CHAPTER 9. COMPUTERS AND DEVICE INTERCONNECTION173dynamic RAM in which a bit of memory is a storage capacitor in either the charged ordischarged condition. The term dynamic refers to the need to periodically renew orrefresh the slowly discharging capacitor.Compared to dynamic memory, static memory has the following advantages. It is simplerto use, about ten times faster and more reliable. On the other hand, it is more expensive,consumes more power and requires more physical space. Because of power consumption inan IC the largest static RAM is 16K bits. The largest dynamic RAM is 260K and hence isused for normal applications while the static RAM is used for special fast applications withinthe same computer. Both types of RAM are volatile, meaning that stored information islost when power is removed from the chip. Some computer designs provide a limited amountof non-volatile read/write RAM storage by using special low-power (and slower) dynamicmemories powered by re-chargeable batteries.9.1.8Read-Only MemoryAnother form of non-volatile random access storage is the read-only memory (ROM). Herea single memory bit is nothing more than a connection that is either open or closed. Themost common ROM types are known as ﬁeld-programmable (as opposed to factory pro-grammable). This programming process consists of stepping through all the bits and settingthe necessary ones by burning open the fuse-like material associated with that bit.There are many varieties of ﬁeld-programmable ROM units:PROM – programmable read-only memory,EPROM – erasable PROM (using ultraviolet light), andEEPROM (E2PROM) – electrically erasable PROMs.The most common uses of ROM memory in a computer are to provide initialization suchas memory tests and disk bootstrapping.9.1.9I/O PortsInput and output ports are the pathways by which the CPU communicates with the worldoutside the computer. The pathway may be either• 1-bit wide (bit-serial),• 8-bit wide (byte-serial), or• 16- to 32-bit wide (word-serial).9.1.10InterruptsReal-time applications require the computer to respond immediately to an external stimulus.The hardware interrupt can be used to suspend the current sequence of instructions, performa speciﬁc and usually short I/O task, then return to the original sequence of instructions.