A memory mangement unit’s (MMU) main responsibility is to define and enforce the boundaries that separate different tasks or processes. To understand the effect of separating tasks, consider two different office environments. In the first one (the formal environment), there is an individual office for each employee. Each employee has a key to the door of his or her office and is considered the owner of that space. The second environment (the informal environment) uses a bullpen setup. There are no walls between employees, and everyone is on the same floor in the same air space. Each one of these configurations has its particular advantages and disadvantages. In the formal environment, you don’t have to worry about one employee bothering another employee because there are walls between them. No matter how noisy one employee gets, the neighboring worker does not hear the disturbance. This is not the case for an informal setup. If an employee is talking loudly on the phone in the open environment, this inconsiderate individual distracts the other workers. If, on the other hand, the employees are considerate, two employees can quickly communicate with each other without needing to leave their seats.
Similarly, if one employee needs to borrow another employee’s stapler, it’s just a toss away. In the formal environment, each time communication needs to take place, the communicating employee needs to go through some series of steps, like making a phone call or walking to the other office.
Each employee can be compared to a block of code (or task) that is designed to perform some specified job in an embedded systems program. The air space equates to the memory space of the target system, and the noise generated by a misbehaving employee can be equated to a bug that corrupts the memory space used by some other task. The MMU in hardware, if configured properly, can be compared to the walls of the formal office environment. Each task is bounded by the limitations placed on it by the MMU, which means that a task that somehow has a bad pointer cannot corrupt the memory space of any other task. However, it also means that when one process wants to talk to another process, more overhead is required.
Later, in this book, you will see that code bounded by the MMU protection is called a process and code not bounded by an MMU is called a task. A few years ago, the full use of an MMU in an embedded system was rare. Even today, most embedded
systems don’t fully use the capabilities of the MMU; however, as embedded systems become more and more complex and CPU speeds continue to rise, the use of an MMU to provide walls between processes is becoming more common.
Similarly, if one employee needs to borrow another employee’s stapler, it’s just a toss away. In the formal environment, each time communication needs to take place, the communicating employee needs to go through some series of steps, like making a phone call or walking to the other office.
Each employee can be compared to a block of code (or task) that is designed to perform some specified job in an embedded systems program. The air space equates to the memory space of the target system, and the noise generated by a misbehaving employee can be equated to a bug that corrupts the memory space used by some other task. The MMU in hardware, if configured properly, can be compared to the walls of the formal office environment. Each task is bounded by the limitations placed on it by the MMU, which means that a task that somehow has a bad pointer cannot corrupt the memory space of any other task. However, it also means that when one process wants to talk to another process, more overhead is required.
Later, in this book, you will see that code bounded by the MMU protection is called a process and code not bounded by an MMU is called a task. A few years ago, the full use of an MMU in an embedded system was rare. Even today, most embedded
systems don’t fully use the capabilities of the MMU; however, as embedded systems become more and more complex and CPU speeds continue to rise, the use of an MMU to provide walls between processes is becoming more common.
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