mmap: add MMAP_LEN parameter

kernel/mmap.asm

@@ -4,33 +4,43 @@
 ;
 ; *** DEFINES ***
 ; MMAP_START: Memory address where the mmap begins
+; Memory address where the mmap stops, exclusively (we aren't allowed to access
+; that address).
+.equ	MMAP_LEN	0xffff-MMAP_START
 
-; Returns absolute addr of memory pointer in HL.
+; Returns absolute addr of memory pointer in HL if HL is within bounds.
+; Sets Z on success, unset when out of bounds.
 _mmapAddr:
 	push	de
+	ld	de, MMAP_LEN
+	call	cpHLDE
+	jr	nc, .outOfBounds	; HL >= DE
 	ld	de, MMAP_START
 	add	hl, de
-	jr	nc, .end
+	cp	a	; ensure Z
-	; we have carry? out of bounds, set to maximum
-	ld	hl, 0xffff
-.end:
 	pop	de
 	ret
+.outOfBounds:
+	pop	de
+	jp	unsetZ
 
-; if out of bounds, will continually return the last char
-; TODO: add bounds check and return Z accordingly.
 mmapGetC:
 	push	hl
 	call	_mmapAddr
+	jr	nz, .end
 	ld	a, (hl)
-	cp	a	; ensure Z
+	; Z already set
+.end:
 	pop	hl
 	ret
 
+
 mmapPutC:
 	push	hl
 	call	_mmapAddr
+	jr	nz, .end
 	ld	(hl), a
-	cp	a	; ensure Z
+	; Z already set
+.end:
 	pop	hl
 	ret