Memory is internal storage areas in the computer system.
just like Our Human Body store “our day to day incidents” inside Our brain.
Computer/Mobile phone store information inside Memory.
The term memory identifies data storage that comes in the form of Silicon-chip’s ( pen-drives, SD cards), and the word storage is used for memory that exists on tapes or Hard- disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual silicon-chips capable of holding data.
Some computers also use virtual memory, which expands physical memory onto a hard disk.
Every computer comes with a certain amount of physical memory, usually referred to as main memory or RAM.
You can think of main memory as an array of boxes, each of which can hold a single byte of information.
So, A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information.
Memory is major part of computers that categories into several types.
Memory is the best essential element of a computer because computer can’t perform simple tasks. The performance of computer mainly based on memory and CPU. Memory is internal storage media of computer that has several names such as majorly categorised into two types, Main memory and Secondary memory.
1. Primary Memory / Volatile Memory.
2. Secondary Memory / Non Volatile Memory.
1. Primary Memory / Volatile Memory:
Primary Memory also called as volatile memory because the memory can’t store the data permanently. Primary memory select any part of memory when user want to save the data in memory but that may not be store permanently on that location. It also has another name i.e. RAM.
Random Access Memory (RAM):
The primary storage is referred to as random access memory (RAM) due to the random selection of memory locations. It performs both read and write operations on memory. If power failures happened in systems during memory access then you will lose your data permanently. So, RAM is volatile memory. RAM categorized into following types.
- DRAM
- SRAM
- DRDRAM
2. Secondary Memory / Non Volatile Memory:
Secondary memory is external and permanent memory that is useful to store the external storage media such as floppy disk, magnetic disks, magnetic tapes and etc cache devices. Secondary memory deals with following types of components.
Read Only Memory (ROM) :
ROM is permanent memory location that offer huge types of standards to save data. But it work with read only operation. No data lose happen whenever power failure occur during the ROM memory work in computers.
ROM memory has several models such names are following.
- PROM
- EPROM
- EEPROM
RAM (random-access memory): This is main memory. When used by itself, the term RAM refers “Random Access Memory” to read and write memory, that is, you can both write data into RAM and read data from RAM.
- SRAM ( Static Ram): Static random access memory uses multiple transistors, typically four to six, for each memory cell but doesn’t have a capacitor in each cell.
- DRAM( Dynamic Ram):Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.
- DRDRAM (Direct Rambus DRAM (DRDRAM) ): Rambus is intended to replace the current main memory technology of dynamic random access memory (DRAM). Much faster data transfer rates from attached devices. Direct Rambus (DRDRAM) provides a two-byte (16 bit) bus rather than DRAM’s 8-bit bus. At a RAM speed of 800 megahertz (800 million cycles per second), the peak data transfer rate is 1.6 billion bytes per second.
ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
- PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
- EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.
- EEPROM (electrically erasable programmable read-only memory): An E2PROM is a special type of PROM that can be erased by exposing it to an electrical charge.
This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
Memory Types Used in Micro-Controller’s/Micro-Processor’s
As memory technology has matured in recent years, the line between RAM and ROM has blurred. Now, several types of memory combine features of both. These devices do not belong to either group and can be collectively referred to as hybrid memory devices. Hybrid memories can be read and written as desired, like RAM, but maintain their contents without electrical power, just like ROM. Two of the hybrid devices, EEPROM and flash, are descendants of ROM devices. These are typically used to store code.
Hybrids
As memory technology has matured in recent years, the line between RAM and ROM has blurred. Now, several types of memory combine features of both. These devices do not belong to either group and can be collectively referred to as hybrid memory devices. Hybrid memories can be read and written as desired, like RAM, but maintain their contents without electrical power, just like ROM. Two of the hybrid devices, EEPROM and flash, are descendants of ROM devices.
These are typically used to store code.
EEPROMs are electrically-erasable-and-programmable. Internally, they are similar to EPROMs, but the erase operation is accomplished electrically, rather than by exposure to ultraviolet light. Any byte within an EEPROM may be erased and rewritten. Once written, the new data will remain in the device forever–or at least until it is electrically erased. The primary trade-off for this improved functionality is higher cost, though write cycles are also significantly longer than writes to a RAM. So you wouldn’t want to use an EE-PROM for your main system memory.
Flash memory combines the best features of the memory devices described thus far. Flash memory devices are high density, low cost, nonvolatile, fast (to read, but not to write), and electrically re-programmable.
These advantages are overwhelming and, as a direct result, the use of flash memory has increased dramatically in embedded systems. From a software viewpoint, flash and EEPROM technologies are very similar. The major difference is that flash devices can only be erased one sector at a time, not byte-by-byte. Typical sector sizes are in the range 256 bytes to 16KB. Despite this disadvantage, flash is much more popular than EEPROM and is rapidly displacing many of the ROM devices as well.
The third member of the hybrid memory class is NVRAM (non-volatile RAM). Nonvolatility is also a characteristic of the ROM and hybrid memories discussed previously.
However, an NVRAM is physically very different from those devices. An NVRAM is usually just an SRAM with a battery backup. When the power is turned on, the NVRAM operates just like any other SRAM. When the power is turned off, the NVRAM draws just enough power from the battery to retain its data. NVRAM is fairly common in embedded systems.
However, it is expensive–even more expensive than SRAM, because of the battery–so its applications are typically limited to the storage of a few hundred bytes of system-critical information that can’t be stored in any better way.
| Type | Volatile? | Writeable? | Erase Size | Max Erase Cycles | Cost (per Byte) | Speed |
| SRAM | Yes | Yes | Byte | Unlimited | Expensive | Fast |
| DRAM | Yes | Yes | Byte | Unlimited | Moderate | Moderate |
| Masked ROM | No | No | n/a | n/a | Inexpensive | Fast |
| PROM | No | Once, with a device programmer | n/a | n/a | Moderate | Fast |
| EPROM | No | Yes, with a device programmer | Entire Chip | Limited (consult datasheet) | Moderate | Fast |
| EEPROM | No | Yes | Byte | Limited (consult datasheet) | Expensive | Fast to read, slow to erase/write |
| Flash | No | Yes | Sector | Limited (consult datasheet) | Moderate | Fast to read, slow to erase/write |
| NVRAM | No | Yes | Byte | Unlimited | Expensive (SRAM + battery) | Fast |
MemoryMap
When working with micro-controller’s, many of the tasks usually consist of controlling the peripherals that are connected to the device, respectively programming the subsystems that are contained in the controller (which by itself communicate with the circuitry connected to the controller).
The AVR series of microcontrollers offers two different ways to perform this task:
- There’s a separate I/O address space available that can be addressed with specific I/O instructions that are applicable to some or all of the I/O address space (in, out, sbi etc.), known as I/O mapping.
- The entire I/O address space is also made available as memory-mapped I/O, i. e. it can be accessed using all the MCU instructions that are applicable to normal data memory. The I/O register space is mapped into the data memory address space with an offset of 0x20 since the bottom of this space is reserved for direct access to the MCU registers. (Actual SRAM is available only behind the I/O register area, starting at either address 0x60, or 0x100 depending on the device.).
This is why there are two sets of addresses in the AVR data sheets. The first address is using I/O mapping and the second address in brackets is the memory mapped approach.
Registers
Registers are special storages with 8 bits capacity and they look like this:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Note the numeration of these bits: the least significant bit starts with zero (20 = 1).
A register can either store numbers from 0 to 255 (positive number, no negative values), or numbers from -128 to +127 (whole number with a sign bit in bit 7), or a value representing an ASCII-coded character (e.g. ‘A’), or just eight single bits that do not have something to do with each other (e.g. for eight single flags used to signal eight different yes/no decisions).
The special character of registers, compared to other storage sites, is that
- they can be used directly in assembler commands,
- operations with their content require only a single command word,
- they are connected directly to the central processing unit called the accumulator,
- they are source and target for calculations.
There are 32 registers in an AVR. They are originally named r0 to r31. However certain instructions will not work in registers r0 to r15 e.g. ldi will only work on registers r16 to r31.
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