diff --git a/c64/kernal/membot.s b/c64/kernal/membot.s
index affd36ce..476eefab 100644
--- a/c64/kernal/membot.s
+++ b/c64/kernal/membot.s
@@ -1,7 +1,14 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
 membot:
 	; p287 C64 Programmers reference guide
-	; For now, we only read it, not allow setting it
-	LDX #<$0800
-	LDY #>$0800
+	;;  http://unusedino.de/ec64/technical/project64/mapping_c64.html
+	bcc membot_set
+	
+	LDX MEMSTR+0
+	LDY MEMSTR+1
+	rts
+membot_set:
+	stx MEMSTR+0
+	stx MEMSTR+1
+	
 	rts
diff --git a/c64/kernal/memtop.s b/c64/kernal/memtop.s
index 9ea705ef..548ba1a9 100644
--- a/c64/kernal/memtop.s
+++ b/c64/kernal/memtop.s
@@ -2,6 +2,14 @@
 memtop:
 	; p288 C64 Programmer's reference guide
 	; but for now we only support reading it
-	LDX #<$9FFF
-	LDY #>$9FFF
+	;;  http://unusedino.de/ec64/technical/project64/mapping_c64.html
+	bcc memtop_set
+	
+	LDX MEMSIZ+0
+	LDY MEMSIZ+1
+	rts
+memtop_set:
+	stx MEMSIZ+0
+	stx MEMSIZ+1
+	
 	rts
diff --git a/c64/kernal/ramtas.s b/c64/kernal/ramtas.s
index 452907a8..c9fa503f 100644
--- a/c64/kernal/ramtas.s
+++ b/c64/kernal/ramtas.s
@@ -45,6 +45,9 @@ ramtas_l2:
 	
 	;;  Work out RAM size and put in MEMSTR and MEMSIZ
 	;; "Compute's Mapping the 64", p54
+	;;  http://unusedino.de/ec64/technical/project64/mapping_c64.html
+	lda #>$0800
+	sta MEMSTR+1
 	lda #$00
 	sta MEMSTR+0
 	sta MEMSIZ+0