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cga: capture cga console contents on boot, make sure cgapos is in range

to capture bios and bootloader messages, convert the contents
on the screen to kmesg.

on machines without legacy cga, the cga registers read out as
0xFF, resuting in out of bounds cgapos. so set cgapos to 0 in
that case.

--- a/sys/src/9/pc/cga.c

+++ b/sys/src/9/pc/cga.c

@@ -169,13 +169,46 @@

 	unlock(&cgascreenlock);

 }

+static void

+cgatokmesg(void)

+{

+	int i, n;

+	char *p;

+

+	ilock(&kmesg.lk);

+	n = kmesg.n;

+	for(i = cgapos-2; i >= 0 && n < sizeof kmesg.buf-UTFmax-1; i -= 2){

+		if((i % Width) == Width-2)

+			n++;

+		n += runelen(cp437[CGASCREENBASE[i]]);

+	}

+	n -= kmesg.n;

+	if(n > 0){

+		memmove(kmesg.buf+n, kmesg.buf, kmesg.n);

+		kmesg.n += n;

+		p = kmesg.buf;

+		for(i += 2; i >= 0 && i < cgapos && p < kmesg.buf+n; i += 2){

+			p += runetochar(p, &cp437[CGASCREENBASE[i]]);

+			if((i % Width) == Width-2)

+				*p++ = '\n';

+		}

+	}

+	iunlock(&kmesg.lk);

+}

+

 void

 screeninit(void)

 {

-

 	cgapos = cgaregr(0x0E)<<8;

 	cgapos |= cgaregr(0x0F);

 	cgapos *= 2;

+

+	if(cgapos >= Width*Height){

+		cgapos = 0;

+		movecursor();

+	}

+

+	cgatokmesg();

 	screenputs = cgascreenputs;

 }