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LSH32 Datasheet(PDF) 5 Page - LOGIC Devices Incorporated |
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LSH32 Datasheet(HTML) 5 Page - LOGIC Devices Incorporated |
5 / 9 page DEVICES INCORPORATED LSH32 32-bit Cascadable Barrel Shifter Special Arithmetic Functions 08/16/2000–LDS.32-Q 5 tially the LSH32s are arranged in multiple rows or banks such that the inputs to successive rows are left- shifted by 16 positions. The outputs of each row are multiplexed onto a three-state bus. The normalization problem then reduces to selecting from among the several banks that one which has the first non-zero bit of the input value among its 16 most significant positions. If the most significant one in the input file was within the upper 16 locations of a given bank, the SO4 output of the most significant slice in that bank will be low. Single clock normalization can thus be accomplished simply by enabling onto the three-state output bus the highest priority bank in which this condition is met. In this way the input word will be normalized regardless of the number of shift positions required to accomplish this. The number of shift positions can be determined simply by concatenation of the SO3–SO0 outputs of the most significant slice in the selected row with the encoded Output Enable-bits determining the row number. Note that lower rows need not be fully populated. This is because they represent left shifts in multiples of 16 positions, and the lower bits of the output word will be zero filled. In order to accomplish this zero fill, the least significant device in each row is always enabled, and the row select is instead connected to the SI4 input. This will force the shift length of the least significant device to a value greater than 15 whenever the row containing that device is not selected. This results in zero fill being accom- plished by the equivalently positioned slice in a higher bank, as shown in the diagram. BLOCK FLOATING POINT With a small amount of external logic, block floating point operations are easily accomplished by the LSH32. Data resulting from a vector operation are applied to the LSH32 with the NORM-input deasserted. The SO4– SO0 outputs fill then represent the normalization shift distance for each vector element in turn. By use of an external latch and comparator, the maximum shift distance encountered across all elements in the vector is saved for use in the next block opera- tion (or block normalization). During this subsequent pass through the data, the shift code saved from the previous pass is applied uniformly across all elements of the vector. Since the LSH32 is not used in the internal normalize mode, this operation can be pipelined, thereby obtaining the desired shift distance for the next pass while simultaneously applying the normalization required from the previous pass. |
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