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門,顧名思義它是一扇門通向令一個(gè)地方，看一下門的結(jié)構(gòu)里面有一個(gè)描述符和一個(gè)偏移值,門中定義了這扇門通向的目的地，當(dāng)我們調(diào)用一個(gè)門的時(shí)候會(huì)到達(dá)這個(gè)地方：
門中的選擇子:門中的偏移值，也即下圖的selector:offset
這是我們代碼中國定義的門的數(shù)據(jù)結(jié)構(gòu)：
#define Gate(Selector,Offset,PCount,Attr)\
.2byte (Offset&0xffff);\
.2byte (Selector);\
.2byte (PCount&0x1f)|((Attr<<8)&0xff00);\
.2byte ((Offset>>16)&0xffff)
?
假設(shè)Selector_Gate_Call是調(diào)用門的描述符，執(zhí)行l(wèi)call $Selector_Gate_Call,$0語句時(shí)偏移值($0)無意義，執(zhí)行此語句后
CS=調(diào)用門中的選擇子
IP=調(diào)用門中偏移值
CS:IP=全局描述符表中第(調(diào)用門中的選擇子>>3)項(xiàng)描述符給出的段基址+調(diào)用門中偏移值
調(diào)用門的使用
定義調(diào)用門的描述符和選擇子，如下面代碼
/*門描述符*/
Descriptor_Gate_Call:Gate(Selector_Code_Gate,0,0,(DA_386CGate+DA_PL0))
/*調(diào)用門選擇子*/
.set?Selector_Gate_Call,(Descriptor_Gate_Call-GDT_START)
?
其中調(diào)用門中的選擇子和描述符如下：
Descriptor_CODE_GATE:Descriptor(0x0,CodeGLen,DA_C+DA_32)
.set? Selector_Code_Gate,(Descriptor_CODE_GATE-GDT_START)
?
Descriptor_CODE_GATE的初始化和代碼段如下：
InitDescrptor(Descriptor_CODE_GATE,LABEL_CODE_G);
LABEL_CODE_G:?
??? movl $((80*12+0)*2),%edi/*第10行，0列*/?
??? movb $0x0c,%ah/*高四位0000表示黑底,低四位1100表示紅字*/?
??? movb $'G',%al/*要顯示的字符*/?
??? movw %ax,%gs:(%edi)????
jmp .
?.set CodeGLen,(. - LABEL_CODE_G)
?
這時(shí)lcall $Selector_Gate_Call,$0就會(huì)看到紅色的'G’打印出來,和ljmp Selector_Code_Gate,$0的效果是一樣的，但是這并不是多此一舉的，這涉及的特權(quán)級(jí)轉(zhuǎn)換的問題，這里我們先不關(guān)心這些。
?
/*初始全局描述符Descriptor_CODE_GATE*/?
InitDescrptor(Descriptor_CODE_GATE,LABEL_CODE_G);
具體代碼如下：
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#define?Descriptor(base,lim,attr)\??.word?lim&0xffff;\??.word?base&0xffff;\??.byte?(base>>16)&0xff;\??.word?((lim>>8)&0xf00)|(attr&0x0f0ff);\??.byte?((base>>24)&0xff)????????/*????*InitDescrptor(Descriptor,SegBase)初始化描述符函數(shù)????*Descriptor：要初始化的描述符????*SegBase：段基址????*/????#define?InitDescrptor(Descriptor,SegBase)\??xor?????%eax,%eax;?\??mov?????%cs,%ax??;?\??shl?????$4,%eax??;?\??addl????$(SegBase),?%eax?;\??movw????%ax,?(Descriptor?+?2);\??shr?????$16,?%eax;\??movb????%al,?(Descriptor?+?4);\??movb????%ah,?(Descriptor?+?7)??????#define?Gate(Selector,Offset,PCount,Attr)\??.2byte?(Offset&0xffff);\??.2byte?(Selector);\??.2byte?(PCount&0x1f)|((Attr<<8)&0xff00);\??.2byte?((Offset>>16)&0xffff)??????DA_386CGate?=?0x8c??DA_PL0?=?0x00??DA_C???=?0x98????DA_32??=?0x4000????DA_DRW?=?0x92????SA_TIL?=?0x4????DA_LDT?=?0x82????????.text????.globl?start????.code16????start:?????jmpl?$0x0,?$code??????????/**-----------------------------------------------------------------?????*?全局描述符表:?GDT?????*-------------------------------*/?????????GDT_START:??????Descriptor_DUMMY:?Descriptor(0x0,0x0,0x0)????Descriptor_CODE32:Descriptor(0x0,0xffffffff,DA_C+DA_32)????Descriptor_VIDEO:?Descriptor(0xb8000,0x0ffff,DA_DRW)????Descriptor_CODE_GATE:Descriptor(0x0,CodeGLen,DA_C+DA_32)????/*門描述符*/???Descriptor_Gate_Call:Gate(Selector_Code_Gate,0,0,(DA_386CGate+DA_PL0))????GDT_END:????????GdtPtr:????????.word?(GDT_END-GDT_START)-1?#?so?does?gdt?????????.long?GDT_START?????#?This?will?be?rewrite?by?code.??????.set????selector_Code32,(Descriptor_CODE32-GDT_START)??.set????Selector_Video,(Descriptor_VIDEO-GDT_START)???????.set??Selector_Code_Gate,(Descriptor_CODE_GATE-GDT_START)??/*調(diào)用門選擇子*/??.set????Selector_Gate_Call,(Descriptor_Gate_Call-GDT_START)?????msg:?????.string?"Hello?world!"??????????????code:????????mov?????%cs,%ax???????????mov?????%ax,%ds?????????mov?????%ax,%es????????????mov?????%ax,%ss??????????mov??$0x8000,%sp??????????/*顯示HelloWorld字符串*/????????mov?$msg???,%ax????????mov?%ax????,%bp????????mov?$12????,%cx????????mov?$0x1301,%ax????????mov?$0x000c,%bx????????mov?$0?????,%dl????????????????int?$0x10????????????????/*初始全局描述符Descriptor_CODE32*/????InitDescrptor(Descriptor_CODE32,LABEL_SEG_CODE32);????????/*初始全局描述符Descriptor_CODE_GATE*/????InitDescrptor(Descriptor_CODE_GATE,LABEL_CODE_G);??????/*加載gdtr即將全局描述符表gdt的首地址和gdt的界限賦給gdtr寄存器*/???????????????lgdt?GdtPtr????????/*關(guān)中斷*/????????cli????????/*打開地址線A20*/????????inb?$0x92,%al????????or??$0x02,%al????????outb?%al,$0x92????????/*設(shè)置cr0寄存器,切換到保護(hù)模式*/????????movl?%cr0,%eax????????or???$1,%eax????????movl?%eax,%cr0????????????/*真正進(jìn)入保護(hù)模式,執(zhí)行此命令后CS=0x8,IP=0*/????????ljmp?$selector_Code32,$0??????????LABEL_SEG_CODE32:????.align??32????.code32????????????movw?$Selector_Video,%ax????????movw?%ax,%gs/*?視頻段選擇子(目的)*/????????movl?$((80*11+79)*2),%edi/*第11行，79列*/????????movb?$0x0c,%ah/*高四位表示黑底,低四位表示紅字*/????????movb?$'P',%al/*顯示的字符*/????????movw?%ax,%gs:(%edi)?????????????????#ljmp?$Selector_Gate_Call,$0??????lcall?$Selector_Gate_Call,$0????????????????jmp?.????????????LABEL_CODE_G:??????????movl?$((80*12+0)*2),%edi/*第10行，0列*/????????movb?$0x0c,%ah/*高四位0000表示黑底,低四位1100表示紅字*/????????movb?$'G',%al/*要顯示的字符*/????????movw?%ax,%gs:(%edi)??????????jmp?.?????.set?CodeGLen,(.?-?LABEL_CODE_G)????????.org?0x1fe,?0x90?????.word?0xaa55??? 執(zhí)行結(jié)果如下圖，可以看到紅色的‘G’打印出來
在LABEL_CODE_G中我們打印完字符就讓程序挺在了這里，這時(shí)候如下兩段代碼的效果是一樣的。
ljmp $Selector_Gate_Call,$0
?lcall $Selector_Gate_Call,$0
但ljmp和lcall是不一樣的，它們都會(huì)改變寄存器CS和IP實(shí)現(xiàn)程序的跳轉(zhuǎn)，但lcall還會(huì)影響堆棧?，段內(nèi)跳轉(zhuǎn)回將ip如棧段間跳轉(zhuǎn)回將cs和ip入棧，等到lret執(zhí)行時(shí)這個(gè)ip(或者cs和ip)會(huì)從堆棧中彈出。
下面我們就實(shí)現(xiàn)一個(gè)帶返回的調(diào)用門：
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#define?Descriptor(base,lim,attr)\??.word?lim&0xffff;\??.word?base&0xffff;\??.byte?(base>>16)&0xff;\??.word?((lim>>8)&0xf00)|(attr&0x0f0ff);\??.byte?((base>>24)&0xff)????????/*????*InitDescrptor(Descriptor,SegBase)初始化描述符函數(shù)????*Descriptor：要初始化的描述符????*SegBase：段基址????*/????#define?InitDescrptor(Descriptor,SegBase)\??xor?????%eax,%eax;?\??mov?????%cs,%ax??;?\??shl?????$4,%eax??;?\??addl????$(SegBase),?%eax?;\??movw????%ax,?(Descriptor?+?2);\??shr?????$16,?%eax;\??movb????%al,?(Descriptor?+?4);\??movb????%ah,?(Descriptor?+?7)??????#define?Gate(Selector,Offset,PCount,Attr)\??.2byte?(Offset&0xffff);\??.2byte?(Selector);\??.2byte?(PCount&0x1f)|((Attr<<8)&0xff00);\??.2byte?((Offset>>16)&0xffff)??????DA_386CGate?=?0x8c??DA_PL0?=?0x00??DA_C???=?0x98????DA_32??=?0x4000????DA_DRW?=?0x92????SA_TIL?=?0x4????DA_LDT?=?0x82????????.text????.globl?start????.code16????start:?????jmpl?$0x0,?$code??????????/**-----------------------------------------------------------------?????*?全局描述符表:?GDT?????*-------------------------------*/?????????GDT_START:??????Descriptor_DUMMY:?Descriptor(0x0,0x0,0x0)????Descriptor_CODE32:Descriptor(0x0,0xffffffff,DA_C+DA_32)????Descriptor_VIDEO:?Descriptor(0xb8000,0x0ffff,DA_DRW)????Descriptor_CODE_GATE:Descriptor(0x0,CodeGLen,DA_C+DA_32)??Descriptor_LDT:???Descriptor(0x0,LDTLen?-?1,?DA_LDT)????/*門描述符*/???Descriptor_Gate_Call:Gate(Selector_Code_Gate,0,0,(DA_386CGate+DA_PL0))????GDT_END:????????GdtPtr:????????.word?(GDT_END-GDT_START)-1?#?so?does?gdt?????????.long?GDT_START?????#?This?will?be?rewrite?by?code.??????/**-----------------------------------------------------------------?????*?GDT中的選擇子???*-------------------------------*/???.set????selector_Code32,(Descriptor_CODE32-GDT_START)??.set????Selector_Video,(Descriptor_VIDEO-GDT_START)???????.set??Selector_Code_Gate,(Descriptor_CODE_GATE-GDT_START)??.set????SelectorLDT,(Descriptor_LDT-GDT_START)??/*調(diào)用門選擇子*/??.set????Selector_Gate_Call,(Descriptor_Gate_Call-GDT_START)????/**-----------------------------------------------------------------???*?局部描述符表:?LDT???*-------------------------------*/??LDT_START:??/*???????????????????????段基址????段界限??????屬性*/??Descriptor_LDT_CODEA:?Descriptor(0,?CodeALen?-?1,?DA_C?+?DA_32)#?Code,?32?位????.set????LDTLen,(.?-?LDT_START)????#LDT?選擇子??.set??SelectorLDTCodeA,(0x0?+?SA_TIL)?????msg:?????.string?"Hello?world!"??????????????code:????????mov?????%cs,%ax???????????mov?????%ax,%ds?????????mov?????%ax,%es????????????mov?????%ax,%ss??????????mov??$0x8000,%sp??????????/*顯示HelloWorld字符串*/????????mov?$msg???,%ax????????mov?%ax????,%bp????????mov?$12????,%cx????????mov?$0x1301,%ax????????mov?$0x000c,%bx????????mov?$0?????,%dl????????????????int?$0x10????????????????/*初始全局描述符Descriptor_CODE32*/????InitDescrptor(Descriptor_CODE32,LABEL_SEG_CODE32);?????/*初始全局描述符Descriptor_LDT*/??InitDescrptor(Descriptor_LDT,LDT_START);??????/*初始全局描述符Descriptor_CODE_GATE*/????InitDescrptor(Descriptor_CODE_GATE,LABEL_CODE_G);??????/*初始LDT?中的描述符Descriptor_LDT_CODEA*/??InitDescrptor(Descriptor_LDT_CODEA,LABEL_CODE_A);????/*加載gdtr即將全局描述符表gdt的首地址和gdt的界限賦給gdtr寄存器*/???????????????lgdt?GdtPtr????????/*關(guān)中斷*/????????cli????????/*打開地址線A20*/????????inb?$0x92,%al????????or??$0x02,%al????????outb?%al,$0x92????????/*設(shè)置cr0寄存器,切換到保護(hù)模式*/????????movl?%cr0,%eax????????or???$1,%eax????????movl?%eax,%cr0????????????/*真正進(jìn)入保護(hù)模式,執(zhí)行此命令后CS=0x8,IP=0*/????????ljmp?$selector_Code32,$0????????????LABEL_SEG_CODE32:????.align??32????.code32????????????movw?$Selector_Video,%ax????????movw?%ax,%gs/*?視頻段選擇子(目的)*/????????movl?$((80*11+79)*2),%edi/*第11行，79列*/????????movb?$0x0c,%ah/*高四位表示黑底,低四位表示紅字*/????????movb?$'P',%al/*顯示的字符*/????????movw?%ax,%gs:(%edi)?????????????????lcall?$Selector_Gate_Call,$0??????????????movw?$SelectorLDT,%ax??????????lldt?%ax/*加載lgtr*/????????????ljmp????$SelectorLDTCodeA,$0????/*?跳入局部任務(wù)?LABEL_CODE_A*/??????????jmp?.????????????LABEL_CODE_G:??????????movl?$((80*12+0)*2),%edi/*第10行，0列*/????????movb?$0x0c,%ah/*高四位0000表示黑底,低四位1100表示紅字*/????????movb?$'G',%al/*要顯示的字符*/????????movw?%ax,%gs:(%edi)??????????????????????lret??.set?CodeGLen,(.?-?LABEL_CODE_G)??????LABEL_CODE_A:??????movw?$0x10,%ax??????movw?%ax,%gs/*?視頻段選擇子(目的)*/??????movl?$((80*12+10)*2),%edi/*第10行，0列*/??????movb?$0x0c,%ah/*高四位0000表示黑底,低四位1100表示紅字*/??????movb?$'A',%al/*要顯示的字符*/??????movw?%ax,%gs:(%edi)????????loop3:??????jmp?loop3??.set?CodeALen,(.?-?LABEL_CODE_A)??????.org?0x1fe,?0x90?????.word?0xaa55???
其結(jié)果是打印完'G'后返回，然后又打印出'A'如下圖：
?
門也是一個(gè)描述符，我們放到了GDT中，我們通過門的選擇子訪問門，門通過它記錄的選擇子和偏移值又把我們傳送到了目的地。
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