l Look-through  read —In look-through read, the cache is checked first. If a miss occurs, the reference is sent to main memory to be serviced. This is known as a serial read policy. l Look-aside  read  —A  look-aside  read  presents both  cache  and  main  memory  with  the  reference simultaneously. Since the cache will respond faster, if a hit occurs, the request can be terminated before main memory responds. This is known as a parallel read policy. CACHE REPLACEMENT POLICIES.— When new data is read into the cache, a replacement policy determines  which  block  of  old  data  should  be  replaced. The objective of replacement policies is to retain data that is likely to be used in the near future and discard data that won’t be used immediately. The replacement policies include the following: . FIFO —The first block that was read into cache is the first one to be discarded. l LRU —The block that hasn’t been used in the longest period of time is replaced by the new block. Blocks are replaced randomly. l Random. — . Optimum— This cache replacement algorithm is psychic and has perfect knowledge of the future. Optimum  replacement  is  what  the  other  three  strive  for, with LRU coming the closest. CACHE  WRITE.—  Since the cache contents area duplicate  copy  of  information  in  main  memory,  writing (instructions  to  enter  data)  to  the  cache  must  eventually be made to the same data in main memory. This is done in two ways as follows: l Write-through cache—  Writing is made to the corresponding data in both cache and main memory. . Write-back cache— Main memory is not updated until the cache page is returned to main memory. READ-ONLY MEMORY (ROM) Every computer comes with a set of software instructions  supplied  by  the  manufacturer.  This  enables the computer to perform its I/O operations. These permanent instructions (routines) reside in a read-only memory   (ROM).   ROM   is   often   referred   to   as firmware: software  permanently  contained  in hardware.  The  instructions  are  considered  permanent or nonvolatile, since they are not erased each time the computer loses power or is turned off. The ROM contains  the  program  that  defines  its  uniqueness compared with all other types of computers. The   ROM   is   programmed   at   the   time   of manufacture and cannot be altered. It is tailored to system  requirements.    It cannot be altered except by removing and replacing it—either a module or IC chip on a board. The contents of the ROM are electrically unalterable. Other variations of ROMs called PROMS can be reprogrammed as required. This and other variations are covered in further detail in chapter 6 on memory. In connection with the ROM, you will hear the term boot procedure used. The ROM initiates the boot procedure—a  sequence  of  steps  followed  when  you turn on the power to the computer or initiate the boot procedure. The steps required to successfully  boot the computer depend on the type of computer. Other terms that have the same meaning as boot include  boot up, booting, or bootstrap. They all refer to the process of loading  the  software. Consult  your  computer’s technical or owner’s manual for the exact procedures for your computer system. We use two types of ROMs to discuss some of the programs associated with the ROM: nondestructive readout (NDRO) memory and basic input/output system (BIOS). Nondestructive Readout (NDRO) Memory A nondestructive readout (NDRO) memory is usually associated with a militarized mainframe or minicomputer. The NDRO is a small module that occupies two or more slots. For mainframes, it is located in the CPU module. For minicomputers, it is located in the chassis that contains the CPU’s pcb’s. The functions of an NDRO are controlled from the computer’s controlling device: a maintenance console or equivalent. The sizes of the NDRO addresses vary with  the  type  of  computer  and  its  requirements. Selection of a particular word in the NDRO is via the NDRO address select, line selector, and current switch logic. AN  NDRO  consists  of  hardwired  circuits  to create the bootstrap programs or a ROM or PROM. Some of the programs contained on an NDRO include the  following: l   Two   bootstrap   programs— Used   to   load programs from peripheral equipments into main memory . Autostart programs 5-17