Understanding the Linux Kernel

2.6. Anticipating Linux 2.4

Linux 2.4 introduces two main changes. The TSS Segment Descriptor associated with all existing processes is no longer stored in the Global Descriptor Table. This change removes the hard-coded limit on the number of existing processes. The limit thus becomes the number of available PIDs. In short, you will not find anymore the NR_TASKS macro inside the kernel code, and all data structures whose size was depending on it have been replaced or removed.

The other main change is related to physical memory addressing. Recent Intel 80x86 microprocessors include a feature called Physical Address Extension (PAE), which adds four extra bits to the standard 32-bit physical address. Linux 2.4 takes advantage of PAE and supports up to 64 GB of RAM. However, a linear address is still composed by 32 bits, so that only 4 GB of RAM can be "permanently mapped" and accessed at any time. Linux 2.4 thus recognizes three different portions of RAM: the physical memory suitable for ISA Direct Memory Access (DMA), the physical memory not suitable for ISA DMA but permanently mapped by the kernel, and the "high memory," that is, the physical memory that is not permanently mapped by the kernel.