Memory, Types and Their Functions

The computer cannot work without the memory. The processor fetches instructions from the memory, data is also stored in the memory. In this chapter, we will discuss memory types, technologies and their properties. The overall view of the computer memory can be represented as the memory hierarchy triangle shown in Fig. 1 where the available size reduces while going up but the access time significantly shortens. It is visible that the fastest memory in the computer is the set of internal registers, next is the cache memory, operational memory (RAM), disk drive, and the slowest, but the largest is the memory available via the computer network - usually the Internet.

In the table 1 you can find the comparison of different memory types with their estimated size and access time.

Before we start talking about details of memory we will mention the term address space. The processor can access instructions of the program and data to process. The question is how many instructions and data elements can it reach? The possible number of elements (usually bytes) represents the address space available for the program and for data. We know from the chapter about von Neumann and Harvard architectures that these spaces can overlap or be separate.

The size of the program address space is determined by the number of bits of the Instruction Pointer register. In many 8-bit microprocessors, the size of IP is 16-bits. It means that the size of address space, expressed in the number of different addresses the processor can reach, is equal to 2^16 = 65536 (64k). It is too small for bigger machines like PC computers so here the number of bits of IP is 64 (although the real number of bits used is limited to 48).

The size of the possible data address space is determined by addressing modes and the size of index registers used for indirect addressing. In 8-bit microprocessors sometimes it is possible to join two 8-bit registers to achieve address space of the same 64k size as for the program. In PC computers program and data address spaces overlap, so the address space for data is the same as for programs.

In this kind of memory, the instructions of the programs are stored. In many computers (like PCs), the same physical memory is used to store the program and the data. This is not the case in most microcontrollers where the program is stored in non-volatile memory, usually Flash EEPROM. Even if the memory for programs and data is common, the code area is often protected from writing to prevent malware and other viruses from interfering with the original, safe software.

In data memory, all kinds of data can be stored. Here we can find numbers, tables, pictures, audio samples and everything that is processed by the software. Usually, the data memory is implemented as RAM, with the possibility of reading and writing. SRAM is a volatile memory so its content is lost if the computer is powered off.

There are different technologies for creating memory elements. The technology determines if memory is volatile or non-volatile, the physical size (area) of a single bit, the power consumption of the memory array, and how fast we can access the data. According to the data retention, we can divide memories into two groups:

• non-volatile,
• volatile.

Non-volatile memories are implemented as:

• ROM
• PROM
• EPROM
• EEPROM
• Flash EEPROM
• FRAM

Volatile memories have two main technologies:

• Static RAM
• Dynamic RAM

Non-volatile memories are used to store the data which should be preserved during the power is off. Their main application is to store the code of the program, constant data tables, and parameters of the computer (e.g. network credentials). The hard disk drive (or SSD) is also the non-volatile memory, although not directly accessible by the processor via address and data buses.

ROM - Read Only Memory is the kind of memory that is programmed by the chip manufacturer during the production of the chip. This memory has fixed content which can't be changed in any way. It was popular as a program memory in microcontrollers a few years ago because of the low price of a single chip, but due to a need for replacement of the whole chip in case of an error in the software and lack of possibility of a software update, it was replaced with technologies which allow reprogramming the memory content. Currently, ROM is sometimes used to store the serial number of a chip.

PROM - Programmable Read Only Memory is the kind of memory that can be programmed by the user but can't be reprogrammed further. It is sometimes called OTP - One Time Programming. Some microcontrollers implement such a memory but due to the inability of software updates, it is a choice for simple devices with a short lifetime of a product. OTP technology is used in some protection mechanisms for preserving from unauthorised software change or reconfiguration of device.

EPROM - Erasable Programmable Read Only Memory is the memory which can be erased and programmed many times. The data can be erased by shining an intense ultraviolet light through a special glass window into the memory chip for a few minutes. After erasing the chip can be programmed again. This kind of memory was very popular a few years ago but due to a complicated erase procedure, requiring a UV light source, was replaced with EEPROM.

EEPROM - Electrically Erasable Programmable Read Only Memory is a type of non-volatile memory that can be erased and reprogrammed many times with electric signals only. They replaced EPROM memories due to ease of use and the possibility of reprogramming without removing the chip from the device. It is designed as an array of MOSFET transistors with, so-called, floating gates which can be charged or discharged and keep the stable state for a long time (at least 10 years). In EEPROM memory every byte can be individually reprogrammed.

Flash EEPROM is the version of EEPROM memory that is similar to EEPROM. Whilst in the EEPROM a single byte can be erased and reprogrammed, in the Flash version, erasing is possible in larger blocks. This reduces the area occupied by the memory, making it possible to create denser memories with larger capacity. Flash memories can be realised as part of a larger chip, making it possible to design microcontrollers with built-in reprogrammable memory for the firmware. This enables firmware updates without the need for any additional tools or removing the processor from the operating device. It enables also remote updates of the firmware.

FRAM - Ferroelectric Random Access Memory is the type of memory where the information is stored with the effect of change of polarity of ferroelectric material. The main advantage is that the power efficiency, access time, and density are comparable to DRAM, but without the need for refreshing and with data retention while the power is off. The main drawback is the price which limits the popularity of FRAM applications. Currently, due to their high reliability, FRAMs are used mainly in specialised applications like data loggers, medical equipment, and automotive “black boxes”.

Volatile memories are used for temporal data storage while the computer system is running. Their content is lost after the power is off. Their main application is to store the data while processed by the program. Their main applications are operational memory (known as RAM), cache memory, and data buffers.

SRAM - Static Random Access Memory is the memory used as operational memory in smaller computer systems and as the cache memory in bigger machines. Its main benefit is very high operational speed. The access time can be as short as a few nanoseconds making it the first choice when speed is required: in cache memory or as processor's registers. The drawbacks are the area and power consumption. Every bit of static memory is implemented as a flip-flop and composed of six transistors. Due to its construction, the flip-flop always consumes some power. That is the reason why the capacity of SRAM memory is limited usually to a few hundred kilobytes, up to a few megabytes.

DRAM - Dynamic Random Access Memory is the memory used as operational memory in bigger computer systems. Its main benefit is very high density. The information is stored as a charge in a capacitance created under the MOSFET transistor. Because every bit is implemented as a single transistor it is possible to implement a few gigabytes in a small area. Another benefit is the reduced power required to store the data in comparison to static memory. But there are also drawbacks. DRAM memory is slower than SRAM. The addressing method and the complex internal construction prolong the access time. Additionally, because the capacitors which store the data discharge in a short time DRAM memory requires refreshing, typically 16 times per second, it is done automatically by the memory chip, but additionally slows the access time.