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realemu: ignore access to CMOS/RTC address/data registers

the kernel wont allow access to i/o ports 0x70/0x71, so
ignore the access. reads return 0xFF. this fixes vesa on
lenovo e540.

--- a/sys/src/cmd/aux/realemu/main.c

+++ b/sys/src/cmd/aux/realemu/main.c

@@ -29,6 +29,7 @@

 static int cputrace;

 static int porttrace;

 static Pit pit[3];

+static uchar rtcaddr;

 static vlong pitclock;

@@ -186,6 +187,12 @@

 	case 0x65:	/* A20 gate */

 		w = 1 << 2;

 		break;

+	case 0x70:	/* RTC addr */

+		w = rtcaddr;

+		break;

+	case 0x71:	/* RTC data */

+		w = 0xFF;

+		break;

 	case 0x80:	/* extra dma registers (temp) */

 	case 0x84:

 	case 0x85:

@@ -243,6 +250,11 @@

 	case 0x63:

 	case 0x64:	/* KB controller input buffer (ISA, EISA) */

 	case 0x65:	/* A20 gate (bit 2) */

+		break;

+	case 0x70:	/* RTC addr */

+		rtcaddr = w & 0xFF;

+		break;

+	case 0x71:	/* RTC data */

 		break;

 	case 0x80:

 	case 0x84: