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4.3 Instruction Encoding

Defoe is a 64-bit compressed VLIW architecture. In an uncompressed VLIW system, MultiOps have a fixed length. When suitable operations are not available to fill the issue slots within a MultiOp, NOPs are inserted into those slots. A compressed VLIW architecture uses variable length MultiOps to get rid of those NOPs and achieve high code density. In the Defoe, individual operations are encoded as 32-bit words. A special stop bit in the 32-bit word indicates the end of a MultiOp. Common arithmetic operations have an immediate mode, where a sign or zero extended 8-bit constant may be used as an operand. For larger constants of 16, 32 or 64 bits, a special NOP code may be written into opcode field of the next operation and the low order bits may be used to store the constant. In that case, the predecoder concatenates the bits from 2 or more different words to assemble a constant. Figure 2 depicts the instruction format.

Figure 2: Instruction Encoding
\includegraphics{figures/fig2.eps}


next up previous contents
Next: 4.4 Instruction Dispersal and Up: 4 Defoe: An Example Previous: 4.2 Registers and Predication   Contents
Binu K. Mathew