front end. To further decouple the memory access from execution, each of the two pipelines was associated with two addressable [[FIFO (computing and electronics)|queues]] that effectively performed limited register renaming.<ref name=zs1>{{cite journal |last1=Smith |first1=James E. |author1-link=James E. Smith (engineer) |title=Dynamic Instruction Scheduling and the Astronautics ZS-1 |journal=Computer |url=https://course.ece.cmu.edu/~ece740/f13/lib/exe/fetch.php?media=00030730.pdf |pages=21–35 |doi=10.1109/2.30730 |date=July 1989 |volume=22 |issue=7 |s2cid=329170 }}</ref> A similar decoupled architecture had been used a bit earlier in the Culler 7.<ref>{{cite web |last1=Smotherman |first1=Mark |title=Culler-7 |url=https://people.computing.clemson.edu/~mark/culler.html |website=[[Clemson University]]}}</ref> The ZS-1's ISA, like IBM's subsequent POWER, aided the early execution of branches.