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REAL-TIME SYSTEMS DESIGN AND ANALYSIS phần 3

Điều này đặc biệt quan trọng cho các ứng dụng đa phương tiện mà không bị gián đoạn vận chuyển dữ liệu quan trọng và thời gian giao hàng trong thời gian làm giảm nhu cầu để đệm tốn kém. Điều này làm cho nó lý tưởng cho các thiết bị cần phải chuyển mức độ cao của dữ liệu trong thời gian thực, chẳng hạn như máy ảnh, VCR, và truyền hình. | 82 3 REAL-TIME OPERATING SYSTEMS void int3 void interrupt handler 3 save context save context on stack task3 execute task 3 restore context restore context from stack Procedure save saves certain registers to a stack area whereas restore restores those registers from the stack. In practice save and restore would actually take two arguments a pointer to data structure representing the context information and a pointer to the stack data structure which will be discussed later. In the case of the context data structure the programming language compiler must provide a mechanism to extract the current contents of the general registers PCs and so forth.2 Finally both save and restore must adjust the stack pointer which is illustrated later. 3.1.3 Preemptive-Priority Systems A higher-priority task is said to preempt a lower-priority task if it interrupts the lower-priority task. Systems that use preemption schemes instead of round-robin or first-come-first-served scheduling are called preemptive-priority systems. The priorities assigned to each interrupt are based on the urgency of the task associated with the interrupt. For example the nuclear power station monitoring system is best designed as a preemptive-priority system. While the handling of intruder events is critical for example nothing is more important than processing the core over-temperature alert. Prioritized interrupts can be either fixed priority or dynamic priority. Fixed-priority systems are less flexible since the task priorities cannot be changed. Dynamic-priority systems can allow the priority of tasks to be adjusted at runtime to meet changing process demands. Preemptive-priority schemes can suffer from resource hogging by higher-priority tasks. This can lead to a lack of available resources for lower-priority tasks. In this case the lower-priority tasks are said to be facing a problem called starvation. A special class of fixed-rate preemptive-priority interrupt-driven systems called rate-monotonic .