iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
A very large instruction word (VLIW) is a wide processor instruction that encodes several operations intended to run in parallel. The compiler selects independent operations and places them into the word’s slots; the processor executes the packaged work using its functional units. The same architecture is also commonly called Very Long Instruction Word.
What does VLIW mean?
VLIW is an instruction-set architecture approach, not the name of one processor model. Sources use both “Very Large Instruction Word” and “Very Long Instruction Word” for the same idea: a single wide word contains multiple operations rather than one conventional scalar operation. The University of Texas at Austin’s Electrical and Computer Engineering glossary describes the compiler as specifying which instructions are fetched together in a VLIW word: University of Texas glossary.
How does a VLIW instruction work?
- The compiler finds independent operations. It analyzes the program’s data dependencies and determines which operations can proceed at the same time. If one operation needs the result of another, they cannot be scheduled together as independent work.
- The compiler places operations into slots. The wide instruction word has fields or slots for operations. The compiler packages compatible work into those positions, making the intended parallelism explicit in the instruction stream.
- The processor executes the packaged work. Its functional units perform the operations in the bundle concurrently, subject to the architecture’s supported operations and resources.
This division of labor is central to VLIW: the compiler schedules parallel work, while the processor executes the resulting bundle. The compiler therefore must account for dependencies before the program runs.
Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesHow is VLIW different from scalar instruction execution?
| Aspect | Scalar instruction execution | VLIW |
|---|---|---|
| Operations encoded | A conventional instruction represents one operation. | A wide word encodes several operations. |
| Where parallel work is identified | The instruction does not itself package multiple independent operations as one explicit bundle. | The compiler groups independent operations into the word’s slots. |
| Encoding and decoding | Instructions need not reserve a wide word for a set of parallel operations. | The encoding uses more bits to make parallel operations explicit; this can simplify hardware decoding of that work. |
| Insufficient independent work | Not applicable to a multi-slot bundle in the same way. | If a fixed-slot bundle has fewer independent operations than slots, some positions may be unused or represented by no-ops. |
The tradeoff is not simply “more parallelism.” VLIW moves responsibility for finding and scheduling independent work toward the compiler and spends instruction bits to describe that work explicitly. Whether a bundle is full depends on the program’s dependencies and the architecture’s available slots.
#1 Best Overall
- Equipped with a 240 MHz dual-core processor, 16 MB of flash memory, 8 MB of PSRAM, and 512 KB of SRAM, this device offers rapid response times and ample storage resources, ensuring the smooth operation of complex programs and multitasking applications. It provides reliable support for image processing, machine vision, and multifunctional projects.
- Equipped with a 1MP USB camera module featuring a 65° field of view, it delivers natural and clear images. It features a fixed-focus lens design and supports lens rotation for focus adjustment, making it suitable for image recognition, real-time video capture, and automation control scenarios.
- The camera module supports video output up to 1280×720@15fps, as well as 854×480@20fps, 800×480@20fps, 640×480@30fps, 480×320@30fps, and 320×240@30fps, allowing users to flexibly select the appropriate video capture resolution based on specific application requirements.
- Featuring a 44-pin expansion interface and dual Type-C ports, it offers flexible GPIO expansion capabilities along with convenient data transfer and power delivery, enabling connection to a wide range of peripheral devices. It also provides flexible data communication capabilities, supporting stable data transfer and device interconnectivity to meet the needs of smart devices and IoT applications.
- The camera supports two connection methods—USB and 4-pin cable—facilitating prototyping, functional testing, and project integration. It also supports the UVC protocol and OTG connectivity, enabling easy connection to compatible devices. Compatible with Arduino IDE Windows 10/11 Android systems, it simplifies programming, debugging, and application development.
How wide is a VLIW word?
There is no single width shared by all VLIW designs. The following figures are examples from particular sources, not a universal specification:
| Example | Width | What the source describes |
|---|---|---|
| Harvard University CS 146 lecture illustration | 112–168 bits | Seven operation fields, each 16–24 bits wide; an illustrative calculation, not a general VLIW width. Harvard lecture material. |
| U.S. Patent No. 5,530,817, issued 1996 | 128 bits | One example instruction word with five operation fields. Patent record. |
| Technische Hochschule Mittelhessen IA-64 overview | 128 bits | A bundle of three 41-bit instructions plus a 5-bit template. IA-64 overview. |
What do unused slots and no-ops mean?
Some VLIW formats use a fixed number of operation slots. When the compiler cannot find enough independent work to fill them, remaining positions may be left unused or filled with no-op operations, which do no useful computation. This is a possible consequence of fixed-slot bundling, not a rule that applies identically to every VLIW encoding. It also illustrates why the word’s width alone does not tell you how much useful work a particular program can perform in parallel.
Rank #2
- 2PCS CH32V103C8T6 core board RSIC-V development learning board CH32V
Are there examples of VLIW processors?
Louisiana State University teaching material names Itanium and Tera as examples of VLIW. These are architectural examples in a course context; that reference does not establish current product availability or market status. LSU course material.
Quick Recap
Rank #3
- PS & PL I/O Extension (FMC, Pmod, XADC)
- Onboard USB JTAG programming
- Development Boards & Kits - ARM
- Onboard USB JTAG programming
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
