This section provides information on applying and using the
bounce buffer patch on the Linux 2.4 kernel. The bounce buffer
patch, written by Jens Axboe, enables device drivers that
support direct memory access (DMA) I/O to high-address physical
memory to avoid bounce buffers. The Linux 2.4 kernel includes
configuration options for specifying the amount of physical
memory in the target computer. By default, the configuration is
limited to the amount of memory that can be directly mapped into
the kernel's virtual address space starting at PAGE_OFFSET. On
i386 systems the default mapping scheme limits kernel-mode
addressability to the first gigabyte (GB) of physical memory,
also known as low memory. Conversely, high memory is normally
the memory above 1 GB. High memory is not directly accessible or
permanently mapped by the kernel. Support for high memory is an
option that is enabled during configuration of the Linux kernel.
When DMA I/O is performed to or from high memory, an area is
allocated in low memory kno