Delaying physical register allocation through virtual-physical registers

Abstract

We report ellipsometric measurements made on semiconductor samples using photon-correlated beams produced by the process of spontaneous parametric down-conversion. Such a source yields higher accuracy than its quantum-limited conventional counterpart. We also show that our approach has the added advantage of not requiring an external reference sample for calibration. I. BACKGROUNDSince all optical measurements are limited by quantum noise, which dominates at low light levels, there has been a strong interest in developing nonclassical optical sources with sub-Poisson photon statistics that offer sub-shot-noise accuracy. One implementation that has been considered for metrology applications is based on the use of two optical beams, each with Poisson-distributed photons, but also with a fully correlated joint photon counting distribution. Such correlated-photon beams have been generated, for example, by spontaneous parametric down-conversion (SPDC) in a nonlinear optical crystal, and used for applications including quantum cryptography [1], teleportation [2], and metrology [3,4]. If one of the beams is reflected from, or transmitted through, a sample, then measurement of the photon coincidence rate, together with the mean photon counts in each beam, yield estimates of the sample reflectance/transmittance with accuracy greater than the conventional measurement using a single beam [5][6][7][8][9]. In this paper, we consider the use of photon-correlated beams in ellipsometry.Ellipsometry [10-15] is a technique in which the polarization of light is used to determine the optical properties of a material (sample) and infer information such as the thickness of a thin film. The sample is characterized by two parameters = arctan͉ r 2 / r 1 ͉ and ⌬ = arg͑r 2 / r 1 ͒ where r 1 and r 2 are the sample's eigenpolarization complex reflection coefficients [11]. In a conventional ellipsometer, these parameters are extracted by manipulation of the polarization state of the incident or the reflected/transmitted light and measurement of the optical intensities or the photon counting rates. Clearly, such measurements are limited by shot noise, particularly at low light intensities or when using ellipsometers employing a nulling technique. The use of photon-correlated beams in ellipsometry has been previously reported and referred to as "quantum ellipsometry" [16,17]. It was shown that this technique alleviates the need for calibration using an external reference sample.In this paper, we report experimental quantum ellipsometric measurements made on standard optical samples. We also estimate the accuracy advantage attained by the use of quantum relative to conventional ellipsometry. Section II of the paper reviews the theory of correlated-photon ellipsometry, a form of quantum ellipsometry. Although correlated-photon pairs may be generated by a variety of means, correlatedphoton ellipsometry in this paper refers to the use of photon pairs generated by SPDC. In Sec. III we present experimental results obtained from two semiconductor sa...

Microarchitectural Support for Speculative Register Renaming

“…The Physical Register File (PRF) is located in a critical path and can limit the processor clock frequency [2][11], so several directions have been proposed to reduce its access latency, such as reducing the number of ports [2][5] [9][12] [24], reducing the number of physical registers [3] [4][10] [20][22] [28], or using a few, narrow entries to encode frequent values [18].…”

Out-of-Order Commit Processors

“…In [22], the authors propose a mechanism that allows early recycling of registers, while at the same time providing precise exceptions. In [21], a technique that delays allocation of physical registers to instructions to the execution stage is presented. The mechanism uses tags, named virtual registers, to keep track of dependences between instructions and to convert these tags into real physical registers when the instruction produces its result.…”

Exploiting value locality in physical register files

“…Different mapping strategies can be used to provide different ways of storing values and different ways of allowing consumer instructions to get the appropriate value. An early proposal for optimizing the use of physical registers was the concept of virtual-physical registers [8,12]. Here a virtual name (a virtual-physical register) is assigned to a value during the rename stage, but an actual physical register (i.e., storage) is not allocated to hold the value until it has been produced.…”

“…The pioneering work in this area was the work by González et al on virtual-physical registers [8,12]. Delaying the allocation of a physical register reduces the lifetime of a physical register and thereby the physical register requirements.…”

“…An orthogonal approach for physical register design is to make more efficient use of the storage by changing the storage management strategy. Pioneering work by González et al proposed the concept of virtualphysical (VP) registers [8,12]. VP registers make more efficient use of the physical register storage by delaying the allocation of a physical register until the time the value is ready, as opposed to the time the architectural register is renamed in the conventional management strategy.…”