Field emission from two special kinds of emitters has been studied in detail: intentionally introduced particles of controlled geometry, and sites produced by intentional mechanical damage to the cathode surface. We found that the size of particles seems to play no role in their threshold field but their shapes are a determinant factor since spherical particles do not emit for fields up to 120 MV m-1. The method of creation of damage sites, and the similarity of their emission on Nb and Au substrates, suggest the possibility that emission comes from geometrical protrusions. A model of superposed geometrical protrusions is proposed to explain the enhanced field emission behaviour of this particular type of surface defect.

Loop tiling is a well-known loop transformation generally used to expose coarse-grain parallelism and to exploit data reuse at the cache level. Tiling can also be used to exploit data reuse at the register level and to improve a program's ILP. However, previous proposals in the literature (as well as commercial compilers) are only able to perform multidimensional tiling for the register level when the iteration space is rectangular. In this article we present a new general algorithm to perform multidimensional tiling for the register level in both rectangular and nonrectangular iteration spaces. We also propose a simple heuristic to determine the tiling parameters at this level. Finally, we evaluate our method using as benchmarks typical linear algebra algorithms having nonrectangular iteration spaces and compare our proposal against hand-optimized vendor-supplied numerical libraries and against commercial compilers able to perform optimizing code transformations such as inner unrolling, unroll-and-jam, and software pipelining. Measurements were taken on three different superscalar microprocessors. Results will show that our method outperforms the native compilers (showing speedups of 2.5 in average) and matches the performance of vendor-supplied numerical libraries. The general conclusion is that compiler technology can make it possible for nonrectangular loop nests to achieve as high performance as hand-optimized codes.

Tiling is a well-known loop transformation, which is basically used to expose coarse-grain parallelism and to exploit data reuse at the cache level. However, it can also be used to exploit data reuse at the register level and to improve programs's ILP. Previous work on tiling and also commercial compilers are able to perform tiling for the register level in more than one dimension when the iteration space is rectangular. Non-rectangular iteration spaces are commonly found in linear algebra algorithms or can arise as a result of applying previous transformations such as loop skewing. In this paper we evaluate the technique we present in [11] which is able to perform tiling for the register level in more than one dimension in both rectangular and non-rectangular iteration spaces. We use typical linear algebra algorithms having non-rectangular iteration spaces as benchmarks and compare our proposal against commercial preprocessors able to perform optimizing code transformations such as inner unrolling, outer unrolling and software pipelining. We will also present quantitative data showing the benefits of tiling only for the register level, tiling only for the cache level and tiling for both levels simultaneously. Results measured on a ALPHA 21164 processor show that tiling for both cache and register levels improves upon commercial compilers and preprocessors by factors in the range of 1.3 to 6.3.

The electron field emission from various kinds of particles ( approximately 20 mu m) placed on plane cathodes has been measured in a SEM modified by the addition of a scanning anode tip. Results are similar for cathodes of Au (oxide free) and Nb (with thin insulating native oxide). In general no emission is found from insulating particles (Al2O3, SiO2) or particles with insulating oxides (Nb,Ti) under fields at least as high as 100 MV m-1, while oxide free particles (Ag, Au) and those with a conducting oxide (Fe) tend to emit strongly at much lower fields. Anodizing the Nb cathode surface to >200 nm produces no significant change in emission; after initial emission the particle/substrate contact is found to be conducting. With rare exceptions, these results are consistent with a classic metallic projection model.