Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Reexamination Certificate
2000-12-29
2003-04-08
Lane, Jack A. (Department: 2188)
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
C711S162000, C713S320000
Reexamination Certificate
active
06546472
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to data storage in computer systems. More particularly, the present invention relates to the manipulation of data that has been stored in random access memory during periods when the computer system is placed in a low power mode to minimize the latency that otherwise occurs when the system enters a low power mode and resumes operation from a low power mode.
2. Background of the Invention
Almost all computer systems include a processor and a system memory. The system memory functions as the working memory of the computer system, where data is stored that has been or will be used by the processor and other system components. The system memory typically includes banks of dynamic random access memory (DRAM) circuits. According to normal convention, a memory controller interfaces the processor to a memory bus that connects electrically to the DRAM circuits. The system memory provides storage for a large number of instructions and/or a large amount of data for use by the processor, providing faster access to the instructions and/or data than would otherwise be achieved if the processor were forced to retrieve data from a disk or drive.
Because system memory typically is constructed of dynamic random access memory circuits, the contents of the memory are volatile. To preserve the integrity of the data stored in system memory, a periodic refresh signal must be sent to each memory cell to refresh the voltage levels of each cell, where the data is stored. Failure to timely refresh the memory cells of system memory causes the data to be lost. Thus, when power is turned off to a volatile memory device, the contents of that memory are lost. Data that is to be stored long-term on a computer system thus is stored in other non-volatile memory devices. Most computer systems include a hard drive that is capable of permanently storing data on magnetic tape. Other removable drives, such as zip drives, CD-ROMs, DVD-ROMs, and the like, may also be used for long-term storage of data. In these types of media, the data is preserved, even when power is removed from the computer system.
Almost all portable computers, and some desktop computers, may be placed in a low power state to preserve power. Preservation of power is especially important in portable computers, where operating power may be provided from batteries. To extend the life of batteries in portable computers, and thus extend the amount of time that a user can operate a portable computer without recharging the batteries or finding an electrical source, most portable computers are capable of going into a sleep mode where minimal power is consumed. The sleep mode permits the computer system to be placed in standby, so that operation can resume when the user is ready, without requiring the system to boot.
As power management of portable computer systems has evolved, two different low power modes have been developed and used commercially. The first low power state is referred to as the “suspend” mode or “Suspend to RAM” mode. In the suspend mode, the system memory remains powered while the system is taken to a non-operational state. The advantage of keeping the system memory powered is that when operation is resumed, the system is ready within a very short period for operation, in the state last used by the operator. Thus, resuming from a suspend mode only takes a few seconds, because very little system context is moved. Suspend to RAM generally is preferred as a bookmark feature because of its “instant on” low latency resume time. Suspend to RAM is also called the S
1
, S
2
, or S
3
power state by the ACPI nomenclature.
Conventional Suspend to RAM works by stopping the clocks to the system, while leaving the entire system power on. Because the power used by the system depends on the system clock speeds, removing the clock signals significantly lowers the system power. Suspend to RAM often is referred to as “Power on Suspend.” When the system resumes operation from Suspend to RAM, the clocks may simply be started to restore system operation. Another form of Suspend to RAM stores the context of certain system devices to system memory. Examples of the device contexts that may be saved include peripherals such as audio controllers, the state of the processor, the contents of the processor cache, and the like. Once the context of these devices is stored to system memory, the clocks to those devices are stopped and power is removed. The system memory, however, remains powered to maintain its contents. To resume operation, the system BIOS or operating system restores the context of the peripherals from system memory, and then system operation is resumed.
The second low power mode is known as “hibernation” or “hibernation to disk”, which is referred to as the S
4
power state by the ACPI nomenclature. In this mode, which is the lowest power mode of the computer system other than power-off, the computer system consumes minimal energy. The hibernation mode can be analogized to a no-power bookmark of the existing state of the computer system. When the hibernation mode is entered, the system hardware state is copied to the hard drive in a predetermined sequence. Because the hard drive is non-volatile memory, all power can then be removed from the system. Upon resume, the system is powered on, and control is given to the system ROM. A sequence called POST performs initial configuration of the system. During the POST sequence, the BIOS checks a bit in non-volatile memory to detect if the system had been placed in the hibernation state the last time system power was turned off. If the BIOS determines that it is resuming from a hibernation state, it will start to restore the system in a predetermined way. First, the system peripherals may be configured by reading the data from the hard drive. After system peripherals are configured, the system memory may be restored to the state that existed just prior to placing the system in hibernation. The BIOS algorithm will restore the system DRAM, filling DRAM memory in a predetermined sequence. After the entire state of the system has been restored by the ROM-based restoration algorithm, control of the system is handed to the operating system and the system user. Hibernation to disk thus allows the user to set a bookmark so that they can leave the system in a very low or no power state, and then return to the exact environment where they turned off the computer system. Because the context of the system is stored in non-volatile memory (on the hard drive), it provides reliable storage of the data in system memory, even during the low power state.
A delay period is encountered as the system context is stored to the hard drive, and again when the context is reloaded from the hard drive back to the system memory. The time required to access data from the hard drive is significantly longer than accessing data from system memory. Thus, there is a perceptible delay that occurs when data is loaded from the hard drive to the system memory after the hibernation mode is exited. This delay is exacerbated by increasingly large system memories. In many instances the suspend or resume process can require more than one minute to execute. This delay period is unworkable for systems that must be capable of instantly turning on to handle events.
Some have suggested that the data in system memory be compressed before hibernation occurs, to reduce the amount of data that must be copied and retrieved. Unfortunately, compression algorithms require extensive CPU power, and thus the act of compressing the system memory may actually require more time than simply writing uncompressed data to the hard drive. The Windows 2000 Operating System uses the knowledge of memory usage, so that unused areas of memory are not saved to disk during a hibernation event, as compared to previous systems, which typically stored the ent
Atkinson Lee W.
Nguyen Vu T.
Peters Mark W.
Conley & Rose, P.C.
Hewlett--Packard Development Company, L.P.
Lane Jack A.
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