1. phys_io 与 io_pg_offst
我们在移植BSP的时候需要填充 machine_desc 结构体，其中有两个字段 phys_io 和 io_pg_offst，如下红色加粗部分：
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
在linux2.6.38中已经没有phys_io与io_pg_offs这两个变量，后面的文章会分析这个问题，现在就来分析linux2.6.35中在machine_desc结构体中，有关phys_io和io_pg_offst变量的作用以及使用方法，在介绍phys_io和io_pg_offst变量的作用之前，我们先来熟悉一些machine_desc这结构体：
[cpp]
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); };