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Figure  6-5.Memory  address  with  an  8-bit  word.
Memory Fault Detection

Electronics Technician Volume 06-Digital Data Systems
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Memory Cycle Main memory is a read/write memory that allows data to be retrieved (read) and stored (written) in what is known as the memory cycle. The memory cycle includes reading the data out of memory and/or writing the data into memory, either by a read/write operation or by separate read and write operations. The memory cycle is based on fixed (constant) time periods for reading and/or writing data from and into memory. As soon as read and/or write operations are initiated, almost  simultaneously,  address translation  occurs, then the read and/or write cycle or cycles begin. MEMORY ADDRESS TRANSLATION.— One of the most important processes that must occur before a read or write cycle begins is the memory address translation. Look at memory as a sequence of memory locations starting at address 0 and extending to the maximum memory address available to the requester (CPU or IO(C)). Receiving or sending mail uses a similar concept. Before mail can be received or sent, there must bean address on the envelope. Memory uses the same principle. Memory logic identifies the memory address where a memory word is to be read from or written into in memory. A memory address can be anyone of the entire range of memory addresses (0 to maximum). To identify the desired memory address, the memory logic uses a register designated as the address register and/or translators or decoders. The memory  logic  receives  the  logical  address  from  the CPU or I/O and temporarily stores it in the address register, and then converts it to a physical address that can be read from or written into. Memory  Address  Register  and/or  Translator (Decoder).—  The  address  register  and/or  translator identifies the exact location from which to read the bits or write them. The contents of the address register or translator identifies the memory address. The memory logic is designed to make its selection based on the type of memory it uses. It can be designed to identify a memory address of a single memory pcb or it can be designed to identify an address located in one of four or more memory modules. Memory  Address  Word.—   As stated in the architecture of memory, the word contained in the memory  address  can  be  one  or  more  bits,  most computers have words with at least 8 bits and some up to 128 bits in length. Variations of reading and/or writing from and to memory can include the upper or lower half of the word, or any other variation within the design of a given computer. Variations are based on the instruction types and the program. Also, if a computer is identified as an 8-bit computer and a 16-bit word is required  for  a  read  or  write  operation,  then  two consecutive memory addresses would have to be used to  complete  the  operation.  There  are  many  other variations;  the  instruction  repertoire  set  of  your computer and the technical manual will provide details of  your  computer’s  memory  operations  and  limitations. Here  are  two  examples  of  memory  address translation. For the first example, use figure 6-6 as a reference;  it  shows  a  4-bit  memory  address.  The memory address register or translator contains 168 as follows: Figure 6-6.—4-bit address. 6-6






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