diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index f45890aa..228f7007 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -515,18 +515,17 @@ HBX_ROM:
         JP      P,HBX_ROM               ; BIT 7 INDICATES RAM
         OUT     (MPCL_ROM),A            ; ENSURE ROM PAGE OUT OF MEMORY BEFORE SWITCH
 ;
-	PUSH	HL			; FORCE TOP 32K
-	LD	HL,(HCB + HCB_RAMBANKS)	; TO BE IN FIRST CHIP
-	SRL	L
-	XOR	L			; MAYBE PRECALCULATE THIS
-	POP	HL			; AT INITIALIZATION
+
+HBX_MMMSK .EQU	$+1			; FORCE TOP 32K		; MASK POPULATED
+	XOR	%00000000		; TO BE IN FIRST CHIP	; DURING INITIALIZATION
+
 ;
 ;#IF (RAMSIZE == 256)
-;	XOR	%00000100		; TOP 32K IS ALWAYS IN FIRST CHIP	; 256K = 8 banks = 00001000
+;	XOR	%00000100		; TOP 32K IS ALWAYS IN FIRST CHIP
 ;#ENDIF
 ;
 ;#IF (RAMSIZE == 1024)
-;	XOR	%00010000		; TOP 32K IS ALWAYS IN FIRST CHIP	; 1024K = 32 banks = 00100000
+;	XOR	%00010000		; TOP 32K IS ALWAYS IN FIRST CHIP
 ;#ENDIF
 ;
         OUT     (MPCL_RAM),A            ; SET RAM PAGE SELECTOR
@@ -534,7 +533,7 @@ HBX_ROM:
 HBX_ROM:
         OUT     (MPCL_RAM),A            ; ENSURE RAM PAGE OUT OF MEMORY BEFORE SWITCH
         OUT     (MPCL_ROM),A            ; SET ROM PAGE SELECTOR
-
+;
 HBX_RAMX:
 ;
   #IF (INTMODE == 1)
@@ -1315,6 +1314,18 @@ Z280_INITZ:
 	LD	BC,HBX_SIZ
 	LDIR
 ;
+;	RUNTIME MEMORY SIZE ADJUSTMENT
+;
+#IF (MEMMGR == MM_MBC)
+	LD	HL,HCB + HCB_RAMBANKS	; IN NUMBER OF RAMBANKS DETECTED FOR MBC
+	LD	A,%11101011		; IS 4 (128KB) OR 16 (512KB) THEN
+	AND	(HL)			; ZERO THE LAST BANK MASK OTHERWISE
+	JR	Z,MB128512		; CALCULATE THE LAST BANK MASK (BANKS/2)
+	RRA				; 256K = %00000100, 1024K = %00010000
+MB128512:
+	LD	(HBX_MMMSK),A		
+#ENDIF
+;
 ; IF APPBOOT, RESTORE CURRENT BANK ID
 ;
 #IFDEF APPBOOT