Quantitative Measurements of FPGA Utility in Special and General Purpose Processors
Barry S. Fagin
Thayer School of Engineering
Hanover, NH 03755
We present experimental results on FPGA use in special and general purpose processors, using as case studies a computational accelerator for gene sequence analysis, an integer implementation of the DLX microprocessor, and a real-time signal processor for rocket telemetry. All these devices have been successfully prototyped, and are now completely functional. We present detailed analysis of our experience with FPGAs in these machines, describing savings in chip count, power consumption, area, and cost. For all quantities except cost, measured savings were typically an order of magnitude improvement over discrete IC implementations.
Recent work at the Thayer School of Engineering has investigated the use of FPGAs in a variety of digital systems. We present here our results on FPGA usage in three advanced designs: a special purpose processor for gene sequence analysis, known as the GSP, an implementation of Patterson and Hennessy's DLX 32-bit microprocessor architecture, and a real-time signal processor for rocket telemetry know as the Plasma Frequency Tracker.
We first provide an overview of the laboratory where these machines were constructed, and then give an overview of the three processors. Architectural tradeoffs concerning the use of FPGAs are discussed. We then present our quantitative data and analyze it in detail. We compare our results in light of the special purpose and general purpose nature of the respective designs, and note similarities and differences. Finally, we offer our conclusions on the utility of FPGAs for processor design.
Throughout this paper we assume the reader is familiar with field programmable gate arrays. For background information on the devices used, the reader is referred to  and .
2.0 The Thayer Rapid Prototyping Facility
The systems described here were built at the Thayer Rapid Prototyping Facility, a laboratory for the rapid construction and evaluation of digital systems. The RPF emphasizes in-house prototype development using semi-custom ASICs and printed circuit boards to avoid the lengthy turnaround time of a VLSI-based approach. This approach uses a new printing technology developed by Direct Imaging Inc. to produce PCB's in-house, supporting 5 mil traces at 5 mil spacing, with up to 8 trace layers. The goal of the RPF is to use this technology to produce prototypes as quickly as possible.
The RPF has been involved in several successful experiments in rapid digital system design. In addition to GSP, DLX, and the Plasma Frequency Tracker, other systems include an FHT transform engine and a hardware monitor for the 68000. For further information on these and other projects, the reader is referred to references  through . All projects described in this paper have been prototyped and are fully functional.
3.0 The Processors
To better understand the use of FPGAs in our designs, we give brief overviews of each of the systems under study. The GSP and the Plasma Frequency Tracker are special purpose devices, while DLX is a general purpose 32-bit microprocessor.
3.1 The Gene Sequence Processor
One of the more recent systems to be prototyped at the RPF is the Gene Sequence Processor, a computational accelerator for gene sequence comparison. The GSP consists of two boards that connect to the NuBus slots of a Mac II f/x computer, increasing its performance by 15x for certain problems. Gene sequence comparison is essentially a dynamic programming algorithm with a particularly defined cost function. The GSP contains extra memory and special hardware to support the fast execution of DP algorithms with these functions on sequences of nucleotides. For more information on gene sequence comparison, the reader is referred to .
A block diagram of the Gene Sequence Processor is shown in Figure 1. The GSP consists of five basic subsystems: DRAM, SRAM, a counter/comparator, the host interface, and the Gene ALU.
The GSP contains 8 megabytes of DRAM, organized as a 4M x 16-bit words. The DRAM holds the dynamic programming array for the alignment computation. The sequences to be compared, cost function coefficients, and sequence lengths are downloaded into a 4k x 8 SRAM. The counter/comparator is used to generate DRAM addresses for the dynamic programming calculation, and to halt the computation when the last entry in the dynamic programming array has been calculated. The processor interface is responsible for all interaction with the host. Currently, this interface is implemented with TI NuBus chips, although general interfaces can be designed to interact with a variety of hosts.
3.2 The Plasma Frequency Tracker
The Plasma Frequency Tracker, or PFT, is one of several instruments scheduled to go on board a NASA rocket flight in January 1993. Its primary purpose is to detect the presence of Langmuir solitons, high energy events in the ionospheric plasma. The PFT consists of three boards: an onboard computer, an interface board, and the output board. The onboard computer is a dedicated Q88/B microprocessor used for data acquisition and analysis. The interface board handles data format conversion between the rocket-borne experiment and the microprocessor, and consists primarily of an Altera EPLD and an AD575 analog to digital converter. The output board contains four Actel FPGAs, a 12-bit A/D converter, and some analog circuitry. For our purposes, the most interesting part of the output board is the Soliton Detector. A block diagram of this subsystem is shown in Figure 2.
The Soliton Detector receives data from two sources: a radio receiver, and a companion instrument that calculates plasma resonance frequencies. Data from the radio receiver goes through a 12-bit A/D converter and is converted to signed magnitude representation. The Master Counter is a 20-bit counter used for grouping data, along with the Counter Register Bank, a 10 x 20-bit FIFO. The Data Register Bank is a 19 x 12-bit FIFO used in conjunction with the Window Detector and the Length Generator to implement the soliton detection algorithm. Soliton events are indicated by large changes in the plasma electric field, typically about 60dB greater than background emissions. These changes can be distinguished from noise by their bipolar nature; true soliton events will have both positive and negative magnitude changes. Thus soliton detection is accomplished by first examining the most significant 4 magnitude bits of the sampled data. If any of these bits are non-zero, a 1 is shifted in to one of two 19-bit shift registers, depending on the sign of the sample. If at any time both shift registers contain a 1, the beginning of a soliton event has been detected. (Soliton events have typical durations of several milliseconds; the PFT system is designed to record the entire event). This triggers a change in the gain state of the experiment, to accommodate the increased magnitude of the electric field, and sends the appropriate information to the router. The router coordinates multiple inputs and hands them off to the Altera EP1900 on the interface board for output format conversion.
While the output format conversion could have been handled by an Actel FPGA similar to those employed elsewhere, an EP1900 was employed for reasons of speed and expandability. Although EPLD's do not offer the integration of FPGA's, they are considerably faster. The parallel-to-serial conversion performed by the output converter requires output bit rates 12 times faster than input, placing severe performance constraints on the selected technology. While the current sampling rate specified by the telemetry experiment is within the limits of existing FPGA technology, factor increases of 2 or 4 are not. To accommodate possible rate changes in future experiments, therefore, we decided to employ a smaller and faster EPLD.
For more information on the Plasma Frequency Tracker, the reader is referred to .
3.3 The DLX Microprocessor
For our case study in general purpose processor implementation, we selected the DLX microprocessor. The DLX is a 32-bit CPU, described in detail in . This architecture is emerging as a standard machine to illustrate basic RISC principles. For readers unfamiliar with DLX, we give a brief overview here.
DLX is a complete microprocessor architecture, with 32 general purpose registers and a hard-wired zero in R0. Memory is Big Endian byte addressable, and all instruction accesses are aligned. The integer DLX integer instruction set has three basic classes: data transfer, arithmetic/logical, and control flow. (A floating point extension of DLX is described in , which we did not implement). The DLX instruction set is highly streamlined; the number of instructions and instruction formats is small, and instruction decoding is simple.
A block diagram of the Thayer DLX datapath is shown in Figure 3. The Thayer DLX employs a 2-bus architecture, driven by a 32 x 32 register file, a 32-bit ALU, 32K of static ram, and a so-called "universal unit", or UU. (A 32-bit instruction register, IR, holds the current instruction). We chose a 2-bus architecture for three reasons: 1) to match our available register files, which shared input and output pins, 2) to improve the routability of the board, and 3) to simplify the machine. This decision reflects a consistent willingness to tradeoff performance for the increased likelihood of producing a working prototype under time constraints. For more details about this and other design decisions regarding the prototyping of DLX, the reader is referred to . We discuss the Universal Unit in the next section.
4.0 FPGA Usage
The GSP, PFT, and DLX processors all make use of FPGAs to implement various design subsystems. The GSP employs five devices to implement all non-memory subsystems, the PFT uses four, while DLX employs one. FPGA utilization in each of these processors is shown in Table 1. The name of the FPGA in the design is given, along with its functionality and the device chosen.
We chose the Actel family of FPGAs for a number of reasons. Our initial experiences with routing software indicated that placement and routing of designs would be very easy on Actel parts. Additionally, the PFT was designed as a rocket payload, an environment well suited to non-volatile write-once devices. Finally, for this study we wanted to keep the device family constant across implementations to prevent technology-related effects from biasing the results.
All our systems employed the ACT1 series of FPGAs. Some of these devices are available with different pin counts and routing resources, as shown in Table 2. The part number of each device appears in column 1; the last two digits indicate the number of pins (not all of which are available to the user). All the devices we used were packaged in Plastic Leadless Chip Carriers, or PLCC's.
Actel FPGAs contain two principal resources: logic blocks for user circuitry and i/o blocks for pins. The total number of these blocks for each device appears in the second and third columns of Table 2. The area of each device in square inches appears in the next column, followed by the cost. Resource and area information are taken from . Cost figures are for small quantities, obtained from our Actel distributor in April 1992.
4.1 FPGAs in the GSP
FPGA utilization in the GSP is shown in the first section of Table 3. The GSP employs five Actel devices: three 1020's and two 1010's.
We see from Table 1 that the GSP required very little glue logic: most of the logical subsystems of the GSP proved implementable with FPGAs. Comparing Figure 1, we see that the 16-bit Gene ALU was successfully bit sliced into two bytes, each using one 1020 FPGA, while another 1020 implemented the counter/comparator subsystem. The larger of the 1010's served as the interface to the SRAM, while the smaller was used for a 10-bit 2-1 mux. This device is the only one that could have conceivably been replaced with its TTL equivalents. The remaining subsystems of Figure 1 are either RAM modules or the host interface. This latter component was implemented with standard NuBus chips, as their seemed little be gained from reimplementing an existing commercial product.
We note that since FPGAs are used for the basic comparison, other algorithms can be implemented by simply replacing the "actelbi4" parts with other FPGAs, or by using reprogrammable parts and reconfiguring them dynamically. We are investigating this as a topic for future research.
4.2 FPGAs in the PFT
FPGA utilization in the PFT is shown in the second section of Table 3. Only a single FPGA device type is employed; all FPGA's in the PFT are ACT1020-68's.
Like the GSP, the PFT was able to utilize FPGA's quite successfully. Virtually all the logic on the output board was implemented with FPGA's; due to space constraints on the rocket payload, the device literally could not have been built without them. The FPGA "dagc_contl" is used for the control section of the board; it contains the sign converter, window detector, length generator, and router. "master3a" holds the master 20-bit counter on the board, along with some related interface logic. "cntrega" holds the counter register bank, while "regbank" holds the data register bank.
4.3 FPGAs in DLX
Unlike the special purpose machines, the DLX was able to employ only a single FPGA, due to the increased complexity of its component subsytems. Referring to Figure 3, we see that DLX, like most general purpose microprocessors, required a 32-bit register file and an ALU. Both of these components were sufficiently complex as to make FPGA implementation difficult. Additionally, these subsystems could be implemented with a small number of low cost commercial ICs, negating much of the chip count reduction advantage of FPGA implementation.
We found, however, that the Universal Unit of Figure 3 was an attractive target for FPGA implementation. The UU is implemented with an Actel1020 FPGA, initially adopted to implement a 32-bit barrel shifter. As the design progressed, we discovered that more and more logic could be added to it without increasing chip count or power consumption. Thus the shifter became the UU, containing both the nontrivial sign-extension logic required by the DLX instruction set and the memory alignment circuitry. The ability to incorporate new logic into our design quickly and easily was crucial to its success; the use of a field programmable gate array was absolutely essential.
5.0 Empirical Measurements
We have analyzed the utility of each of the 9 FPGAs described in Table 1, and discuss our results here. We present measurements on IC equivalent chip count, pin count, area, power, and cost, along with measurements of logic and i/o utilization for each device.
IC equivalence measurements are shown in Table 3. All IC information is taken from  and , while FPGA data was obtained from . We describe each column of Table 3 below.
Chip count: We have estimated the equivalent number of discrete IC's replaced by an FPGA by going through the schematic and mapping the design onto discrete components. This process is approximate, in that a design specifically targeted for discrete IC's can differ in subtle ways from an FPGA implementation. Designs for FPGAs will be influenced by the items in the FPGA macro library, which may differ from the IC's listed in a databook. Logic polarities may be changed, and parts of the design that are highly tuned to a particular FPGA macro could change substantially if implemented in discrete IC's. Nonetheless, our inspection of our designs suggests that these effects will not make a significant difference in the numbers presented here, nor do they affect our qualitative conclusions.
Area: Area values in Table 3 are shown in square inches. These figures are obtained by summing the area of each of the IC's used in determining the chip count. We note that this figure represents component area only. FPGA's reduce interconnect area as well, as suggested by the next column.
Pins: The pin column is the sum of all the pins of each of the IC's used in the chip count. Reduction of pin count is an important advantage of FPGAs due to the resultant savings in interconnect area and system complexity.
Power consumption: With the resultant savings in chip count comes a corresponding savings in power consumption. Figures here use the maximum power supply current for each IC in the chip count as reported in  and , since this is the value that would be used by conservative designers working within a tight power budget. Power consumption is reported in milliwatts.
Cost: The total cost of the IC's replaced by each FPGA is shown in the last column of Table 3. Our costs are based on small quantity pricing, from a standard IC catalog.
Table 4 shows the reduction in area, power, pins, and cost due to FPGAs, obtained from data in Tables 1 through 3. We see that most of the results for area, pin count, and power reduction are quite impressive, approximating an order of magnitude reduction in each quantity. Those with more modest reductions represent an inappropriate use of the technology, and in our case were poor design choices.
Power figures for FPGAs are approximate, and are calculated via an analytical formula based on clock rates, logic module switching frequency, and i/o switching frequency. This formula is given in ; we follow the recommendations there in making conservative worst-case power estimates for an FPGA design.
Contrary to our initial expectations, the component costs of discrete IC's and FPGAs were roughly equivalent. We note, however, that component costs are only a fraction of total system costs. FPGAs reduce costs in other ways, include shrinking total area, design time, and system complexity.
Finally, Table 5 shows the utilization of logic blocks and i/o blocks for each device. We present these values separately as each may serve as a limiting factor in FPGA use. Devices with 295 or 546 logic blocks were available, along with 34, 57, or 69 i/o blocks.
From the data in Tables 1 through 5, we observe the following benefits from using FPGAs in processor design:
1) Reduced chip count. FPGA's replaced 89, 150, and 34 IC's on the GSP, PFT, and DLX processors respectively. For the GSP, discrete IC's would have required a minimum of three extra boards. For the PFT, construction with discrete IC's would have been impossible in light of the space constraints of the rocket payload. Even the single FPGA on the DLX saved 34 chips, and made possible a single board implementation.
We note, however, that a point of diminishing returns exists. "actelmx4", for example, does not represent a cost-effective use of FPGAs due to the small number of TTL equivalent IC's it replaces. The use of FPGA's in this manner is an artifact of the "bucketing" approach to design that FPGA's encourage. FPGA devices may be regarded as "buckets" into which varying portions of the design may be placed and then removed as constraints and performance goals change. The least effective FPGAs in our designs once held substantially more complex pieces of the system, left over from previous design iterations.
2) Reduced board area. A reduction in chip count brings with it a reduction in area, as shown in Table 4. This reduction is at least a factor of 7 for all but 2 FPGAs we examined. Note that this only a reduction in component area; the savings in interconnect area, while not directly measurable, would be even more significant.
3) Reduced power consumption. In every instance we examined, FPGAs consumed substantially less power than the components they replaced, even those FPGAs that were otherwise poor design choices. Reduction factors in power consumption ranged from 9.8 up to 65.49. Clearly FPGA's are a very attractive technology for systems on a tight power budget.
Qualitatively, we have observed that the use of FPGAs brings with it increased flexibility in the design process. FPGAs in a design can be viewed as unspecified components of the project whose contents change as the design evolves. It was common in the design of the GSP, for example, to migrate various pieces of the design on and off FPGAs as the design changed. On the DLX board, the "uuchip" FPGA originally contained only the logic for the barrel shifter. Control logic was later added, then removed to make room for sign extension and memory alignment circuitry.
It is difficult to specify detailed aspects of this "bucketing" process, in which FPGAs serve as buckets in which pieces of the design can be inserted and removed as needs permit. We can say, however, that its advantages appear to be equivalent to modular programming in software system design. Like software modules, portions of the design placed on FPGAs interact with the larger system through well-defined variables, in this case the FPGA input and output signals. We note, however, that in comparison with hardware engineers, software engineers have always had more flexibility of deciding how to partition their systems, and of changing that partition as system requirements evolve. FPGAs now appear to offer hardware engineers a similar flexibility, and thus may reduce the "redesign gap" between hardware and software systems.
The use of FPGAs also reduces bringup time. All our FPGAs worked correctly the first time they were programmed. The reliability of the devices eliminates many sources of errors during system bringup. The use of FPGAs to replace large numbers of discrete components meant fewer connections, fewer nets, and easier debugging.
We do note, however, some disadvantages that appeared on all designs:
1) Routing difficulties due to FPGA pin densities. The larger FPGA devices are the most attractive from the point of view of system integration, but they place severe demands on routing software due to their high pin densities. We hope to address this problem through the use of multi-layer boards and improved routing software.
2) Reduction in performance. For the GSP and DLX, the critical path goes through an FPGA; faster versions could be built if FPGAs were not used. The performance consequences are not as severe for the GSP, as its highly tuned architecture can make up for a slower critical path. For the DLX, however, no such advantage is possible, due to its general purpose architecture. The DLX board is therefore quite slow, running at 2 MHz. Performance was not an issue for the Plasma Frequency Tracker, which ran off an externally specified 500kHz clock.
Both the GSP and DLX presented us with a design choice many researchers in rapid prototyping face: "make it fast" versus "make it now". Since our interest is in the rapid production of functioning systems, we opted for the latter. Our work clearly indicates, however, that FPGAs introduce time penalties into digital systems that performance-critical designs may not be able to accommodate. This is due to both internal delays as signals are routed through the various logic blocks of the device, and on/off chip delays when signals are used elsewhere in the system. At this point in time, there appears to be a definite tradeoff between performance and flexibility, with FPGAs sacrificing the former to achieve the latter. We believe, however, that technological advances will reduce this tradeoff, and propose a detailed investigation of the relationship between these two qualities in FPGAs as a subject for future research. For further results on the effects of FPGAs on digital system performance, the reader is referred to  and .
Both the special purpose processors were better able to utilize FPGAs than the general purpose one. We believe this is due to their reduced subsystem complexity.
Special purpose processors tend to be less complex than general purpose ones; their designs discard all superfluous components, leaving only what is absolutely necessary to solve a narrowly defined problem. Accordingly, their subsystems, the logical candidates for FPGA implementation, are less complex than those of general purpose devices, which require suitable hardware to solve many basic computing tasks. This suggests that the subcomponents of a special purpose system will be easier to implement than those of a general purpose one.
Finally, we note that logic block utilization is not the only limiting factor in subsystem partitioning with FPGAs. When examining candidate devices, pin counts may become more important. As shown in Table 5, 6 of the 9 devices were i/o-limited rather than logic limited. We also found it difficult to combine logic efficiency with pin efficiency; in no case were we able to obtain 90% utilization or higher of both logic modules and i/o.
7.0 Conclusions and Future Work
Our work has outlined some of the advantages and disadvantages in employing FPGAs in digital systems, and presented quantitative data in support of our conclusions. Our results suggest that FPGAs may be more appropriate for special purpose devices than general purpose ones, but that both can make excellent use of FPGAs.
Five years ago, the rapid construction of working devices of the complexity described in this paper would have been impossible in an academic setting. Our ability to produce these systems and perform the appropriate experiments is due completely to FPGAs. We believe that as the technology advances further, systems of ever increasing complexity will become feasible to prototype.
Readers interested in applying these results to similar projects should examine various subsystems of their designs for possible FPGA implementation. Relevant parameters include complexity/gate count, I/O requirements, and performance. It is important to emphasize that both gate count and I/O are important factors in subsytem analysis, as either one can render an FPGA implementation impossible. Subsystems that are not in the critical path, or do not require aggressive clock rates, are also preferable, as current FPGAs introduce performance penalties as the price for design flexibility and reduced chip count. The challenge to designers is to match subsystem requirements with an FPGA such that the gate and pin utilization on the device is maximized. We believe this to be an important area for future CAD research.
Work at the Thayer RPF is advancing on a number of fronts. Some of our current projects include 1) multi-layer design techniques, 2) an investigation of different FPGA device families, 3) a more advanced computational accelerator for molecular genetics, and 4) a pipelined DLX implementation. It is expected that FPGAs will be an important part of these projects, and that their accompanying technology will have advanced considerably by the time these projects are prototyped. We intend to further test the hypotheses advanced here, to see if they hold under more advanced technological assumptions.
The author gratefully acknowledges the assistance of Prof. James LaBelle and the Dartmouth College Physics Department, as well as Gill Watt, Evan Gewirtz, and Pichet Chintrakulchai for their work on the development of the GSP, PFT, and DLX processors. Support for the GSP project and the Thayer Rapid Prototyping Facility was provided by the Whitaker Foundation, Sun Microsystems, Direct Imaging, Actel Corporation, Xilinx Incorporated, National Semiconductor, and Viewlogic. Support was also provided by the National Science Foundation under award #CDA-8921062.
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Figure 1: GSP Block Diagram
Figure 2: Soliton Detector Block Diagram 
Figure 3: Thayer DLX Datapath
Table 1: Processor FPGA Usage
FPGA name function device type
actelbi4 special-purpose ALU ACT1020-68 (2)
actelcc counter/comparator ACT1020-68
actelsri static ram interface ACT1010-68
actelmx4 10 bit 2-1 mux ACT1010-44
dagc_contl sign, window, length, router ACT1020-68
master3a master counter ACT1020-68
cntrega counter register bank ACT1020-68
regbank data register bank ACT1020-68
uuchip universal unit for DLX ACT1020-84
Table 2: FPGA Module Count, Area, and Cost
Device type logic i/o area (in2) cost
ACT1010-44 PLCC 295 34 .43 23.25
ACT1010-68 PLCC 295 57 .91 23.25
ACT1020-44 PLCC 546 34 .43 43.30
ACT1020-68 PLCC 546 57 .91 43.30
ACT1020-84 PLCC 546 69 1.32 43.30
Table 3: IC Equivalents for FPGAs
FPGA name count area (in2) pins power (mW) cost ($)
actelbi4 53 10.54 874 12492.5 69.76
actelcc 26 7.03 502 8241 29.80
actelsri 7 1.61 128 2065 7.28
actelmx4 3 .55 48 345 2.64
dagc_contl 41 9.31 644 7882.5 57.97
master3a 33 14.36 624 5878 53.43
cntreg 34 7.88 624 5000 37.92
regbank 42 9.28 780 6560 46.84
uuchip 34 7.52 923 8768 72.27
Table 4: IC/FPGA Ratios
FPGA name area power pins cost
actelbi4 11.58 65.49 12.85 1.61
actelcc 7.72 19.29 7.38 .69
actelsri 1.77 27.14 1.88 .31
actelmx4 1.29 9.80 1.09 .11
dagc_contl 10.89 15.46 9.47 1.34
master3a 58.71 11.50 9.18 1.23
cntrega 8.66 22.22 9.18 .88
regbank 10.20 24.94 11.47 1.08
uuchip 5.70 24.48 10.99 1.66
Table 5: FPGA Utilization
FPGA name logic/total % logic utilized io/total % i/o utilized
actelbi4 386/546 70.70 44/57 77.19
actelcc 444/546 81.32 55/57 96.49
actelsri 78/295 26.44 51/57 89.47
actelmx4 15/295 5.08 31/34 91.18
dagc_contl 276/546 50.55 56/57 98.24
master3a 501/546 91.76 44/57 77.19
cntrega 490/546 89.74 32/57 56.14
regbank 524/546 95.97 24/57 42.10
uuchip 484/546 88.64 65/69 94.20