The code of the setup( ) assembly language function is placed by the linker immediately after the integrated boot loader of the kernel—that is, at offset 0x200 of the kernel image file. The boot loader can therefore easily locate the code and copy it into RAM, starting from physical address 0x00090200.
The setup( ) function must initialize the hardware devices in the computer and set up the environment for the execution of the kernel program. Although the BIOS already initialized most hardware devices, Linux does not rely on it, but reinitializes the devices in its own manner to enhance portability and robustness. setup( ) performs the following operations:
1. Invokes a BIOS procedure to find out the amount of RAM available in the system.
2. Sets the keyboard repeat delay and rate. (When the user keeps a key pressed past a certain amount of time, the keyboard device sends the corresponding keycode over and over to the CPU.)
3. Initializes the video adapter card.
4. Reinitializes the disk controller and determines the hard disk parameters.
5. Checks for an IBM Micro Channel bus (MCA).
6. Checks for a PS/2 pointing device (bus mouse).
7. Checks for Advanced Power Management (APM) BIOS support.
8. If the kernel image was loaded low in RAM (at physical address 0x00010000), moves it to physical address 0x00001000. Conversely, if the kernel image was loaded high in RAM, the function does not move it. This step is necessary because to be able to store the kernel image on a floppy disk and to save time while booting, the kernel image stored on disk is compressed, and the decompression routine needs some free space to use as a temporary buffer following the kernel image in RAM.
9. Sets up a provisional Interrupt Descriptor Table (IDT) and a provisional Global Descriptor Table (GDT).
10. Resets the floating-point unit (FPU), if any.
11. Reprograms the Programmable Interrupt Controller (PIC) and maps the 16 hardware interrupts (IRQ lines) to the range of vectors from 32 to 47. The kernel must perform this step because the BIOS erroneously maps the hardware interrupts in the range from 0 to 15, which is already used for CPU exceptions (see Section 4.2.2).
12. Switches the CPU from Real Mode to Protected Mode by setting the PE bit in the cr0 status register. The PG bit in the cr0 register is cleared, so paging is still disabled.
13. Jumps to the startup_32( ) assembly language function.