Necesary to make realemu work (and maybe have video in the future).

Reference: /n/patches.lsub.org/patch/realmodemem
Date: Thu Apr 26 16:30:27 CES 2012
Signed-off-by: paurea@lsub.org

--- /sys/src/nix/k10/devarch.c	Thu Apr 12 12:26:27 2012
+++ /sys/src/nix/k10/devarch.c	Thu Apr 26 15:37:05 2012
@@ -35,6 +35,7 @@
 	Qiol,
 	Qbase,
 	Qmapram,
+	Qrealmem,
 
 	Qmax = 16,
 };
@@ -51,6 +52,7 @@
 	"iow",		{ Qiow, 0 },		0,	0660,
 	"iol",		{ Qiol, 0 },		0,	0660,
 	"mapram",	{ Qmapram, 0 },	0,	0444,
+	"realmodemem",	{ Qrealmem, 0 },	0,	0660,
 };
 Lock archwlock;	/* the lock is only for changing archdir */
 int narchdir = Qbase;
@@ -382,7 +384,6 @@
 
 	case Qioalloc:
 		break;
-
 	default:
 		if(c->qid.path < narchdir && (fn = readfn[c->qid.path]))
 			return fn(c, a, n, offset);
@@ -473,7 +474,6 @@
 		for(port = offset; port < offset+n; port += 4)
 			outl(port, *lp++);
 		return n;
-
 	default:
 		if(c->qid.path < narchdir && (fn = writefn[c->qid.path]))
 			return fn(c, a, n, offset);
@@ -531,10 +531,45 @@
 	return readstr(off, a, n, buf);
 }
 
+
+static long
+rmemrw(int isr, void *a, long n, vlong off)
+{
+	if(off < 0 || n < 0)
+		error("bad offset/count");
+	if(isr){
+		if(off >= MB)
+			return 0;
+		if(off+n >= MB)
+			n = MB - off;
+		memmove(a, KADDR((ulong)off), n);
+	}else{
+		/* realmode buf page ok, allow vga framebuf's access */
+		if(off >= MB || off+n > MB &&
+		    (off < 0xA0000 || off+n > 0xB0000+0x10000))
+			error("bad offset/count in write");
+		memmove(KADDR((ulong)off), a, n);
+	}
+	return n;
+}
+
+static long
+rmemread(Chan*, void *a, long n, vlong off)
+{
+	return rmemrw(1, a, n, off);
+}
+
+static long
+rmemwrite(Chan*, void *a, long n, vlong off)
+{
+	return rmemrw(0, a, n, off);
+}
+
 void
 archinit(void)
 {
 	addarchfile("cputype", 0444, cputyperead, nil);
+	addarchfile("realmodemem", 0660, rmemread, rmemwrite);
 }
 
 void