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  }