| has abstract
| - The Heterogeneous Element Processor (HEP) was introduced by in 1982. The HEP's architect was Burton Smith. The machine was designed to solve fluid dynamics problems for the Ballistic Research Laboratory. A HEP system, as the name implies, was pieced together from many heterogeneous components -- processors, data memory modules, and I/O modules. The components were connected via a switched network. A single processor, called a PEM, in a HEP system (up to sixteen PEMs could be connected) was rather unconventional; via a "program status word (PSW) queue," up to fifty processes could be maintained in hardware at once. The largest system ever delivered had 4 PEMs. The eight-stage instruction pipeline allowed instructions from eight different processes to proceed at once. In fact, only one instruction from a given process was allowed to be present in the pipeline at any point in time. Therefore, the full processor throughput of 10 MIPS could only be achieved when eight or more processes were active; no single process could achieve throughput greater than 1.25 MIPS. This type of multithreading processing classifies the HEP as a barrel processor. The hardware implementation of the HEP PEM was emitter-coupled logic. Processes were classified as either user-level or supervisor-level. User-level processes could create supervisor-level processes, which were used to manage user-level processes and perform I/O. Processes of the same class were required to be grouped into one of seven user tasks and seven supervisor tasks. Each processor, in addition to the PSW queue and instruction pipeline, contained instruction memory, 2,048 64-bit general purpose registers and 4,096 constant registers. Constant registers were differentiated by the fact that only supervisor processes could modify their contents. The processors themselves contained no data memory; instead, data memory modules could be separately attached to the switched network. The HEP memory consisted of completely separate instruction memory (up to 128 MBs) and data memory (up to 1 GB). Users saw 64-bit words, but in reality, data memory words were 72-bit with the extra bits used for state, see next paragraph, parity, tagging, and other uses. The HEP implemented a type of mutual exclusion in which all registers and locations in data memory had associated "empty" and "full" states. Reading from a location set the state to "empty," while writing to it set the state to "full." A programmer could allow processes to halt after trying to read from an empty location or write to a full location, enforcing critical sections. The switched network between elements resembled, in many ways, a modern computer network. On the network were sets of nodes, each of which had three links. When a packet arrived at a node, it consulted a routing table and attempted to forward the packet closer to its destination. If a node became congested, any incoming packets were passed on without routing. Packets treated in such a manner had their priority level increased; when several packets vied for a single node, a packet with a higher priority level would be routed before ones with lower priority levels. Another component of the switched network was the sO System, with its own memory and many individual DEC UNIBUS buses attached for disks and other peripherals. The system also had the ability to save the full/empty bits not normally visible directly. The initial IO System performance was shown to be woefully inadequate due to the high latency in starting the IO operations. Ron Natalie (from BRL) and Burton Smith designed a new system out of spare parts on napkins at a local steakhouse and put it into operation in the course of the ensuing week. The HEP's primary application programming language was a unique Fortran variant. In time C, Pascal, and SISAL were added. The syntax of data variables using full-empty bits prepended '$' before their name. So 'A' would name a local variable, but $A would be a locking full-empty variable. Application dead-lock was thus possible. Problematic, failure to '$' could introduce unintended numerical inaccuracy. The first HEP operating system was HEPOS. Mike Muuss was involved in a Unix port for the Ballistic Research Laboratory. HEPOS was not a Unix-like operating system. Although it was known to have poor cost-performance, the HEP received attention due to what were, at the time, several revolutionary features. The HEP had the performance of a CDC 7600-class computer in the Cray-1 era. HEP systems were acquired by the Ballistic Research Laboratory (four PEM system), Los Alamos, the Argonne National Laboratory (single PEM), the National Security Agency, and Germany's Messerschmitt (three PEMS system. Denelcor also delivered a two PEM system to the University of Georgia in exchange for them providing software assistance (the system had also been offered to the University of Maryland). Messerschmitt was the only client to put the HEP into use for "real" applications; the other clients used it for experimenting with parallel algorithms. The BRL system was used to prepare a movie using the BRL-CAD software as its only real application.Faster and larger designs for HEP-2 and HEP-3 were started but never completed. The architectural concept would later be embodied with the code-name Horizon. (en)
- L'Heterogeneous Element Processor (HEP) è un sistema presentato dalla società Denelcor nel 1982 ed è noto per essere stato il primo computer commerciale MIMD. Il sistema HEP come indicato dal nome era composto da un insieme eterogeneo di elementi - processori, moduli di memoria e moduli di I/O collegato tramite una rete informatica. Un processore HEP (fino a 16 ne potevano essere collegati) era un tipo di processore poco convenzionale, tramite una particolare coda fino a 50 processi potevano essere caricati in un singolo processore. Il processore era dotato di una pipeline a otto stadi e dato che l'istruzione di un singolo processo poteva occupare al massimo un singolo stadio della pipeline servivano almeno otto processi per sviluppare la piena potenza di 10 MIPS del sistema. Inoltre ogni processo poteva ottenere al massimo 1.25 MIPS dal processore. Questo tipo di multithreading classificava i moduli HEP come . I singolo processi venivano classificati come livelli utente o livelli supervisore. I livelli utente potevano creare i livelli supervisore, questi erano utilizzati per coordinare i processi a livello utente o per le operazioni di I/O. I processi della stessa classe venivano raccolti in gruppi, vi erano sette gruppi utente e sette gruppi supervisore. Ogni processore in aggiunta alla coda d'istruzioni caricata dalla pipeline contiene una memoria per le istruzioni. Questa memoria è formata da 2048 registri a 64 bit e da 4096 registri "costanti". Questi registri si differenziano dagli altri unicamente per il fatto che solo i processi supervisore sono in grado di modificarli. È da notare che i processori non contengono memoria per i dati, questi vanno caricati nei moduli di memoria che sono inseriti nella rete informatica che crea il computer. Il sistema HEP gestisce la memoria in mutua esclusione. Ad ogni registro e a ogni locazione di memoria è associato uno stato. Questo stato può essere del tipo "libero" o "occupato". Se la locazione viene letta questa viene settata nello stato "libero" mentre se viene scritta questa viene settata allo stato "occupato". Usando questi stati i programmatori potevano scrivere dei programmi che gestissero l'elaborazione di dati comuni. La rete informatica che collega i vari moduli ricorda molto una moderna rete telematica. Ogni nodo a tre collegamenti a altri nodi, quando un pacchetto di dati arriva a un nodo questo consulta una tabella di routing e decide dove smistarlo, nel caso il collegamento di un nodo sia congestionato questo può passare i pacchetti ad altri nodo al fine di aggirare il collo di bottiglia. I pacchetti hanno una priorità il pacchetto a più alta priorità viene servito per prima. Il sistema era noto per l'elevato costo rispetto alle modeste prestazioni ma comunque divenne noto nell'ambiente informatico per la sua rivoluzionaria architettura. Il sistema HEP venne acquisito dai Los Alamos National Laboratory, dagli Argonne National Laboratory, dal Ballistic Research Laboratory e da altri laboratori. Il sistema fu acquisito da molti centri di ricerca nonostante le sue modeste prestazioni per via delle sue peculiarità che facilitavano la realizzazione di programmi paralleli. La tedesca Messerschmitt AG fu l'unica società che utilizzò il computer per applicazioni reali, gli altri enti utilizzavano il sistema per lo studio degli algoritmi paralleli. (it)
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