Lesson 3.5o Memory
· Introduction - Characteristics
Memory;
The external signs of a memory
By studying the lesson you will
· To know the features a memory
; To name the categories and types of memories
· To know the external signals that have a memory
3.5.1 Introduction - Characteristics
Consider a simple computer system, and assume that we have to import from the keyboard 100 one hundred-digit numbers which we want to introduce the computer system and then compute their sum.
To do this, the computer system to store these numbers in memory. The memory size must be large enough to fit and one hundred (100) numbers. The number of numbers that can store a memory depends on how many different places that have the memory.
Figure 3.5.1 Memory
Imagine the memory, as shown in Figure 3.5.1, as a crowd of 100 drawers. Each drawer is a data storage location and has as an address number. So the numbers of drawers was the address of their content corresponds to the data. For example, the address or memory location 93 is the number 37. Generally we can say that each memory location has an address is a number that characterizes to give or get from a given position. A memory then, as many storage locations has many different addresses must be included. The number of different positions and therefore addresses a memory is called memory size.
Let us turn to Figure 3.5.1 and observe that the representation of the address specifies the memory location using two decimal places. With this restriction we can have more than 100 different addresses (from 0 to 99). If we used 3 decimal digits, then the number of memory locations that we could count is 1000, from 000 to 999. So we can say that the size of a memory determines the number of digits used for the show addresses.
In computing the digits we use are binary. So the performance of a memory management is done by binary digits. For example, a memory that uses 3 bits for the representation of management can have 23 = 8 different memory locations.
Assuming that the number of bits of memory address is «b» and memory size "m" then, the relationship
m = 2b
Let us observe Figure 3.5.1 again and see that at 93 the number that is stored is 37 decimal. Also in Figure 3.5.2 we can see some example contents of a memory. All numbers are stored eight binary digits. Each memory location can not imagine that it is divided into 8 positions where we naapothikefsoume a bit. So in our example, the number 00110111, consisting of 8 digits, stored at 93 by placing at each of eight binary digits 00110111.
Figure 3.5.2 Contents of a memory
The number of cells present in a memory location called a memory word length. In our example we use the memory word length is 8 and each cell in a memory location can store one bit, ie the '1 'or '0'. The number of cells in a memory location, called the word length and are usually for 2.
3.5.2 Memory
In the example cited the calculator can distinguish memories that we use for different purposes. For example we use memory to read the CPU commands of the program. Even where there is a memory write some information - data and then read them. Depending on the function and application that will use a memory select the appropriate type. So the memories depending on their choices, divided into the following two categories:
The memories from which we can read and write, called RAM (Random Access Memory Random Access Memory).
The memories from which we can only read their contents, called ROM (Read Only Memory Read-only Memory). The contents of these memories, that their data can not change it.
Memories RAM using two different technologies for storing data. Depending on how the information storage RAM memories are divided into two categories, static and dynamic memories memories.
The static memory registers using Flip-Flop electronic switches to keep the binary information. In Figure 3.5.3 is shown in simple form the operating principle of a static memory. We have two NOT gates that one feeds the other as shown in Fig. So if one day save the value 1, the one with the lower inverter is 0 and then with the above inverter returns the same value of 1. Therefore the price is "held".
Figure 3.5.3 Basic static memory storage unit
Similarly if we had saved the 0 through the inverter will be below 1 and above the inverter returns to its initial value 0. So in both cases the data is kept in its original price. The value stored can be read from the entry-exit since when is selected and then the memory switch is closed. When we write, again the switch should be closed. Now we impose a new order that will open when the switch will keep the two inverters. The static memories have the important advantage that it is very fast. The highlight downside is their price. Static memories because they need lots of space to implement, compared with other memories are akrivoteres.
The dynamic memory use capacitors to store the data as shown in Figure 3.5.4. The binary data stored here is based on the assumption that the logical 1 corresponds to a charged capacitor and the logical 0 corresponds to an uncharged capacitor. Dynamic memories have the advantage of being cheap and have high capacity because of their small size.
Figure 3.5.4 Basic dynamic memory storage unit
It is also low since practically no energy for data retention. The disadvantage of dynamic memory is speed. Still, a charged capacitor to be discharged over time thus losing the value that you saved. For this reason, periodically require dynamic memory refresh their content. The interval between two successive renewals called renewal years. In a renewal process reads the contents of memory and automatically repeated itself. This ensures the renewal of the content. The process of renewal in older technology is a dynamic random external circuit, while the newer type of memory circuit is built on the same piece of memory.
The term "read-only memory» - ROM used to characterize an entire class of memory from which we can only read simple and include ROM, PROM, EPROM, EEPROM, Flash. An important property is that they do not lose their data when they stop feeding.
Simple ROM memories are made from the factory and have specific data that can not be changed. If we use such a memory and we must change the contents, this is impossible. We should throw out and use another memory, the new desired content.
A variation of ROM memory, which gives us a degree of freedom is a PROM (Programmable Read Only Memory). The contents of this memory is not fixed, but the construction can be scheduled later, but only once. A PROM memory after a programmed time, then the content can not be changed.
The memories of reading the contents of which can erase after programming and to reschedule called EPROM (Erasable Programmable Read Only Memory) and EEPROM (Electrically Erasable Programmable Read Only Memory). Here it should be noted that the EPROM and EEPROM memories are memories of RAM. The EPROM and EEPROM memories allow us to plan the content is often but not write to them as if they were memories RAM.
The EPROM and EEPROM memories have different shutdown. EPROM memories in their content erased with ultraviolet light, while in the EEPROM memory content erased the electric way. Finally, a special kind of memory is rewritable and FLASH memories that have similar characteristics to the EEPROM. The difference is that while the EEPROM can erase any memory location we want to FLASH shutdown command clears all entries.
Figure 3.5.5 Memory
3.5.3 The external signs of a memory
A memory communicates with other circuits through three sets of signals. Take as an example of a memory with a capacity of 1024 byte = 1Kbyte. In Figure 3.5.6, which depicts such a memory, we can distinguish the following groups of signals:
Figure 3.5.6 Symbolic representation of memory
The address signals (A0-A9).
The signals of the address are on the treadmill address of the computer system. These signals are denoted by the letter A and an index number that indicates the binary value of bit of the address. The signal with index number 0 corresponds to the smallest bit binary value (LSB-least significant bit) and mark with the index in 9 bit binary with the largest value (MSB-most significant bit).
The data signals (D0-D7)
The data signals are connected to the data path of the computer system. The signals are denoted by the letter D and an index number that indicates the binary value of bit signals as in the address. The data signals are common to the input and output data.
The control signals ().
The control signals are connected to the channel system of the computer system. Like any device that connects computer system on a treadmill, so the memory has a enable signal (Chip Enable). Line above the mark indicates that the memory is activated, ie chosen as the signal voltage is low and remains disabled as a high voltage signal. This signal is often denoted as (Chip Select).
Additional RAM memories have a selection signal to the memory write (Write Enable). Similarly when the signal voltage is low then the memory performs the data entry process, and when the signal voltage is high, then the contents of memory can not be changed.
Finally have the memories RAM selection signal read or write memory (Read Enable). When the signal voltage is low then the memory performs the data reading process, and when the signal voltage is high, then the memory does not display the data in the output.
What did you learn
A memory consists of binary information storage locations. The number of these positions is called the memory size. The amount of memory determines the number of bits used to address.
The number of bits in each memory location is called the word length of memory.
The memories are divided into different categories according to their functions, and their method of construction. The memories are divided into read-only memories (ROM) and read and write memory (RAM).
The main types of memory are read-only ROM - PROM - EPROM - EEPROM.
Memories RAM depending on the manufacturing technology are divided into static and dynamic memories.
Terminology
memory size.
word length of memory
RAM ROM EPROM EEPROM FLASH
static and dynamic random memories
Read enable, Write enable, Chip Enable
Control knowledge.
What we call memory size?
What determines the size of a memory?
What we call memory word length?
What are the basic categories of memory RAM;
What are the advantages and disadvantages of static and dynamic memory?
What are the main categories of "read-only memory?" How distinguished?
What are the control signals of a memory?
