Linux Device Drivers Development

Low memory

The first 896 MB of kernel address space constitutes the low memory region. Early in the boot, the kernel permanently maps this 896 MB. Addresses that result from that mapping are called logical addresses. These are virtual addresses, but can be translated into physical addresses by subtracting a fixed offset, since the mapping is permanent and known in advance. Low memory matches with the lower bound of physical addresses. You could define low memory as being the memory for which logical addresses exist in the kernel space. Most of the kernel memory function returns low memory. In fact, to serve different purposes, kernel memory is divided into a zone. Actually, the first 16 MB of LOWMEM is reserved for DMA use. Because of hardware ...