http://lxr.linux.no/#linux+v2.6.26/include/asm-x86/mach-default/irq_vectors.h
http://lxr.linux.no/#linux+v2.6.32/arch/x86/include/asm/irq_vectors.h
from oldlinux such as v0.11, as analyzed version as Doctor Zhao Jiong did, an old version just use it for the hardware limit, in the old 8259 interrupt controller days, the hardware 8259 is cascaded by 2 chips, and its real capability is only to handle 8 hard interrupt, by cascading style of implementation, there's another number is occupied by cascade, so there's only 7+8 = 15 interrupt numbers, every PC operating systems just re-program the 8259 IC, except for the intact PC standard, its 0x8~0xf for the first, and 0x78~0x7f for the second cascaded one, that can work well under 8086 Intel real mode, the DOS operating system keep it unchanged, and use int 0x21 as the system call service, then Linux re-program it as in the protected programming mode, for CPU internal usage, it reserved 0x0~0x1f 32 interrupt numbers; for the sake of x86 instruction encoding style, "int 0x80" is encoded as "0xcd 0x80", the interrupt number is just occupying one byte, so the CPU hardware limit is 256 int numbers, minus 32 reserved for CPU internal usage, it's 224 left, also as stated in the v2.6.26 irq_vectors.h comments said; Linux just use 0x80 as the system call interface, and shift/re-program 8259 to 0x20~0x27 and 0x28~0x2f; so the other range 0x30~0x7f, and 0x81~0xff are all unused; SMP architect is still strange, it generates special interrupts btween 0xee~0xff (238 ... 255); according to some recent secret unleashed, microsoft windows utilized 0x2e as the interrupt number, so the Linux int numbers changes just kept an reserved space for the unified kernel project; maybe it should be merged into mainstream someday;
15 * 16 * Vectors 0 ... 31 : system traps and exceptions - hardcoded events 17 * Vectors 32 ... 127 : device interrupts 18 * Vector 128 : legacy int80 syscall interface 19 * Vectors 129 ... 237 : device interrupts 20 * Vectors 238 ... 255 : special interrupts 21 *
http://www.oldlinux.org/
