HBIOS Proxy Temp Stack Enhancement

Reuse the bounce buffer area as the temporary stack space required briefly in HBX_INVOKE when transitioning banks.  Increases size of temporary stack space to 64 bytes.

Source/HBIOS/hbios.asm

@@ -268,14 +268,16 @@ HBX_INVOKE:
 	LD	A,(HB_CURBNK)		; GET CURRENT BANK
 	LD	(HB_INVBNK),A		; SAVE INVOCATION BANK
 
-	LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM FOR BANK SWITCH
+	;LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM FOR BANK SWITCH
+	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 	LD	A,BID_BIOS		; HBIOS BANK
 	CALL	HBX_BNKSEL		; SELECT IT
 	LD	SP,HB_STACK		; NOW USE FULL HBIOS STACK IN HBIOS BANK
 
 	CALL	HB_DISPATCH		; CALL HBIOS FUNCTION DISPATCHER
 
-	LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM FOR BANK SWITCH
+	;LD	SP,HBX_TMPSTK		; USE SMALL TEMP STACK FRAME IN HI MEM FOR BANK SWITCH
+	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 	PUSH	AF			; SAVE AF (FUNCTION RETURN)
 
 	LD	A,(HB_INVBNK)		; LOAD ORIGINAL BANK
@@ -636,6 +638,7 @@ HBX_INTFILL	.EQU	(HBX_XFC - HBX_BUFSIZ - $)
 ; INTERBANK COPY BUFFER (64 BYTES)
 ;
 HBX_BUF		.FILL	HBX_BUFSIZ,0
+HBX_BUF_END	.EQU	$
 ;
 ; HBIOS PROXY MGMT BLOCK (TOP 32 BYTES)
 ;