Update md.asm

Further optimization of loops utilizing alternate register AF'

Source/HBIOS/md.asm

@@ -1101,13 +1101,10 @@ MD_FWRIT_R:				; THIS CODE GETS RELOCATED TO HIGH MEMORY
 ;
 	LD	H,C			; SECTOR
 	LD	L,D			; ADDRESS
-;
-	PUSH	AF			; SAVE CURRENT BANK
 ;
 MD_FWRI1:
-	POP	AF			; SELECT BANK
-	PUSH	AF			; TO READ
-	CALL	HBX_BNKSEL
+	CALL	HBX_BNKSEL		; SELECT BANK TO READ
+	EX	AF,AF'			; SAVE CURRENT BANK
 ;
 	LD	C,(IX+0)		; READ IN BYTE
 ;
@@ -1133,13 +1130,14 @@ MD_FW7:	LD	A,(HL)			; FROM THE SAME FLASH ADDRESS.
 ;
 	INC	HL			; NEXT DESTINATION LOCATION
 	INC	IX			; NEXT SOURCE LOCATION
+;
+	EX	AF,AF'			; RESTORE CURRENT BANK
 ;
 	INC	DE			; CONTINUE WRITING UNTIL
 	BIT	4,D			; WE HAVE DONE ONE SECTOR
 	JR	Z,MD_FWRI1
 ;
-	POP	AF			; RETURN TO ORIGINAL BANK
-	JP	HBX_BNKSEL		; WHICH IS OUR RAM BIOS COPY
+	JP	HBX_BNKSEL		; RETURN TO ORIGINAL BANK WHICH IS OUR RAM BIOS COPY
 ;
 ;	RET
 ;