tailieunhanh - Operating-System concept 7th edition phần 9

máy tính để bàn môi trường máy tính rất nhiều sẽ làm tăng chi phí của hệ thống thời gian thực, mà có thể làm cho hệ thống như vậy kinh tế không thực tế. Xem xét bổ sung áp dụng khi xem xét bộ nhớ ảo trong một hệ thống thời gian thực. Cung cấp các tính năng bộ nhớ ảo như mô tả trong Chương 9, | Features of Real-Time Kernels 699 physical 5 memory Figure Address translation in real-time systems. desktop computing environments would greatly increase the cost of real-time systems which could make such systems economically impractical. Additional considerations apply when considering virtual memory in a real-time system. Providing virtual memory features as described in Chapter 9 require the system include a memory management unit MMU for translating logical to physical addresses. However MMUs typically increase the cost and power consumption of the system. In addition the time required to translate logical addresses to physical addresses especially in the case of a translation look-aside buffer TLB miss maybe prohibitive in a hard real-time environment. In the following we examine several appraoches for translating addresses in real-time systems. Figure illustrates three different strategies for managing address translation available to designers of real-time operating systems. In this scenario the CPU generates logical address L that must be mapped to physical address p. The first approach is to bypass logical addresses and have the CPU generate physical addresses directly. This technique known as real-addressing mode does not employ virtual memory techniques and is effectively stating that p equals L. One problem with re al-addressing mode is the absence of memoiỵ protection between processes. Re al-addressing mode may also require that programmers specify the physical location where their programs are loaded into memory. However the benefit of this approach is that the system is quite fast as no time is spent on address translation. Real-addressing mode is quite common in embedded systems with hard real-time constraints. In fact some real-time operating systems running on microprocessors containing an MMU actually disable the MMU to gain the performance benefit of referencing physical addresses directly. A second strategy for translating .