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ARM System Developer’s Guide phần 5

Mã này hoạt động bởi vì trong sổ đăng ký máy tính là pipelined. Máy tính đến từ method_0 khi ARM thực hiện hướng dẫn LDR. Phương pháp trên là rất nhanh, nhưng có một nhược điểm: mã không phải là vị trí độc lập kể từ khi các cửa hàng | 268 Chapter 8 Digital Signal Processing or in integer C Y t isqrt X t 2 d-n The function isqrt finds the nearest integer to the square root of the integer. See Section 7.4 for efficient implementation of square root operations. 8.1.7 SuMMARy How TO Represent a DiGiTAL SiGNAL To choose a representation for a signal value use the following criteria Use a floating-point representation for prototyping algorithms. Do not use floating point in applications where speed is critical. Most ARM implementations do not include hardware floating-point support. Use a fixed-point representation for DSP applications where speed is critical with moderate dynamic range. The ARM cores provide good support for 8- 16- and 32-bit fixed-point DSP. For applications requiring speed and high dynamic range use a block-floating or logarithmic representation. Table 8.2 summarizes how you can implement standard operations in fixed-point arithmetic. It assumes there are three signals x t c t y t that have Qn Qm Qd representations X t C t Y t respectively. In other words X t 2nx t C t 2mc t Y t 2dy t 8.19 to the nearest integer. To make the table more concise we use as shorthand for an operation that is either a left or right shift according to the sign of the shift amount. Formally x s x s if s 0 Table 8.2 Summary of standard fixed-point operations. Signal operation y t x t y t x t c t y t x t -c t y t x t c t y t x t c t y t sqrt x t Integer fixed-point equivalent Y t X t d-n Y t X t d-n C t d-m Y t X t d-n - C t d-m Y t X t C t d-n-m Y t X t d-n m C t Y t isqrt X t 2 d-n 8.2 Introduction to DSP on the ARM 269 x -s if s 0 and rounding is not required x round -s if s 0 and rounding is required round 1 -1-s if 0.5 should round up 1 -1-s -1 if 0.5 should round down You must always check the precision and dynamic range of the intermediate and output values. Ensure that there are no overflows or unacceptable losses of precision. These considerations determine the representations and size to use for .