问题是这样的，我定义了两个tss
 选择子是#define SELECTOR_TSS 0x20
 #define SELECTOR_TSS_NEXT 0x28
 
情况是这样的，我在ring0下
 ltr SELECTOR_TSS
 然后
 jmp SELECTOR_TSS_NEXT : 0
 
但bochs弹出的错误指示是
 00018540031e[CPU0 ] task_switch(exception after commit point): SS not valid or writeable segment
 00018540031e[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x0a)
 00018540031e[CPU0 ] interrupt(): gate descriptor is not valid sys seg (vector=0x08)
 


它们的初始化是这样的
memset(&tss, 0, sizeof(struct tss));
 tss.ss0 = SELECTOR_KERNEL_DS;
 tss.ldt = 0;
 
init_desc(&gdt[INDEX_TSS],  (u32)&tss, sizeof(struct tss) - 1, DA_386TSS);
  = 0x40000000;
 
memset(&tss_next, 0, sizeof(struct tss));
 tss_next.ldt = 0;
 
init_desc(&gdt[INDEX_TSS_NEXT], (u32)&tss_next, sizeof(struct tss) - 1, DA_386TSS);
 tss_ = 0x40000000;
 
tss_next.eip = (u32)TestA;
 tss_next.ss = 0x10;
 tss_next.esp = (u32)task_stack + STACK_SIZE_TOTAL;
 其中
 0x10所代表的gdt是数据段
 base为0，limit为0fffffh，type是c093h
 
init_desc是用来初始化gdt的，代码是
 PUBLIC void init_desc(DESCRIPTOR* p_desc, unsigned int base, unsigned int limit, u16 attr)
 {
 p_desc->limit_low = limit & 0xFFFF;
 p_desc->base_low = base & 0xFFFF;
 p_desc->base_mid = (base >> 16) & 0xFF;
 p_desc->attr1 = attr & 0xFF;
 p_desc->attr2_limit_high = (((limit >> 16) & 0x0F) | (attr >> 8)) & 0xF0;
 p_desc->base_high = (base >> 24) & 0xFF;
 }
 
DESCRIPTOR结构是
 typedef struct descriptor {
 u16 limit_low;
 u16 base_low;
 u8 base_mid;
 u8 attr1;
 u8 attr2_limit_high;
 u8 base_high;
 }DESCRIPTOR;
 
tss结构是
 typedef struct tss {
 u32 backline;
 u32 esp0;
 u32 ss0;
 u32 esp1;
 u32 ss1;
 u32 esp2;
 u32 ss2;
 u32 cr3;
 u32 eip;
 u32 flags;
 u32 eax;
 u32 ecx;
 u32 edx;
 u32 ebx;
 u32 esp;
 u32 ebp;
 u32 esi;
 u32 edi;
 u32 es;
 u32 cs;
 u32 ss;
 u32 ds;
 u32 fs;
 u32 gs;
 u32 ldt;
 u32 iomap;
 }TSS;