Computer Organization and Assembly Language

1. Computer Organization and Assembly Language Memory Organization Computer Organization & Assembly Language By Engr. Bilal Ahmad

2. Early Memory • In the Last Lecture we have discussed the magnetic core memory. • In this Lecture our aim is to understand the Memory Organization, and Interfacing with the Processor. • Memory as on its own is a Technology. • Every Year they are developing so it is very hard to keep track of all the inventions but we will consider how they were introduces and now how are they developed. • Can anyone highlight what can be the two most critical aspects for the memory development? Computer Organization & Assembly Language By Engr. Bilal Ahmad

3. The Magnetic Core Memory • One of earliest types of computer memory was called magnetic core memory. • It was made by weaving fine copper wires through tiny rings of magnetic material in an array. Computer Organization & Assembly Language By Engr. Bilal Ahmad

4. MCM (Continued) • It was much like recording music to the magnetic tape. • When electrical current was sent through the center of one of the magnetic rings, it polarized it with a magnetic charge. • Each of that ring could have a charged that would flowed clockwise or anti clockwise. • Can you identify what will be the use of clockwise or counter clockwise rotation? • The horizontal and vertical wires were used to write data to a specific ring. Computer Organization & Assembly Language By Engr. Bilal Ahmad

5. MCM (Continued) • By putting half the necessary current to polarize the magnetic material on one of the horizontal wires and the same level of the current on the vertical wires, the ring where the two wires intersected had enough total current to modify the ring’s polarity. • What will happen to the polarity of other wires than? • The diagonal wires, called the sense wires, were used to read data. • They had the ability to detect when the polarity of one of the rings was changed. Computer Organization & Assembly Language By Engr. Bilal Ahmad

6. Let’s Recap our understanding so far Computer Organization & Assembly Language By Engr. Bilal Ahmad

7. MCM (Continued) • To read data therefore the bit in question would be written to the horizontal or vertical wires. • If a sense wire detected a change in polarity, the bit that had been stored there must have been opposite from the one just written before. • If there is no change in the polarity there will be no change in the bit, simple is that. • It was a very fragile process as it was difficult and very expensive process. Just to have 1K memory that is 1024 bytes you had to use so many copper wiring. Also referred as (1K x 8) 8 by 8 inches. Computer Organization & Assembly Language By Engr. Bilal Ahmad

8. Organization of Memory Device • The basic configurations of the modern memories are same as MCM, and you know MCM stands for…….. • The rings have been replaced with electronic memory cells such as D-Latch. • The cells are arranged so that each row represents a memory location where the binary value would be stored and the column represents the bits of those memory locations. • This is where the 1K x 8 Terminology came from. • So memory is treated as a matrix where rows identifies the number of memory locations and columns identifies the number of bits in each memory location. Computer Organization & Assembly Language By Engr. Bilal Ahmad

9. Organization of Memory Device • To store or retrieve data from a memory device, the processor must place a binary number called an address or special inputs to the memory device. • The address identifies which row of the memory matrix or array the processor is interested in communicating with, and enables it. • Once a valid address is placed on the address lines, the memory cells from that row are connected to bi directional connections on the memory device that allows data either to be stored to or read from the Latches. The connections are called data lines. Computer Organization & Assembly Language By Engr. Bilal Ahmad

10. Memory Device Organization (Continued) • Three additional lines, chip select, read enable and write enable are used to control the transaction. Computer Organization & Assembly Language By Engr. Bilal Ahmad

11. Memory Device Organization (Continued) • Remember a decoder with n inputs has how many outputs? • One application where digital signals are used to enable a device is to identify the unique conditions to enable an operation. • For example the magnetron in the microwave is enabled only when the timer is running and the start button is pushed on and the oven door is closed. • This method of enabling a device based on the condition of number of inputs is common in digital circuits. Computer Organization & Assembly Language By Engr. Bilal Ahmad

12. Decoders • One common application is in the processors interface to the memory and that is why we are studying Decoders. • It is used to identify which memory device will contain a piece of data. • In the example of microwave, the sentence used to describe the enabling of magnetron joined each of the inputs with the word “AND”. Therefore the enabling will be done using the AND Gate. Computer Organization & Assembly Language By Engr. Bilal Ahmad

13. Other Examples • There are many other examples of digital systems that enables a process based on a specific combination of ones and zeroes from multiple inputs. • An example can be a vending machine that delivers soda when enough money is inserted and a button is pushed and the machine is not out of the selected soda. Computer Organization & Assembly Language By Engr. Bilal Ahmad

14. Exercise for You • I need you to design a two input circuit should called a 1-of-4 decoder. Means that only one output should be enabled one at a time. • It is very simple, all of you can design that as a hint if you have a 2 bit decoder with an input A and B, you can have an output 1 that is only if A and B=0, an output that is 1 when A=0 and B=1, an output 1 when A=1 and B=0 and an output 1 when A and B= 1. • Now can you design, I am sure you can. Computer Organization & Assembly Language By Engr. Bilal Ahmad

15. Active Low Signals • You all are aware that computer systems are composed of numerous subsystems. • Some of which may be idle and some of them may be operating independent of the processor and some might be with the direct contact to the processor. • For the systems that are in direct contact with the processor, only one may be enabled at any one time. • For example although a computer does have multiple memory devices, when an information is stores, it is sent to one of the modules with the other modules must be silent. Computer Organization & Assembly Language By Engr. Bilal Ahmad

16. Active Low Signals Continued • So therefore a scheme is needed to select or enable a specific device or to route data to an appropriate subsystem. • This scheme is simply implemented with a separate binary line that is connected to each subsystem where one of the binary values enables a subsystem and other disables that. • Simply on/ off control. • Previously our discussion was that the on signal is equivalent to logic 1, but for a number of reasons, the standard method of enabling a device is not to send a logic 1. Computer Organization & Assembly Language By Engr. Bilal Ahmad

17. Active Low Signals (Continued) • Instead it is a common practice to enable the device with a Logic 0 and disable it with Logic 1. • This is called the active low signal. • The device is active on Low Signal that is 0 Computer Organization & Assembly Language By Engr. Bilal Ahmad

18. Another Exercise for You • The Decoder that you have designed, modify it so that it must be working as an Active Low Decoder. • Again it is very simple just common sense. Computer Organization & Assembly Language By Engr. Bilal Ahmad

19. Memory Device Organization • So now after getting information on decoders and active low signals you are in the command to understand the address decoder. • An Address Decoder selects exactly one row of the memory array to be active leaving the others inactive. • When the microprocessor places a binary number into the address line, the address decoder selects a single row in the memory array to be written to or read from. • For example if 0b011 = 3 (Decimal) is placed onto the address line, the fourth row of the memory will be connected to the data line. Computer Organization & Assembly Language By Engr. Bilal Ahmad

20. Memory Device Organization (Continued) • The processor uses the inputs read enable and write enable to specify whether it is reading or writing the data. • These signals are active low. When read enable is 0 and when write enable is 0. • Remember these two signals can never be 0 at the same time. • Some times you use R/W (Read / Write) whatever you have to enable you just place a bar on top of that. Computer Organization & Assembly Language By Engr. Bilal Ahmad

21. Memory Device Organization • The last input is chip select. The chip select is an active low signals that enables and disables the memory device. • If the chip select = 0 than the memory activates all its input and output lines and uses them to transfer data. • If it is 1 the memory becomes idle. • The perspective behind that processor rarely communicate with one memory device on its data line. There are so many memories. If there is no chip select means that the no one will understand each other. Take an example of your class when you are all talking no one can understand what is going on. Computer Organization & Assembly Language By Engr. Bilal Ahmad