Linux Kernel Architecture

Mauerer runapp01.tex V1 - 09/04/2008 6:08pm Page 1123

Appendix A: Architecture Specifics

Newer versions of the architecture use a much larger register set, which the kernel saves only as needed.
Consequently, the corresponding elements are held in thethread_structstructure, as shown here:

include/asm-x86/processor_32.h

struct thread_struct {

/* cached TLS descriptors. */

struct desc_struct tls_array[GDT_ENTRY_TLS_ENTRIES];

unsigned long esp0;

unsigned long sysenter_cs;

unsigned long eip;

unsigned long esp;

unsigned long fs;

unsigned long gs;

/* Hardware debugging registers */

unsigned long debugreg[8]; /* %%db0-7 debug registers */

/* fault info */

unsigned long cr2, trap_no, error_code;

/* floating point info */

union i387_union i387;

/* virtual 86 mode info */

struct vm86_struct __user * vm86_info;

unsigned long screen_bitmap;

unsigned long v86flags, v86mask, saved_esp0;

unsigned int saved_fs, saved_gs;

/* IO permissions */

unsigned long *io_bitmap_ptr;

unsigned long iopl;

/* max allowed port in the bitmap, in bytes: */

unsigned long io_bitmap_max;

};

Depending on the processor version, the coprocessor provides various register sets as specified in
i387_union:^1

include/asm-x86/processor_32.h

union i387_union {

struct i387_fsave_struct fsave;

struct i387_fxsave_struct fxsave;

struct i387_soft_struct soft;

};

Old 80386 and 80486SX processors that do not have a hardware-based coprocessor employ kernel-
side software emulation whose state is held ini387_soft_struct. Machines whose coprocessors sup-
port only the classic registers (that is, eight 10-byte-wide floating-point registers) store their data in
i387_fsave_struct. Because most registers are never used alone but are always read or written to
block-by-block, the kernel puts them in a contiguous memory area provided in the structure by an array
of suitable size:

include/asm-x86/processor_32.h

struct i387_fsave_struct {

long cwd;

long swd;

(^1) Coincidentally, the definition ofi387_unionfor kernel 2.6.17 is located in line 387.