reactos操作系统实现(143)

VgaIsPresent函数通过读取图形模式的索引寄存器和数据寄存器来检查VGA显示卡是否安装，实现代码如下：

#001 BOOLEAN

#002 NTAPI

#003 VgaIsPresent(VOID)

#004 {

#005 UCHAR VgaReg,VgaReg2,VgaReg3;

#006 UCHAR SeqReg,SeqReg2;

#007 UCHAR i;

#008

读取VAG的基地址寄存器。

#009 /* Read the VGA Address Register */

#010 VgaReg = READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE);

#011

选择索引为4的映射选择寄存器。

#012 /* Select Read Map Select Register */

#013 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,4);

#014

读取回来是否还是索引为4的值，如果不是，就说明显示卡不存在。

#015 /* Read it back...it should be 4 */

#016 if (((READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE)) & 0xF) != 4) return FALSE;

#017

读取VGA数据寄存器值。

#018 /* Read the VGA Data Register */

#019 VgaReg2 = READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF);

#020

打开所有显示平面。

#021 /* Enable all planes */

#022 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,3);

#023

如果读取显示平面个数不为3时，说明也出错。

#024 /* Read it back...it should be 3 */

#025 if (READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF) != 0x3)

#026 {

#027 /* Reset the registers and fail */

#028 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0);

#029 return FALSE;

#030 }

#031

选择位屏蔽寄存器。

#032 /* Select Bit Mask Register */

#033 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,8);

#034

读取是否设置了位屏蔽寄存器，如果不是就出错了。

#035 /* Read it back...it should be 8 */

#036 if (((READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE)) & 0xF) != 8)

#037 {

#038 /* Reset the registers and fail */

#039 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,4);

#040 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0);

#041 return FALSE;

#042 }

#043

读取位屏蔽寄存器的值。

#044 /* Read the VGA Data Register */

#045 VgaReg3 = READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF);

#046

检查设置位屏蔽寄存器的值是否成功。

#047 /* Loop bitmasks */

#048 for (i = 0xBB; i; i >>= 1)

#049 {

#050 /* Set bitmask */

#051 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,i);

#052

#053 /* Read it back...it should be the same */

#054 if (READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF) != i)

#055 {

#056 /* Reset the registers and fail */

#057 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0xFF);

#058 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,4);

#059 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0);

#060 return FALSE;

#061 }

#062 }

#063

#064 /* Select Read Map Select Register */

#065 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,4);

#066

#067 /* Read it back...it should be 3 */

#068 if (READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF) != 3)

#069 {

#070 /* Reset the registers and fail */

#071 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0);

#072 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,8);

#073 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,0xFF);

#074 return FALSE;

#075 }

#076

恢复所有修改过的寄存器值。

#077 /* Write the registers we read earlier */

#078 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,VgaReg2);

#079 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,8);

#080 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CF,VgaReg3);

#081 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3CE,VgaReg);

#082

读取序列寄存器，如果也成功，说明VGA显示卡已经可以使用。

#083 /* Read sequencer address */

#084 SeqReg = READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C4);

#085

#086 /* Select memory mode register */

#087 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C4,4);

#088

#089 /* Read it back...it should still be 4 */

#090 if (((READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C4)) & 7) != 4)

#091 {

#092 /* Fail */

#093 return FALSE;

#094 }

#095

#096 /* Read sequencer Data */

#097 SeqReg2 = READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C5);

#098

#099 /* Write null plane */

#100 WRITE_PORT_USHORT((PUSHORT)VgaRegisterBase + 0x3C4,0x100);

#101

#102 /* Write sequencer flag */

#103 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C5,SeqReg2 ^ 8);

#104

#105 /* Read it back */

#106 if ((READ_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C5)) != (SeqReg2 ^ 8))

#107 {

#108 /* Not the same value...restore registers and fail */

#109 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C5,2);

#110 WRITE_PORT_USHORT((PUSHORT)VgaRegisterBase + 0x3C4,0x300);

#111 return FALSE;

#112 }

#113

#114 /* Now write the registers we read */

#115 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C5,SeqReg2);

#116 WRITE_PORT_USHORT((PUSHORT)VgaRegisterBase + 0x3C4,0x300);

#117 WRITE_PORT_UCHAR((PUCHAR)VgaRegisterBase + 0x3C4,SeqReg);

#118

检测VGA兼容显示卡已经安装。

#119 /* VGA is present! */

#120 return TRUE;

#121}

（编辑：李大同）

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