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--- en:multiasm:cs:chapter_3_6 [2026/02/27 14:14] – [Table] jtokarz
+++ en:multiasm:cs:chapter_3_6 [2026/03/01 13:57] (current) – [Memory, Types and Their Functions] ktokarz
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+====== Memory, Types and Their Functions======
+The computer cannot work without memory. The processor fetches instructions from memory, and data is stored there as well. In this chapter, we will discuss memory types, technologies and their properties.
+The overall view of computer memory can be represented by the memory hierarchy triangle shown in Fig. {{ref>memtriangle}}, where available size decreases as one moves up, but access time decreases significantly. 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.
+<figure memtriangle>
+{{ :en:multiasm:cs:memory_triangle.png?300 |Memory hierarchy triangle}}
+<caption>Memory hierarchy triangle</caption>
+</figure>
+In the table {{ref>memhierarchy}}, you can find the comparison of different memory types with their estimated size and access time.
+<table memhierarchy>
+<caption>Parameters of memory in the hierarchy</caption>
+^ Memory type  ^ Average size   ^ Access time                              ^
+| Registers    | few kilobytes  | <1 ns                                    |
+| Cache        | few megabytes  | <10 ns                                   |
+| RAM          | few gigabytes  | <100 ns                                  |
+| Disk         | few terabytes  | <100 us                                  |
+| Network      | unlimited      | a few seconds, but can be unpredictable  |
+</table>
+===== Address space =====
+Before we start talking about memory details, we will mention the term //address space//. The processor can access both instructions and data in the program to process. The question is: how many instructions and data elements can it reach? The number of possible elements (usually bytes) represents the address space available for the program and 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 the address space, expressed in the number of distinct addresses the processor can reach, is 2^16 = 65536 (64k). It is too small for larger machines, such as personal computers, so the IP address is 64 bits (although only 48 are used).
+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, it is sometimes possible to join two 8-bit registers to achieve an address space of the same 64k size as for the program. In personal computers, the program and data address spaces overlap, so the data address space is the same as the program address space.
+===== Program memory =====
+In this kind of memory, programs' instructions are stored. In many computers (like PCs), the same physical memory is used to store both programs and data. This is not the case in most microcontrollers, where the program is stored in non-volatile memory, usually Flash EEPROM. Even if memory for programs and data is common, the code area is often protected from write access to prevent malware and other viruses from interfering with the original, safe software.
+===== Data memory =====
+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, allowing read and write access. RAM is a volatile memory, so its content is lost if the computer is powered off.
+===== Memory technologies =====
+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 =====
+Non-volatile memories are used to store data that should be preserved when power is off. Their main application is to store the program's code, constant data tables, and computer parameters (e.g., network credentials). The hard disk drive (or SSD) is also a non-volatile memory, although it is not directly accessible to the processor via the address and data buses.
+===== ROM =====
+ROM (Read-Only Memory) is the type of memory that is programmed by the chip manufacturer during chip production. 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 the 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 a chip's serial number.
+===== PROM =====
+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 memory, but because software updates are not possible, it is a choice for simple devices with a short product lifetime. OTP technology is used in some protection mechanisms to prevent unauthorised changes to software or reconfiguration of the device.
+===== EPROM =====
+EPROM (Erasable Programmable Read-Only Memory) is a memory that 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 because of a complicated erase procedure that required a UV light source, it was replaced by EEPROM.
+===== 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 EPROMs because of their ease of use and the ability to reprogram them 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 =====
+Flash EEPROM is a type of non-volatile 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, enabling denser memories with greater capacity. Flash memories can be integrated into a larger chip, enabling the design of microcontrollers with built-in reprogrammable memory for firmware. This enables firmware updates without the need for any additional tools or removing the processor from the operating device. It also allows remote firmware updates.
+===== FRAM =====
+FRAM (Ferroelectric Random Access Memory) is a type of memory where the information is stored with the effect of a change of polarity of ferroelectric material. The main advantage is that power efficiency, access time, and density are comparable to DRAM, without the need for refreshing and with data retention while power is off. The main drawback is the price, which limits the popularity of FRAM applications. Currently, due to their high reliability, FRAMs are mainly used in specialised applications like data loggers, medical equipment, and automotive "black boxes".
+===== Volatile memories =====
+Volatile memory is used to store temporary data while the computer system is running. Their content is lost after the power is off. Their main application is to store the data while the program processes it. Their main applications are operational memory (known as RAM), cache memory, and data buffers.
+===== SRAM =====
+SRAM (Static Random Access Memory) is the memory used as operational memory in smaller computer systems and as cache memory in larger 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 the 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 why the capacity of SRAM memory is usually limited to a few hundred kilobytes to a few megabytes.
+===== DRAM =====
+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 data compared 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 that store data discharge quickly, DRAM memory requires refreshing, typically 16 times per second; this is done automatically by the memory chip, but also slows access time.