runtime/stdlib: add MaxAvail function

lib/runtime.s

@@ -1792,6 +1792,38 @@ MAX_XT:
 	FPADJ 4
 	RET
 
+; find largest free chunk on heap
+; args: none
+; returns: size of largest free chunk in bytes
+MAXAVAIL:
+	FPADJ -4
+	LOADC 0
+	STORE 0	; start with zero as result
+
+	LOADCP _HEAP_ANCHOR
+MXAV_L:
+	DUP	; dup chunk ptr for later
+	INC 4	; move to size field
+	LOADI	; load chunk size
+	LOAD 0  ; compare with current result value
+	CMPU.S0 LE ; compare with keeping first arg on stack
+	CBRANCH MXAV_NEXT ; if smaller or equal, no change
+	STORE 0		; else store as new value
+	BRANCH MXAV_NEXT2
+MXAV_NEXT:
+	DROP
+MXAV_NEXT2:
+	LOADI	; load next ptr
+	DUP
+	LOADCP _HEAP_ANCHOR ; compare with anchor
+	CMPU NE
+	CBRANCH MXAV_L       ; if not equal, loop
+MXAV_XT:
+	DROP	; drop chunk ptr
+	LOAD 0	; put result value on stack
+	FPADJ 4
+	RET
+
 ; check if a pointer is part of the free list
 ; args: pointer returned by MEM_ALLOC
 ; throws runtime error if the pointer is found