| Popis: |
Publisher Summary This chapter introduces some of the basic hardware concepts involving memory that are typically found on an embedded board, specifically the different types of board memory, and the basic electrical elements that are used in building them. It is memory that is typically located outside the processor, or that can both be either integrated into the processor or located outside the processor. This includes other types of primary memory, such as Read-Only Memory (ROM), level-2+ cache, and main memory, and secondary/tertiary memory, which is the memory that is connected to the board but not the master processor directly, such as CD-ROM, floppy drives, hard drives, and tape. Primary memory is typically a part of a memory subsystem made up of three components: memory IC, address bus, and data bus. Further ROM is a type of nonvolatile memory that can be used to store data on an embedded system permanently, typically through a smaller onboard battery source that is separate from the board's main power source. The type of data stored on ROM in an embedded system is the software required by the device to function in the field after being shipped out of the factory. With RAM, commonly referred to as main memory, any location within it can be accessed directly and randomly, rather than sequentially from some starting point, and its content can be changed more than once—the number of times depending on the hardware. Unlike ROM, contents of RAM are erased if the board loses power, which means RAM is volatile. The chapter also introduces some of the key performance issues that revolve around board memory. |
