arch/arm/kernel/head.S中:
00079: bl__lookup_machine_type@ r5=machinfo
00080: movsr8, r5@ invalid machine (r5=0)?
00081: beq__error_a@ yes, error a

79行: 跳轉(zhuǎn)到__lookup_machine_type函數(shù),在__lookup_machine_type中,會(huì)把struct machine_desc的基地址(machine type)存儲(chǔ)在r5中
80,81行: 將r5中的 machine_desc的基地址存儲(chǔ)到r8中,并判斷r5是否是0,如果是0,說明是無效的machine type,跳轉(zhuǎn)到__error_a(出錯(cuò))

__lookup_machine_type 函數(shù)
下面我們分析__lookup_machine_type 函數(shù):

arch/arm/kernel/head-common.S中:

00176: .long__proc_info_begin
00177: .long__proc_info_end
00178: 3:.long.
00179: .long__arch_info_begin
00180: .long__arch_info_end
00181:
00182:
00193: .type__lookup_machine_type, %function
00194: __lookup_machine_type:
00195: adrr3, 3b
00196: ldmiar3, {r4, r5, r6}
00197: subr3, r3, r4@ get offset between virt&phys
00198: addr5, r5, r3@ convert virt addresses to
00199: addr6, r6, r3@ physical address space
00200: 1:ldrr3, [r5, #MACHINFO_TYPE]@ get machine type
00201: teqr3, r1@ matches loader number?
00202: beq2f@ found
00203: addr5, r5, #SIZEOF_MACHINE_DESC@ next machine_desc
00204: cmpr5, r6
00205: blo1b
00206: movr5, #0@ unknown machine
00207: 2:movpc, lr

193, 194行: 函數(shù)聲明
195行: 取地址指令,這里的3b是向后symbol名稱是3的位置,即第178行,將該地址存入r3.
和上面我們對(duì)__lookup_processor_type 函數(shù)的分析相同,r3中存放的是3b處物理地址.
196行: r3是3b處的地址,因而執(zhí)行完后:
r4存的是 3b處的地址
r5存的是__arch_info_begin 的地址
r6存的是__arch_info_end 的地址

__arch_info_begin 和 __arch_info_end是在 arch/arm/kernel/vmlinux.lds.S中:

00034:__arch_info_begin = .;
00035:*(.arch.info.init)
00036:__arch_info_end = .;

這里是聲明了兩個(gè)變量:__arch_info_begin 和 __arch_info_end,其中等號(hào)后面的"."是location counter(詳細(xì)內(nèi)容請(qǐng)參考ld.info)
這三行的意思是: __arch_info_begin 的位置上,放置所有文件中的 ".arch.info.init" 段的內(nèi)容,然后緊接著是 __arch_info_end 的位置.

kernel 使用struct machine_desc 來描述 machine type.
在 include/asm-arm/mach/arch.h 中:

00017: struct machine_desc {
00018:
00022: unsigned intnr;
00023: unsigned intphys_io;
00024: unsigned intio_pg_offst;
00026:
00027: const char*name;
00028: unsigned longboot_params;
00029:
00030: unsigned intvideo_start;
00031: unsigned intvideo_end;
00032:
00033: unsigned intreserve_lp0 :1;
00034: unsigned intreserve_lp1 :1;
00035: unsigned intreserve_lp2 :1;
00036: unsigned intsoft_reboot :1;
00037: void(*fixup)(struct machine_desc *,
00038: struct tag *, char **,
00039: struct meminfo *);
00040: void(*map_io)(void);
00041: void(*init_irq)(void);
00042: struct sys_timer*timer;
00043: void(*init_machine)(void);
00044: };
00045:
00046:
00050: #define MACHINE_START(_type,_name)
00051: static const struct machine_desc __mach_desc_##_type
00052: __attribute_used__
00053: __attribute__((__section__(".arch.info.init"))) = {
00054: .nr= MACH_TYPE_##_type,
00055: .name= _name,
00056:
00057: #define MACHINE_END
00058: };

內(nèi)核中,一般使用宏MACHINE_START來定義machine type.
對(duì)于at91, 在 arch/arm/mach-at91rm9200/board-ek.c 中:
00137: MACHINE_START(AT91RM9200EK, "Atmel AT91RM9200-EK")
00138:
00139: .phys_io= AT91_BASE_SYS,
00140: .io_pg_offst= (AT91_VA_BASE_SYS >> 18) & 0xfffc,
00141: .boot_params= AT91_SDRAM_BASE + 0x100,
00142: .timer= &at91rm9200_timer,
00143: .map_io= ek_map_io,
00144: .init_irq= ek_init_irq,
00145: .init_machine= ek_board_init,
00146: MACHINE_END

197行: r3中存儲(chǔ)的是3b處的物理地址,而r4中存儲(chǔ)的是3b處的虛擬地址,這里計(jì)算處物理地址和虛擬地址的差值,保存到r3中
198行: 將r5存儲(chǔ)的虛擬地址(__arch_info_begin)轉(zhuǎn)換成物理地址
199行: 將r6存儲(chǔ)的虛擬地址(__arch_info_end)轉(zhuǎn)換成物理地址
200行: MACHINFO_TYPE 在 arch/arm/kernel/asm-offset.c 101行定義, 這里是取 struct machine_desc中的nr(architecture number) 到r3中
201行: 將r3中取到的machine type 和 r1中的 machine type(見前面的"啟動(dòng)條件")進(jìn)行比較
202行: 如果相同,說明找到了對(duì)應(yīng)的machine type,跳轉(zhuǎn)到207行的2f處,此時(shí)r5中存儲(chǔ)了對(duì)應(yīng)的struct machine_desc的基地址
203行: (不相同), 取下一個(gè)machine_desc的地址
204行: 和r6進(jìn)行比較,檢查是否到了__arch_info_end.
205行: 如果不相同,說明還有machine_desc,返回200行繼續(xù)查找.
206行: 執(zhí)行到這里,說明所有的machind_desc都查找完了,并且沒有找到匹配的, 將r5設(shè)置成0(unknown machine).
207行: 返回