Reserved Page Frames

The kernel's code and data structures are stored in a group of reserved page frames. A page contained in one of these page frames can never be dynamically assigned or swapped to disk.

As a general rule, the Linux kernel is installed in RAM starting from the physical address 0x00100000 — i.e., from the second megabyte. The total number of page frames required depends on how the kernel is configured. A typical configuration yields a kernel that can be loaded in less than 2 MBs of RAM.

Why isn't the kernel loaded starting with the first available megabyte of RAM? Well, the PC architecture has several peculiarities that must be taken into account. For example:

• Page frame 0 is used by BIOS to store the system hardware configuration detected during the Power-On Self-Test (POST ); the BIOS of many laptops, moreover, write data on this page frame even after the system is initialized.

• Physical addresses ranging from 0x000a0000 to 0x000fffff are usually reserved to BIOS routines and to map the internal memory of ISA graphics cards. This area is the well-known hole from 640 KB to 1 MB in all IBM-compatible PCs: the physical addresses exist but they are reserved, and the corresponding page frames cannot be used by the operating system.

• Additional page frames within the first megabyte may be reserved by specific computer models. For example, the IBM ThinkPad maps the 0xa0 page frame into the 0x9f one.

In the early stage of the boot sequence (see Appendix A), the kernel queries the BIOS and learns the size of the physical memory. In recent computers, the kernel also invokes a BIOS procedure to build a list of physical address ranges and their corresponding memory types.

Later, the kernel executes the setup_memory_region( ) function, which fills a table of physical memory regions, shown in Table 2-1. Of course, the kernel builds this table on the basis of the BIOS list, if this is available; otherwise the kernel builds the table following the conservative default setup. All page frames with numbers from 0x9f (lowmemsize( )) to

0x100 (high_memory) are marked as reserved.

Table 2-1. Example of BIOS-provided physical addresses map















ACPI data







A typical configuration for a computer having 128 MB of RAM is shown in Table 2-1. The physical address range from 0x07ff0000 to 0x07ff2fff stores information about the hardware devices of the system written by the BIOS in the POST phase; during the initialization phase, the kernel copies such information in a suitable kernel data structure, and then considers these page frames usable. Conversely, the physical address range of 0x07ff3000 to 0x07ffffff is mapped on ROM chips of the hardware devices. The physical address range starting from 0xffff0000 is marked as reserved since it is mapped by the hardware to the BlOS's ROM chip (see Appendix A). Notice that the BIOS may not provide information for some physical address ranges (in the table, the range is 0x000a0000 to

0x000effff). To be on the safe side, Linux assumes that such ranges are not usable.

To avoid loading the kernel into groups of noncontiguous page frames, Linux prefers to skip the first megabyte of RAM. Clearly, page frames not reserved by the PC architecture will be used by Linux to store dynamically assigned pages.

Figure 2-12 shows how the first 2 MB of RAM are filled by Linux. We have assumed that the kernel requires less than one megabyte of RAM (this is a bit optimistic).

Figure 2-12. The first 512 page frames (2 MB) in Linux 2.4

Figure 2-12. The first 512 page frames (2 MB) in Linux 2.4

The symbol _text, which corresponds to physical address 0x00100000, denotes the address of the first byte of kernel code. The end of the kernel code is similarly identified by the symbol _etext. Kernel data is divided into two groups: initialized and uninitialized. The initialized data starts right after _etext and ends at _edata. The uninitialized data follows and ends up at _end.

The symbols appearing in the figure are not defined in Linux source code; they are produced while compiling the kernel. 13!

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  • Costanzo
    What is reserved page frame?
    2 years ago

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