A Computer system can be classified into following groups:
- Internal memory
- Primary memory
- Secondary memory
INTERNAL MEMORY refers to the registers of CPU which hold temporary results. These registers are very fast.
PRIMARY MEMORY is the storage area where all the programs are executed. The CPU can directly access only those items that are stored in primary memory. Primary memory is fast but slower than internal registers by factor of 10. In short period of time addresses generated by CPU have the tendency to get clustered around small regions in main memory. This phenomenon is known as locality of reference. Hence these days a small but fast memory called cache memory is installed between CPU and primary memory.
SECONDARY MEMORY refers to storage medium like hard disks, CDs etc
Memory has two major access modes:
- Sequential access mode: Here memory can be accessed only in serial order i.e. if we have to access 4th memory location then we first would have to move to first three locations and skip them and only then we can access the 4th location. This is also called serial access mode. Here access time depends on the location. If location is away from the starting point of the memory then access time would be more. E.g. magnetic tape
- Random access mode: Any memory location can be accessed at random i.e. if we have to access 4th location then we can directly go to 4th location and access it. So access time is independent of the location we are accessing.
There is also a memory known as semi random access memory in which we have both the modes e.g. memory is divided in different tracks or sectors where we have a memory head for each sector. So we can move to any sector randomly but within the sector we can only move serially.
Memory can also be categorized as follow:
Passive memory: The memory in which no processing activities takes place. In this type of memory only 2 types of operations can be carried out: read & write. e.g. all the memories we use in daily life.
Active memory: This intelligent memory system has a small processor associated with every memory cell and this processor can perform operations like increment & addition and hence can improve the performance of the main processor.
PRIMARY MEMORY
This is a volatile memory (i.e. memory would lose its data whenever there is power loss). This can be of two types as shown:
This is a static RAM where two transistors are used to store a single bit and they can hold data without external assistance as far as power is supplied to the circuit. The circuit is also known as flip-flop or bi-stable multi-vibrator. If Q is high we get high as an input to the transistor T1 and hence Q bar is low and as Q bar is low we get LOW as input to transistor T2 and hence Q is HIGH. We can see how output is maintained by the circuit itself. So we don’t need t refresh the circuit again and again.
DRAM
This is dynamic RAM where we store the bits in capacitors and we have to charge the capacitors after some time again and again as the charge stored on the capacitors gets leaked out. This is called dynamic RAM as we have to refresh the circuit again and again. Due to the fact that capacitors have to be charged again and again, there is a dedicated circuitry required just to refresh the capacitors. DRAM is cheaper that SRAM.
ROM (Read Only Memory)
This is a non volatile memory which is used to store permanent codes. Mainly this is of following types:
Mask ROM: This type of ROM is used by the manufacturers for programming and is mass produced, hence are quiet in-expensive.
PROM (Programmable Read only memory): This is the type of ROM which can be programmed by the user in the working environment but can not be re-programmed.
EPROM (Erasable programmable ROM): This type of ROM can be reprogrammed hence called so. This type of ROM is required when programs are in the development stage.
FLASH MEMORY
Flash memory is a solid-state, non-volatile, rewritable memory that works like RAM and a hard-disk drive combined. It resembles conventional memory, coming in the form of discrete chips, modules, or memory cards. Just like with DRAM and SRAM, bits of electronic data are stored in memory cells, and just like with a hard disk drive, flash memory is non-volatile, retaining its data even when the power is turned off. Although there are advantages over both RAM (its non-volatility) and hard disk (the absence of moving parts) but following are the reasons why flash memory is not a viable replacement for either.
- Because of its design, flash memory must be erased in blocks of data rather than single bytes like RAM.
- It has a higher cost
- The memory cells in a flash chip have a limited lifespan of around 100,000 write cycles, thus making it an inappropriate alternative to RAM for use as a PC s main memory.
CACHE MEMORY
This memory is used as an intermediate between CPU and RAM. It is fastest form of storage. It basically decreases the delay for interaction of CPU and RAM. First data is transferred to cache memory and then from cache memory data is used by CPU. The cache memory is implemented using SRAM. This is used to bridge the performance gap between processor and the RAM.
WRITE BACK CACHE: In this cache memory whatever the changes are made to the cache are not transferred immediately to main memory but changes are transferred to main memory only when corresponding data is to be replaced in the cache. It avoids unnecessary writes to the memory.
WRITE THROUGH CACHE: In this type of cache memory what ever changes are made to cache are transferred immediately to the main memory. This policy is easily implemented but may lead to unnecessary writes to the memory.
MEMORY HIERARCHY
Memory system is often designed using different technologies to achieve the optimum performance. So memory system is a multilevel organization. The following a typical computers’ memory hierarchy organization:
HIT RATIO (h)
It is defined as relative number of successful references to the cache memory. In rough words, if h=0.9, then it means that if we access cache memory for a particular data, then 9 out of 10 times we’ll be able to get it.
Q- Calculate the average access time if cache memory access time is 150ns and memory access time is 900 ns and we have the hit ratio for cache memory as h=0.8. Assume every time we go to main memory we’ll be get the data we want.
Ans: tav= h * tc + (1 – h)*(tc + tm)
As when there is cache hit then we’ll need to go to cache memory only for data hence only time tc is taken otherwise we would have to access main memory after unsuccessfully accessing cache memory so total time taken in this case would be tc + tm
So tav= 0.8 * 150 + (1 – 0.8) * 900 = 120 + 180 = 300 ns
If there had been no cache memory then average time taken would have been 900 ns as every time we would have to access main memory for data so tav= 900ns
So we see there is a 3 times increase in the performance of the system by employing CACHE memory in the hierarchy.
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