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games/mix: fix implementation of MOVE instruction (thanks nicolagi)

Plan 9 memcpy(2) uses the same implementation as memmove(2) to handle
overlapping ranges.  Hovewer, the MIX MOVE instruction, as described
in TAOCP, specifically does not do this.  It copies words one at a
time starting from the lowest address.

This change also expands the address validation to check that all
addresses within the source and destination ranges are valid before
proceeding.

--- a/sys/src/games/mix/mix.c

+++ b/sys/src/games/mix/mix.c

@@ -729,14 +729,20 @@

 mixmove(int s, int f)

 {

 	int d;

+	u32int *p, *q, *pe;

 	if(f == 0)

 		return;

-

+	if(s < 0 || s >= 4000 || s+f < 0 || s+f > 4000)

+		vmerror("Bad src range MOVE %d:%d", s, s+f-1);

 	d = mval(ri[1], 0, MASK2);

-	if(d < 0 || d > 4000)

-		vmerror("Bad address MOVE %d", d);

-	memcpy(cells+d, cells+s, f*sizeof(u32int));

+	if(d < 0 || d >= 4000 || d+f < 0 || d+f > 4000)

+		vmerror("Bad dst range MOVE %d:%d", d, d+f-1);

+	p = cells+d;

+	q = cells+s;

+	pe = p+f;

+	while(p < pe)

+		*p++ = *q++;

 	d += f;

 	d &= MASK2;

 	ri[1] = d < 0 ? -d|SIGNB : d;