在iodefine.h 文件中，对IO寄存器进行了定义，例如对port0的定义：
struct st_port0 {
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} PDR;
char wk0[31];
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} PODR;
char wk1[31];
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} PIDR;
char wk2[31];
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} PMR;
char wk3[31];
union {
unsigned char BYTE;
struct {
unsigned char :3;
unsigned char B4:1;
unsigned char :1;
unsigned char B2:1;
unsigned char :1;
unsigned char B0:1;
} BIT;
} ODR0;
char wk4[63];
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} PCR;
char wk5[31];
union {
unsigned char BYTE;
struct {
unsigned char :5;
unsigned char B2:1;
unsigned char B1:1;
unsigned char B0:1;
} BIT;
} DSCR;
};
其中的char wk0[31]; char wk1[31];等是表示什么意思（功能）？
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