前一篇介绍了汇编函数__lookup_processor_type，这一篇介绍__lookup_architecture_type函数
函数__lookup_architecture_type介绍：
每个机器(一般指的是某一个电路板)都有自己的特殊结构，如物理内存地址，物理I/O地址，显存起始地址等等，
这个结构为struct machine_desc，定义在asm-arm/mach/arch.h中:
struct machine_desc {
/*
* Note! The first four elements are used
* by assembler code in head-armv.S
*/
unsigned intnr;/* architecture number*/
unsigned intphys_ram;/* start of physical ram */
unsigned intphys_io;/* start of physical io*/
unsigned intio_pg_offst;/* byte offset for io page table entry*/
const char*name;/* architecture name*/
unsigned intparam_offset;/* parameter page*/
unsigned intvideo_start;/* start of video RAM*/
unsigned intvideo_end;/* end of video RAM*/
unsigned intreserve_lp0 :1;/* never has lp0*/,
unsigned intreserve_lp1 :1;/* never has lp1*/
unsigned intreserve_lp2 :1;/* never has lp2*/
unsigned intsoft_reboot :1;/* soft reboot*/
void(*fixup)(struct machine_desc *,
struct param_struct *, char **,
struct meminfo *);
void(*map_io)(void);/* IO mapping function*/
void(*init_irq)(void);
};
这个结构一般都定义在(以arm平台为例)kernel\arch\arm\mach-xxx\xxx.c中，是用宏来定义的，以mainstone的开发板为例：
定义在kernel\arch\arm\mach-pxa\mainstone.c文件中，如下所示：
MACHINE_START(MAINSTONE, "Intel DBBVA0 Development Platform")
MAINTAINER("MontaVista Software Inc.")
BOOT_MEM(0xa0000000, 0x40000000, io_p2v(0x40000000))
FIXUP(fixup_mainstone)
MAPIO(mainstone_map_io)
INITIRQ(mainstone_init_irq)
MACHINE_END
这些宏也定义在kernel/include/asm-arm/mach/arch.h中，以MACHINE_START为例：
#define MACHINE_START(_type,_name) \
const struct machine_desc __mach_desc_##_type \
__attribute__((__section__(".arch.info"))) = { \
.nr = MACH_TYPE_##_type, \
.name = _name,
展开之后结构的是：
__mach_desc_MAINSTONE = {
.nr = MACH_TYPE_MAINSTIONE,
.name = "Intel DBBVA0 Development Platform",
中间的1行__attribute__((__section__(".arch.info"))) = {说明将这个结构放到指定的段.arch.info中，这和前面的
.proc.info是一个意思，__attribute__((__section__的含义参考GNU手册。后面的宏都是类似的含义，这里就不再一一
介绍。下面开始说明源码：
第1行实现r4指向2b的地址，2b如__lookup_processor_type介绍的第19行，将machine_desc结构中的数据存放到r2, r3, r5, r6, r7。
读取__mach_desc_MAINSTONE结构中的nr参数到r5中，如第7行，比较r5和r1中的机器编号是否相同，如第8行，
r5中的nr值MACH_TYPE_MAINSTONE定义在kernel\include\asm-arm\mach-types.h中：
#define MACH_TYPE_MAINSTONE 303
r1中的值是由bootloader传递过来的，这在<>中有说明，
如果机器编号相同，跳到15行执行，r5=intphys_ram，r6=intphys_io，r7=intio_pg_offst，并返回。如果
不同则将地址指针增加，在跳到7行继续查找，如10--12行的代码，如果检索完所有的machine_desc仍然没
有找到则将r7清零并返回。
/*
* 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:
* r2, r3, r4 corrupted
* r5 = physical start address of RAM
* r6 = physical address of IO
* r7 = byte offset into page tables for IO
*/
1 __lookup_architecture_type:
2 adr r4, 2b
3 ldmia r4, {r2, r3, r5, r6, r7} @ throw away r2, r3
4 sub r5, r4, r5 @ convert addresses
5 add r4, r6, r5 @ to our address space
6 add r7, r7, r5
7 1: ldr r5, [r4] @ get machine type
8 teq r5, r1
9 beq 2f
10 add r4, r4, #SIZEOF_MACHINE_DESC
11 cmp r4, r7
12 blt 1b
13 mov r7, #0 @ unknown architecture
14 mov pc, lr
15 2: ldmib r4, {r5, r6, r7} @ found, get results
16 mov pc, lr
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