MACHINE_START and MACHINE_END Macro define

在linux-2.6.38中，/arch/arm/mach-w90x900/mach-nuc950evb.c文件的最后部分，有如下宏定义:
MACHINE_START（W90P950EVB, "W90P950EVB"）
.phys_io = W90X900_PA_UART,
.io_pg_offst = （（（u32）W90X900_VA_UART） >> 18） & 0xfffc,
.boot_params =  0x100,
.map_io  = nuc950evb_map_io,
.init_irq = nuc900_init_irq,
.init_machine = nuc950evb_init,
.timer  = &nuc900_timer,
MACHINE_END 而MACHINE_START和MACHINE_END宏定义在arch/arm/include/asm/mach/arch.h，如下：

struct machine_desc {
/*
* Note！ The first four elements are used
* by assembler code in head.S, head-common.S
*/
unsigned int  nr;  /* architecture number */
unsigned int  phys_io; /* start of physical io */
unsigned int  io_pg_offst; /* byte offset for io
       * page tabe entry */ const char  *name;  /* architecture name */
unsigned long  boot_params; /* tagged list  */ unsigned int  video_start; /* start of video RAM */
unsigned int  video_end; /* end of video RAM */ unsigned int  reserve_lp0 :1; /* never has lp0 */
unsigned int  reserve_lp1 :1; /* never has lp1 */
unsigned int  reserve_lp2 :1; /* never has lp2 */
unsigned int  soft_reboot :1; /* soft reboot  */
void   （*fixup）（struct machine_desc *,     struct tag *, char **,   struct meminfo *）;
void   （*map_io）（void）;/* IO mapping function */
void   （*init_irq）（void）;
struct sys_timer *timer;  /* system tick timer */
void   （*init_machine）（void）;
};/*
* Set of macros to define architecture features. This is built into
* a table by the linker.
*/
#define MACHINE_START（_type,_name）
static const struct machine_desc __mach_desc_##_type
__used
__attribute__（（__section__（".arch.info.init"））） = {
.nr  = MACH_TYPE_##_type,
.name  = _name,#define MACHINE_END
};#endif 将前面定义的MACHINE_START和MACHINE_END展开后得到:
static const struct machine_desc __mach_desc_W90P950EVB __used __attribute__（（__section__（".arch.info.init"））） = {
.nr = MACH_TYPE_W90P950EVB, /* architecture number */
.name = "W90P950EVB"", /* architecture name */
.phys_io = W90X900_PA_UART, /* start of physical io */
.io_pg_offst = （（（u32）W90X900_VA_UART） >> 18） & 0xfffc,
.boot_params = 0x100, /* tagged list */
.map_io = nuc950evb_map_io, /* IO mapping function */
.init_irq = nuc900_init_irq,
.init_machine = nuc950evb_init,
.timer = &nuc900_timer,
}MACH_TYPE_W90P950EVB定义在arch/arm/include/asm/mach-types.h内，值为1860 十六进制为：0x744.
#define MACH_TYPE_W90P950EVB             1860这个值是机器的类型值，编译时由arch/arm/tool/mach- types里面定义的数据生成的。
/* arch/arm/tool/mach-types */
w90x900   MACH_W90X900  W90X900   1860而在arch/arm/kernel/head-common.S文件中的__lookup_machine_type函数中有如下的定义：/*
* linux/arch/arm/kernel/head-common.S
*
* Copyright （C） 1994-2002 Russell King
* Copyright （c） 2003 ARM Limited
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*
*/#define ATAG_CORE 0x54410001
#define ATAG_CORE_SIZE （（2*4 + 3*4） >> 2）
#define ATAG_CORE_SIZE_EMPTY （（2*4） >> 2） .align 2
.type __switch_data, %object
__switch_data:
...................中间省略 .long processor_id                   @ r4
.long __machine_arch_type   @ r5
.long __atags_pointer            @ r6
.long cr_alignment                  @ r7
.long init_thread_union + THREAD_START_SP @ sp ...................中间省略/*
* Look in <asm/procinfo.h> and arch/arm/kernel/arch.[ch] for
* more information about the __proc_info and __arch_info structures.
*/
.align 2
3: .long __proc_info_begin
.long __proc_info_end
4: .long .
.long __arch_info_begin
.long __arch_info_end/*
* Lookup machine architecture in the linker-build list of architectures.
* Note that we can"t use the absolute addresses for the __arch_info
* lists since we aren"t running with the MMU on （and therefore, we are
* not in the correct address space）. We have to calculate the offset.
*
* r1 = machine architecture number
* Returns:
* r3, r4, r6 corrupted
* r5 = mach_info pointer in physical address space
*/
__lookup_machine_type:
#ifdef CONFIG_ARCH_W90X900     //这个就是前面的1860,十六进制为:0x744
       mov      r1, #0x700
       add      r1, r1, #0x40
       add      r1, r1, #0x04
#endif
adr r3, 4b
ldmia r3, {r4, r5, r6}
sub r3, r3, r4   @ get offset between virt&phys
add r5, r5, r3   @ convert virt addresses to
add r6, r6, r3   @ physical address space
1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
teq r3, r1    @ matches loader number？
beq 2f    @ found
add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc
cmp r5, r6
blo 1b
mov r5, #0    @ unknown machine
2: mov pc, lr
ENDPROC（__lookup_machine_type）