Abstract:
A method and apparatus for allocating a variable amount of memory to a device coupled to a bus are disclosed. In one embodiment, a first register contains a first value. A second register (e.g., Base Address Register) contains a second value that represents a default amount of memory to allocate to the device. The second value is automatically altered to indicate an updated amount of memory to allocate based on the first value. The ability to alter the default memory value substantially eliminates memory holes, which are unused portions of memory that have been allocated to devices coupled to the bus.

Description:
[0001]    The embodiments disclosed herein relate to memory allocation, and more particularly to varying the amount of memory allocated to a device coupled to a bus.  
         BACKGROUND  
         [0002]    Conventional computer systems generally include a central processing unit (“CPU”) providing primary control of the computer system. The CPU is connected through a host bus to a Host/PCI (e.g., Peripheral Component Interconnect) bus bridge. The Host/PCI bus bridge is connected to a PCI bus, which is connected to peripheral components such as local area networks, graphics cards, and mass media devices. The Host/PCI bus bridge interfaces among the CPU, the main memory, and the peripheral components.  
           [0003]    Different bus architectures abound in the current state of computer technology. An example of a modern bus is the PCI bus. The PCI bus is a high performance, low latency system bus, generally defined by the PCI Special Interest Group (“SIG”) in PCI Local Bus Specification, Revision 2.3 (released Mar. 20, 2002). The PCI bus will be used to illustrate some of the principles behind and operation of the various embodiments disclosed herein. However, those principles may also be applied to other bus architectures.  
           [0004]    When a device is connected to a PCI bus, an amount of memory is allocated to (e.g., reserved for) the device, if necessary. The manufacturer of the device generally sets the amount of memory to be allocated to the device, which is currently based on the number of read-only bits in a base address register (“BAR”) associated with the device. In a typical system, the number of read-only bits is hardwired and cannot be altered.  
           [0005]    In order to determine the number of read-only bits in the BAR, the BIOS (e.g., basic input/output system) or other software writes a series of all 1&#39;s to the BAR and then reads the resultant BAR. When the resultant BAR is read, the read-only bits return a value of 0 (e.g., usually a series of 0&#39;s in the lower bits of the relevant field of the BAR). The BIOS can then allocate memory to the device based on the number of read-only bits in the BAR.  
           [0006]    In systems that employ a PCI bus, the amount of memory to be allocated to each device is set during enumeration, which is the procedure used by software (e.g., generally the BIOS or Operating System) to determine which devices are connected to the PCI bus and how much memory each device requires. Generally, the default amount of memory indicated as necessary by the BAR is equal to the largest possible amount of memory that the device can consume during operation. The default setting for some devices can be large, which is problematic if the device actually requires less than the entire allocated amount of memory.  
           [0007]    In instances where the device does not require all of the allocated memory, a memory hole may be created such that a certain amount of memory space is unused. Naturally, unused memory reduces the efficiency and overall throughput of the system.  
           [0008]    Besides allocating the largest possible amount of memory space to a device, another solution is to allow the device to calculate the amount of memory required by the device and configure the number of read-only bits in the BAR accordingly. However, this requires additional logic or pins and places a burden on devices that require variable amounts of memory to have information readily available regarding the quantity and size of all devices coupled to the bus before enumeration. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    Various embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an,” “one,” “the,” “other,” “various,” or “alternative” embodiments in this disclosure are not necessarily to the same embodiment, and such references mean at least one.  
         [0010]    [0010]FIG. 1 is a flow chart of a method for automatically altering a default value in a register, wherein the altered value indicates an amount of memory to be allocated to a device coupled to a bus.  
         [0011]    [0011]FIG. 2 is an embodiment of a first register used to indicate which bits of a second register (not shown) should be set as read-only bits.  
         [0012]    [0012]FIG. 3 is an embodiment of a system that uses a first register to automatically alter how much memory a second register should indicate is to be allocated to a device coupled to a bus.  
         [0013]    [0013]FIG. 4 shows how a mask register can be used to set specific bits within the BAR as read-only bits.  
     
    
     DETAILED DESCRIPTION  
       [0014]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. It will be apparent to one skilled in the art that the embodiments may be practiced without some of these specific details. In other instances, certain structures and devices are omitted or simplified in order to avoid obscuring the details of the various embodiments.  
         [0015]    The following description and the accompanying drawings provide examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are not intended to provide an exhaustive list of all possible implementations.  
         [0016]    Various methods disclosed herein can be implemented by using a machine to read instructions stored on a machine-readable medium and to perform functions in accordance with those instructions. A machine-readable medium includes any mechanism that provides (e.g., stores and/or transmits) information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; and electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, and digital signals).  
         [0017]    Referring now to FIG. 1, a flow chart for a method of allocating an amount of memory to a device coupled to a bus is shown. At block  10 , a first value is written to a first register.  
         [0018]    At block  12 , a second value in a second register is automatically altered. The alteration of the second value is based on the first value. In various embodiments, the second register is a configurable base address register (“BAR”). Once configured according to the second value, the BAR contains, among other information, the actual pointer to the location in the memory map that has been assigned to the relevant device by the operating system.  
         [0019]    In one embodiment, a user can set the size of memory to be allocated to the device by setting the first value. For example, the user can access a setup menu during the boot process of a personal computer. Within the setup menu (e.g., prior to enumeration), the user can set the first value.  
         [0020]    The first value can be used in several different manners to alter the second value. For example, in various embodiments, the first value is a mask value that indicates which bits of a second register are to be set as read-only bits. Using the mask value of the first register, the appropriate bits of the second register are set as read-only bits. Thus, the second register (e.g., BAR) can be programmed to indicate a variable amount of memory to be allocated to a specific device(s) in a manner that is still PCI-compliant.  
         [0021]    At block  14 , memory is allocated to the device based on the altered second value in the second register. In embodiments in which the amount of memory allocated is based on the number of read-only bits in the second value, the BIOS or other software determines how much memory should be allocated to the device after the second value is altered. In an embodiment, this determination is accomplished by evaluating which bits in the altered second value are read-only bits (as described above). In various embodiments, determining includes conducting an enumeration process according to the PCI protocol.  
         [0022]    [0022]FIG. 2 shows an embodiment of a 32-bit mask register that is used to configure the BAR to indicate an amount of memory to be allocated to a device coupled to a bus. However, other size registers (e.g., 64-bit) are contemplated by the embodiments disclosed herein. The functionality of the various embodiments can be extended to allocate anywhere from 16 Bytes to 1 Gigabyte prior to enumeration using a 32-bit BAR. Larger regions can be reserved if a 64-bit BAR is used.  
         [0023]    The mask register shown in FIG. 2 has several fields. For example, the field containing bits  0  through  21  and the field containing bits  30  and  31  are both reserved. Thus, bits  22  through  29  are used to signify which particular bit positions are read/writable (e.g., signified by a 1) or read-only (e.g., signified by a 0).  
         [0024]    In various embodiments, the bits are set starting with bit  22  and progressing to the left until bit  29  is reached. Initially, a sequential series of 0&#39;s is set to indicate the number of read-only bits. After the series of 0&#39;s is completed, the remaining available bits (e.g., up to bit  29 ) are set with a sequential series of 1&#39;s. The 1&#39;s and 0&#39;s should not be mixed (e.g., 10011010) when writing the bits to the mask register.  
         [0025]    [0025]FIG. 4 shows an example of how bits  22  through  29  of FIG. 2 can be used as a mask to set the appropriate bits of the BAR as read-only. For example, mask register  32  contains four 0&#39;s in the four least significant bit positions. Default BAR  34  contains six 0&#39;s in the six least significant bit positions. Thus, if mask register  32  is not used to alter the default BAR value, a memory hole could be created since the default BAR value would allocate a larger portion of memory to the device than may be required (e.g., indicated by fewer 0&#39;s in mask register  32 ). In various embodiments, a logical operation (e.g., AND, OR, etc.) is carried out to obtain the final value shown in resultant BAR  34 ′. As described above, the amount of memory indicated in the BAR upon enumeration is then allocated to the device.  
         [0026]    [0026]FIG. 3 shows a system that includes the first and second registers, discussed above, in order to allocate a variable amount of memory to a device coupled to a bus. Specifically, system  18  includes processor  20  (e.g., CPU), memory  24  coupled to processor  20  via chipset  22 , and bus  26  coupled to processor  20  via chipset  22 . The first and second registers, among others, are represented by registers  30  located within processor  20 .  
         [0027]    In various embodiments, memory  24  is flash memory. However, other types of non-volatile memory could be used. For example, pseudo-static RAM; various types of ROM such as EPROM (erasable programmable ROM) and EEPROM (electrically erasable programmable ROM); and any PCI device could be used. Most new PCI devices act like memory devices and could use the embodiments disclosed herein to change minimum requirements.  
         [0028]    Although not shown in FIG. 3, system  18  includes circuitry to automatically alter the second value based on the first value and/or circuitry to set at least one bit of the second value as a read-only bit, as indicated by the first value. In addition, system  18  includes circuitry (not shown) to allocate an amount of memory to a device coupled to the bus based on the altered second value.  
         [0029]    [0029]FIG. 3 also shows video card  28  coupled to bus  26 . Although not shown, other devices may also be coupled to bus  26 . For example, suitable devices that may be coupled to a PCI bus include, among others, an Ethernet port, a Small Computer System Interface Host Bus Adapter (e.g., SCSI HBA), and a PCI/ISA (e.g., Industry Standard Architecture) bus bridge. In various embodiments, bus  26  may be at least one of a PCI bus, a PCI-X bus, and a PCI Express bus.  
         [0030]    Alternatively, non-PCI bus architectures may also be used. For example, AGP (e.g., Accelerated Graphics Port) may be used in conjunction with the embodiments disclosed herein. AGP is designed specifically for the throughput demands of three-dimensional graphics and creates a dedicated point-to-point channel so that the graphics controller can have direct access to main memory.  
         [0031]    Likewise, Plug and Play ISA may be used in conjunction with the embodiments disclosed herein. Plug and Play ISA advantageously allows the operating system to automatically configure expansion boards so that users do not need to manually configure devices connected to the bus.  
         [0032]    Other bus architectures besides PCI, AGP, and Plug and Play ISA may be used so long as they allow the operating system to automatically set up the memory map instead of using a fixed map.  
         [0033]    By utilizing another register to configure the BAR before enumeration, the amount of memory allocated to a device coupled to the bus can be varied, which can reduce or eliminate memory holes that are often created when a large amount of memory is allocated to a device based on a default value.  
         [0034]    It is to be understood that even though numerous characteristics and advantages of various embodiments have been set forth in the foregoing description, together with details of structure and function of the various embodiments, this disclosure is illustrative only. Changes may be made in detail, especially matters of structure and management of parts, without departing from the scope of the various embodiments as expressed by the broad general meaning of the terms of the appended claims.