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Memory A memory unit is a device to which binary information is transferred for storage and from which information is retrieved when needed for processing. (a) RAM → random -access memory with both read & write operations. Examples are: Static RAM (SRAM), Dynamic RAM (DRAM). (b) ROM → read-only memory with read operation only. Examples are: PROM, EPROM, EEPROM. A memory stores information in groups of bits called words. A memory word is a group of 1s and 0s. A group of 8 bits is called a byte. Most computer memories use words that are multiple of byte. The capacity (size) of memory is usually stated as total number of bytes that the unit can store. 2 10 =1K, 2 20 =1M, 1G=2 30 ……..2 16 =...K; 2 21 =...M; 2 32 =….G. “the number of address lines k is independent of the number of bits in each word”. Read Only Memory (ROM) A ROM is considered a programmable device where binary information can be stored and kept even when power supply is switched off. A ROM consists of a decoder and array of fused (programmed) OR gates. This is shown in the following Figure. Page 2
The required paths in ROM may be programmed in different ways: 1-mask programming ROM; 2-PROM; 3-EPROM (ultraviolet light); EEPROM (like flash memory) Fig. Internal logic of a 32 X 8 ROM The ROM truth table shows the pattern of the fuses as programmed inside the ROM. The following truth table is an example for the 32 X 8 ROM. Page 3
Page 1: Lecture 14: Programmable Logic Devi
Page 6 and 7: Fig. PLA with three inputs, four pr
Page 8 and 9: 3 programmable AND gates in each se
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Lec14

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