Once  ROM  operations  are  completed,  the  computer is ready for normal operational use. For all this to take place, ROM uses circuits in the computer that we have already  discussed. In some cases, they are circuits specific for ROM operations. They include: . . . . Registers and flip-flops Timing Control signals Internal bus READ-ONLY MEMORY TYPES Types of ROMs include the basic  ROM that once manufactured cannot be written on again. Other types, called programmable read-only memories (PROMs), can be written on again and again. Read-Only Memory ROMs are prepared at the factory. They are not meant to be changed by the user or the technician. They are  only  to  be  changed  when  a  newer  version  is authorized and supplied to replace the old one. Programmable Read-Only memory (PROM) A   PROM   is   a   programmable   ROM.   Once programmed  it  acts  like  a  ROM.  It  can  be field-programmed by an authorized technician. Each cell is identified by selecting the row and the column just like locating an address in read/write memory. There  are  two  types  of  PROMs—erasable   and nonerasable. Erasable PROMS can be erased and reprogrammed. Nonerasable PROMS  cannot be changed once they are programmed. There are a couple of ways to create or erase the ones in the array; electrically or with ultraviolet (UV) light. Some  PROMS  are  electrically  programmed  but erased  with  UV  light.  Others  are  erased  electrically  and programmed with the UV light. ELECTRICAL.— An electric charge can be used to either blow fusible links permanently in a cell or used on a special transistor with two gates. With the special transistors, the gate between the memory cell and the column wire is disabled by the electrical charge. UV LIGHT.—  UV light is used to erase data in a cell by exposing the IC die to the UV light for a few minutes (usually less than 30 minutes). UV light can also be used to restore ones to a cell by dissipating the electrical charge that disabled the gate. ELECTRICALLY   ALTERABLE   OR ERASABLE PROM ( E A P R O M    OR EEPROM).— The EAPROM or EEPROM can be programmed (modified) or erased while it is still in the circuit and used like a nonvolatile read/write memory. EAPROMs/EEPROMs use an electric charge to erase the  ones.  Some  types  of  EAPROMs/EEPROMs  are more versatile; individual cells can be reprogrammed by reversing the voltage used to create a zero. There are some timing constraints that cause the part to need more time for erasure or programming than is needed to read data from the part. Some EAPROM/EEPROMs have a word or byte erase mode. ULTRAVIOLET-ERASABLE  PROM  (UV EPROM OR EPROM).— UV  EPROMs/EPROMs trap a charge (1) in the cells to represent the data. To release the charge, the cells are exposed to the UV light for 30 minutes or less. UV EPROMs/EPROMs are usually programmed out of circuit. Figure 6-38 is an example of a 2K × 8 UV EPROM; block diagram and pin assignments. SUMMARY—COMPUTER  MEMORIES This chapter introduced you to memory types. The following  information  summarizes  important  points you should have learned. MEMORY— The main memory of a computer is used for storing programs, data, calculations, and operands. MEMORY  MODULES—  Memory  modules  are made up of multiple pcb’s (support circuitry) and memory  components  (stacks  [core  or  film]  and semiconductor pcb’s) to form one memory module or unit. Memory modules are interchangeable with other modules of the same type and size in the same computer set. Each module provides a fixed number of memory words with a fixed number of bit positions for each word. MEMORY  ARCHITECTURE—  Memories  are typically organized in square form so that they have an equal number of rows (x) and columns (y). Each intersection of a row and a column comprises a memory word address. Each memory address will contain a memory word. MEMORY  OPERATIONS—  Memory  opera- tions operate on a request, selection, and initiate basis. A memory request or selection and a memory word 6-30