In this dissertation, a novel SIMD extension called Modified MMX (MMMX) for multimedia computing is presented. Specifically, the MMX architecture is enhanced with the extended subwords and the matrix register file techniques. The extended subwords technique uses SIMD registers that are wider than the packed format used to store the data. The extended subwords technique avoids data type conversion overhead and increases parallelism in SIMD architectures. This is because promoting the subwords of the source SIMD registers to larger subwords before they can be processed and demoting the results again before they can be written back to memory incurs conversion overhead. The matrix register file technique allows to load data that is stored consecutively in memory into a column of the register file, where a column corresponds to the corresponding subwords of different registers. In other words, this technique provides both row-wise as well as column-wise accesses to the media register file. It is a useful approach for matrix operations that are common in multimedia processing. In addition, in this work, new and general SIMD instructions addressing the multimedia application domain are investigated. It does not consider an ISA that is application specific. For example, special-purpose instructions are synthesized using a few general-purpose SIMD instructions. The performance of the MMMX architecture is compared to the performance of the MMX/SSE architecture for different multimedia applications and kernels using the sim-outorder simulator of the SimpleScalar toolset. Additionally, three issues related to the efficient implementation of the 2D Discrete Wavelet Transform (DWT)on general-purpose processors, in particular the Pentium 4, are discussed. These are 64K aliasing, cache conflict misses, and SIMD vectorization. 64K aliasing is a phenomenon that happens on the Pentium 4, which can degrade performance by an order of magnitude. It occurs if two or more data items whose addresses differ by a multiple of 64K need to be cached simultaneously. There are also many cache conflict misses in the implementation of vertical filtering of the DWT, if the filter length exceeds the number of cache ways. In this dissertation, techniques are proposed to avoid 64K aliasing and to mitigate cache conflict misses. Furthermore, the performance of the 2D DWT is improved by exploiting the data-level parallelism using the SIMD instructions supported by most general-purpose processors.