Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
2000-06-14
2003-05-13
Nguyen, Than (Department: 2187)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C711S101000, C711S102000, C711S167000, C711S168000, C711S169000
Reexamination Certificate
active
06564285
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of the architecture of computer systems. More particularly, the present invention relates to computer systems that use a large-block erasable non-volatile semiconductor memory as main memory.
BACKGROUND OF THE INVENTION
As modern computer programs have become increasingly more sophisticated, modern personal computer systems have also had to become more sophisticated in order to accommodate these computer programs. Computer programs are made up of a larger number of code instructions than they once were and on average, require access to larger files of data that are read from, and written to, when executing the programs.
Typically, the heart of a personal computer system is a central processing unit (CPU) that resides on a microprocessor chip. New microprocessor chips that operate at increasingly high operating speeds are constantly being developed in order to permit personal computers to execute the larger programs in a timely manner. Usually, these microprocessor chips are developed using CMOS (complementary metal-oxide semiconductor) technology. The greatest amount of power consumption for CMOS chips occurs on the leading and trailing edges of dock pulses (i.e. when a clock signal transitions from a low voltage state to a higher voltage state and vice versa).
When the operating speed of the microprocessor is increased, the number of clock pulses in a particular time period increases thereby increasing the power consumption of the microprocessor during this time period. Furthermore, more heat is generated by the microprocessor and must be dissipated in order to prevent the damage of components within the computer system.
Both power consumption and heat dissipation pose serious problems when designing a personal computer system. This is especially true in the case of mobile computers that are typically powered by batteries. The more power that the computer consumes, the less time that the computer can operate off of a given sized battery. Therefore, as the operating speed of the computer is increased, a designer is faced with several unattractive alternatives.
If the same sized batteries are used, then the effective operating time for the computer system must decrease when the operating speed is increased. On the other hand, if the effective operating time is to remain constant then it is necessary to either add additional batteries, thereby increasing the bulk and weight of the computer, or to use. an exotic and therefore expensive battery technology (or both).
The trend in mobile computers is towards smaller, faster, less expensive and lighter units. Thus, the need to add additional batteries, or more expensive batteries is a significant disadvantage. This disadvantage is exacerbated by the need to add cooling fans, or to implement other cooling techniques, in order to dissipate the additional heat that is generated by the high speed microprocessors.
Additionally, because the microprocessors are operating at a higher speed, they can execute more instructions in a given amount of time, and therefore can also process a greater amount of data during that period. A bottle neck has developed in computer systems having fast microprocessors that can prevent the higher speed of the microprocessor to be utilized effectively. This bottle neck is the bus (or buses) that provide instructions for the microprocessor to execute and the data that the microprocessor will use when executing the instructions.
If the next instruction to be executed is not available when the microprocessor needs it, then the microprocessor must wait idly (i.e. insert wait cycles) while the required instruction is retrieved and provided to the microprocessor. Furthermore, if the next instruction to be executed requires data that is not immediately available to the microprocessor, the microprocessor must also idle until the data has been retrieved. During this idle time, the microprocessor clock continues to toggle thereby needlessly consuming power and generating heat that must be dissipated.
In order to decrease the frequency with which the microprocessor encounters these wait cycles, many modern high performance microprocessors have a small internal cache, called a primary cache. Instructions that are likely to be executed and data that is likely to be needed by the executing instructions are stored in the internal cache so that they may be accessed immediately by the CPU of the microprocessor.
The sequential nature of computer programs is such that when a particular instruction within the program is executed, it is highly probable that the next instruction to be executed will be the instruction that follows the currently executing instruction. Therefore, when an instruction is to be executed, the cache is checked to determine whether a copy of the required instruction is immediately available within the cache. If a copy of the required instruction is stored within the cache (called a cache hit), then the copy of the instruction can be supplied to the CPU immediately from the cache and there is no need for the CPU to wait while the instruction is retrieved to the microprocessor chip from wherever it is stored in the computer system.
On the other hand, if a copy of the required instruction is not stored within the cache (called a cache miss), then the CPU must wait while the instruction is retrieved to the microprocessor chip from wherever it is stored within the computer system. Actually, rather than only retrieving the next instruction to be executed, a cache line is formed by retrieving the next instruction to be executed and a certain number of instructions following the next instruction to be executed. That way, if the subsequent instructions are in fact required to be executed, they will be immediately available to the CPU from within the cache line of the cache. Because of the sequential nature of programs, the benefits of caching also applies to data used by the programs.
Because the internal cache is filled a cache line at a time, many microprocessors can accept data in a burst mode. In a typical burst read, the microprocessor specifies the first address of the data or instructions to be read into a cache line. Then, the data or instructions that are stored at the addresses of the cache line are sent sequentially from where they are stored within the computer system to the microprocessor.
Frequently the internal cache of the microprocessor is formed using static random access memory (SRAM). Because each SRAM cell is formed by six to eight transistors, there is only room on a microprocessor chip for a relatively small SRAM cache. Furthermore, SRAM is volatile meaning that SRAM retains the information stored as long as there is enough power to run the device. If power is removed, the contents of the SRAM cache are lost.
Some microprocessors are dynamic, meaning that if power is removed from them, when power is restored they cannot return directly to the state they were in when the power was removed. When power is restored the microprocessor must be reinitialized, and at least some of the processing progress previously made will probably be lost.
Other microprocessors are static, meaning that they can be placed in an energy saving deep powerdown mode, and then be returned relatively quickly to the state they were in immediately before they entered the deep powerdown mode.
As mentioned earlier, data and instructions are stored within the computer system and provided to the microprocessor over one (or more) bus systems. Because most types of relatively fast random access memory are both volatile and relatively expensive, a typical computer system stores code and data on relatively inexpensive, nonvolatile memory store such as a floppy disk or hard disk.
The typical computer system also has a main memory made of volatile memory because the nonvolatile memory has a relatively slow access speed. When a program is to be executed, the computer system uses a technique known as shadowing to copy the code and data required to execute the program fro
Dipert Brian Lyn
McCormick Bruce
Mills Duane R.
Pashley Richard D.
Sambandan Sachidanandan
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