Add BOOT_LEAVE_MEMORY (desired memory left unused) and BOOT_MIN_AVAIL_MEMORY

(minimum desired free memory), default them to 5M and 10M respectively.
On a machine with more than (BOOT_LEAVE_MEMORY + BOOT_MIN_AVAIL_MEMORY) free
BOOT_LEAVE_MEMORY will be left unused.
Allows a kernel to be booted from SparkFS on a machine with more than 15M
of free memory (SparkFS needs to be able to allocate memory for the
uncompressed kernel)

sys/arch/acorn32/stand/lib/srt0.S

@@ -1,4 +1,4 @@
-/*	$NetBSD: srt0.S,v 1.2 2004/05/18 23:29:30 gavan Exp $	*/
+/*	$NetBSD: srt0.S,v 1.3 2004/06/13 20:50:14 abs Exp $	*/
 
 /*-
  * Copyright (c) 2001 Ben Harris
@@ -33,6 +33,23 @@
  * (see srt1.c)
  */
 
+/* Minimum desired free memory */
+
+#ifndef BOOT_MIN_AVAIL_MEMORY
+#define BOOT_MIN_AVAIL_MEMORY	10 * 1024 * 1024
+#endif
+
+/* If enough above BOOT_MIN_AVAIL_MEMORY, leave BOOT_LEAVE_MEMORY unused */
+/* Avoids sparkfs failing on memory if we try to read a kernel from it */
+
+#ifndef BOOT_LEAVE_MEMORY
+#define BOOT_LEAVE_MEMORY	5 * 1024 * 1024
+#endif
+
+/*
+ * Both BOOT_LEAVE_MEMORY and BOOT_MIN_AVAIL_MEMORY + BOOT_LEAVE_MEMORY
+ * must fit into an immediate ATM constant - 8 bits with arbitrary rotation
+ */
 
 #include <machine/asm.h>
 #include <riscoscalls.h>
@@ -43,6 +60,10 @@ ENTRY(_start)
 	mvn	r1, #0
 	swi	Wimp_SlotSize
 	add	r0, r0, r2
+#if (BOOT_LEAVE_MEMORY > 0)
+	cmp	r0, #BOOT_MIN_AVAIL_MEMORY + BOOT_LEAVE_MEMORY
+	subhi	r0, r0, #BOOT_LEAVE_MEMORY
+#endif 
 	mvn	r1, #0
 	swi	Wimp_SlotSize
 #endif