runtime: add MemAvail function

lib/runtime.s

@@ -1764,6 +1764,33 @@ MEM_DUMP_L0:
 	DROP
 	RET
 
+; calculate total free heap space
+; args: none
+; returns: cumulative size of all free chunks in bytes
+MEMAVAIL:
+	FPADJ -4
+	LOADC 0
+	STORE 0	; start with zero as result
+
+	LOADCP _HEAP_ANCHOR
+MAV_L:
+	DUP	; dup chunk ptr for later
+	INC 4	; move to size field
+	LOADI	; load chunk size
+	LOAD 0  ; add to current result value
+	ADD
+	STORE 0
+
+	LOADI	; load next ptr
+	DUP
+	LOADCP _HEAP_ANCHOR ; compare with anchor
+	CMPU NE
+	CBRANCH MAV_L       ; if not equal, loop
+MAX_XT:
+	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

lib/stdlib.inc

@@ -148,6 +148,7 @@ procedure appendchar(var s:string; aChar:char); external;
 procedure strmoveup(var s:string;index,length,delta:integer); external;
 procedure strmovedown(var s:string;index,length,delta:integer); external;
 procedure RuntimeError(var s:string); external;
+function MemAvail:integer; external;
 
 (* from stdlib *)
 function copy(s:string[256];index,count:integer):string[256]; external;