Update md.asm

Source/HBIOS/md.asm

@@ -938,7 +938,7 @@ MD_I_SZ	.EQU	$-MD_FIDENT_R		; SIZE OF RELOCATABLE CODE BUFFER REQUIRED
 ;
 MD_FERAS_R:				; THIS CODE GETS RELOCATED TO HIGH MEMORY
 ;
-	PUSH	AF			; SAVE CURRENT BANK
+	EX	AF,AF'			; SAVE CURRENT BANK
 	LD	A,B			; SELECT BANK
 	CALL	HBX_BNKSEL		; TO PROGRAM
 ;
@@ -979,7 +979,7 @@ MD_WT4:	LD	A,(HL)			; DO TWO SUCCESSIVE READS
 	JR	MD_WT6
 MD_WT5:	LD	C,L			; SET SUCCESS STATUS
 ;
-MD_WT6:	POP	AF			; RETURN TO ORIGINAL BANK
+MD_WT6:	EX	AF,AF'			; RETURN TO ORIGINAL BANK
 	JP	HBX_BNKSEL		; WHICH IS OUR RAM BIOS COPY
 ;
 ;	RET