Very Long Instruction Word - Backward Compatibility

Backward Compatibility

When silicon technology allowed for wider implementations (with more execution units) to be built, the compiled programs for the earlier generation would not run on the wider implementations, as the encoding of the binary instructions depended on the number of execution units of the machine.

Transmeta addresses this issue by including a binary-to-binary software compiler layer (termed Code Morphing) in their Crusoe implementation of the x86 architecture. Basically, this mechanism is advertised to recompile, optimize, and translate x86 opcodes at runtime into the CPU's internal machine code. Thus, the Transmeta chip is internally a VLIW processor, effectively decoupled from the x86 CISC instruction set that it executes.

Intel's Itanium architecture (among others) solved the backward-compatibility problem with a more general mechanism. Within each of the multiple-opcode instructions, a bit field is allocated to denote dependency on the previous VLIW instruction within the program instruction stream. These bits are set at compile time, thus relieving the hardware from calculating this dependency information. Having this dependency information encoded into the instruction stream allows wider implementations to issue multiple non-dependent VLIW instructions in parallel per cycle, while narrower implementations would issue a smaller number of VLIW instructions per cycle.

Another perceived deficiency of VLIW architectures is the code bloat that occurs when not all of the execution units have useful work to do and thus have to execute NOPs. This occurs when there are dependencies in the code and the functional pipelines must be allowed to drain before subsequent operations can proceed.

Since the number of transistors on a chip has grown, the perceived disadvantages of the VLIW have diminished in importance. The VLIW architecture is growing in popularity, particularly in the embedded market, where it is possible to customize a processor for an application in an embedded system-on-a-chip. Embedded VLIW products are available from several vendors, including the FR-V from Fujitsu, the BSP15/16 from Pixelworks, the ST231 from STMicroelectronics, the TriMedia from NXP, the CEVA-X DSP from CEVA, the Jazz DSP from Improv Systems, and Silicon Hive. The Texas Instruments TMS320 DSP line has evolved, in its C6xxx family, to look more like a VLIW, in contrast to the earlier C5xxx family.