Раздел 6 Второе занятие

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Large-scale Integration: Memories

There are a number of types of memory which can be used as ICs in digital electronics. These include:

1. Random-Access Memory (RAM), wherein each memory word is accessed for reading or writing via a specific address, access time being approximately equal for any combination of succes­sive locations.

2. Serial Access Memory (SAM), wherein the memory consists of a circular shift register (serial output connected to serial input). A counter keeps track of the “address” of the bit available for read­ing and writing (the serial output and input bits). To read or write a given address, the register is shifted until the counter matches the desired address: clearly a large change of address takes longer than a short one.

3. Read-Only Memory (ROM), in which the binary contents are wired in at the factory as a step in the IC manufacturing proce­dure. These act like RAMs, except it is not possible to change the contents. Although it is possible to obtain custom-designed ROMs, they are too expensive for production in small quantities.

4. Programmable ROMs (PROMs), which can be written using spe­cial equipment. These hold their contents until erased with high- intensity ultraviolet light and re-programmed.

5. Programmed Logic Arrays (PLAs), some of which can also be programmed, do not have a full-scale memory complement, but are an expensive way of making a ROM-like device, in which not all input codes correspond to defined outputs, and a given output can be specified by more than one input code.

RAMs are used for temporary data storage because they are vol­atile: that is, their contents are lost if power is removed. ROMs, PROMs, and PLAs are non-volatile, but cannot be written on during normal operation.