Designing Embedded Hardware

Memory

Memory is used to hold data and software for the processor. There is a variety of memory types, and often a mix is used within a single system. Some memory will retain its contents while there is no power, yet will be slow to access. Other memory devices will be high capacity, yet will require additional support circuitry and will be slower to access. Still other memory devices will trade capacity for speed, giving relatively small devices, yet are capable of keeping up with the fastest of processors.

Memory can be organized in two ways, either word-organized or bit-organized. In the word-organized scheme, complete nybbles, bytes, or words are stored within a single component, whereas with bit-organized memory, each bit of a byte or word is allocated to a separate component (Figure 1-10).

Figure 1-10. Eight bit-organized 8 x 1 devices and one word-organized 1 x 8 device

Memory chips come in different sizes, with the width specified as part of the size description. For instance, a DRAM (dynamic RAM) chip might be described as being 4M x 1 (bit-organized), whereas a SRAM (static RAM) may be 512k x 8 (word-organized). In both cases, each chip has exactly the same storage capacity, but they are organized in different ways. In the DRAM case, it would take eight chips to complete a memory block for an 8-bit data bus, whereas the SRAM requires only one chip.

However, because the ...