Implement "safe mode" boot process

Lots of ways this could be done. I have chosen to set it up so it just boots with a minimum driver set.

On SBC test  by setting BT_REC_TYPE to BT_REC_SBC02 and use the 1-bit jumper (crtact) jumper to enable.

Options for other platforms needs to be considered.

Source/HBIOS/hbios.asm

@@ -911,6 +911,38 @@ HB_START1:				; BNKCALL ARRIVES HERE, BUT NOW RUNNING IN RAM BANK
 	LD	(HL),$4D
   #ENDIF
 #ENDIF
+;
+;==================================================================================================
+;	RECOVERY MODE
+;==================================================================================================
+;
+;	PLATFORM SPECIFIC CODE FOR DETECTING RECOVERY MODE SWITCH
+;
+#IF	(BT_REC_TYPE!=BT_REC_NONE)
+ #IF (BT_REC_TYPE==BT_REC_FORCE)
+	LD	A,1			; SET FOR RECOVERY MODE
+	LD	(HB_BOOT_REC),A		; SAVE FOR LATER
+ #ENDIF
+ #IF (PLATFORM == PLT_SBC)
+  #IF (BT_REC_TYPE==BT_REC_SBC01)
+ 	LD	A,%00100000		; DISABLE RTC AND
+	OUT	(RTCIO),A		; DRQ DRIVER READ
+	IN	A,(RTCIO)		; BIT 0 (DRQ). 
+	CPL				; PULLED HIGH
+	AND	1			; IS RECOVERY MODE
+	LD	(HB_BOOT_REC),A		; SAVE FOR LATER
+  #ENDIF
+  #IF (BT_REC_TYPE==BT_REC_SBC02)
+	IN	A,(RTCIO)		; RTC PORT, BIT 6 HAS THE
+	BIT	6,A			; STATE OF CONFIG JUMPER
+	LD	A,1			; JUMPER INSTALLED 
+	JR	Z,SAVE_REC_M		; IS RECOVERY MODE
+	LD	A,0
+SAVE_REC_M:
+	LD	(HB_BOOT_REC),A		; SAVE FOR LATER
+  #ENDIF
+ #ENDIF
+#ENDIF
 ;
 	DIAG(%00001111)
 ;
@@ -1156,7 +1188,17 @@ HB_CPU2:
 	LD	(HL),D
 	LD	B,PC_INITTBLLEN
 	POP	DE								; POP (1) DE IS ADDRESS OF TOP OF TABLE
-	CALL	CALLLIST		; PROCESS THE PRE-INIT CALL TABLE
+
+#IF	(BT_REC_TYPE!=BT_REC_NONE)
+	LD	A,(HB_BOOT_REC)		; IF WE ARE IN RECOVERY MODE
+	OR	A			; POINT TO THE RECOVER MODE
+	JR	Z,NOT_REC_M0		; INITIALIZATION TABLE
+	LD	B,PC_INITRLEN
+	LD	DE,PC_INIT_REC
+NOT_REC_M0:
+
+#ENDIF
+	CALL	CALLLIST							; PROCESS THE PRE-INIT CALL TABLE
 ;
 #IF 0
 ;
@@ -1329,8 +1371,19 @@ HB_SPDTST:
 ; PERFORM DEVICE INITIALIZATION
 ;
 	CALL	NEWLINE
+
+#IF	(BT_REC_TYPE!=BT_REC_NONE)
+	LD	A,(HB_BOOT_REC)		; IF WE ARE IN RECOVERY MODE
+	OR	A			; POINT TO THE RECOVER MODE
+	JR	Z,NOT_REC_M1		; INITIALIZATION TABLE
+	LD	B,HB_INITRLEN
+	LD	DE,HB_INIT_REC
+	JR	IS_REC_M1	
+NOT_REC_M1:
+#ENDIF
 	LD	B,HB_INITTBLLEN
 	LD	DE,HB_INITTBL
+IS_REC_M1:
 	CALL	CALLLIST
 ;
 ; RECORD HEAP CURB AT THE CURRENT VALUE OF HEAP TOP.  HEAP CURB
@@ -1431,8 +1484,31 @@ CALLLIST:
 	DJNZ	CALLLIST
 CALLDUMMY:
 	RET
+
+#IF	(BT_REC_TYPE!=BT_REC_NONE)
 ;
 ;==================================================================================================
+;   TABLE OF RECOVERY MODE INITIALIZATION ENTRY POINTS
+;==================================================================================================
+;
+; USE "CALLDUMMY" IF NO ENTRY REQUIRED
+;
+PC_INIT_REC:
+#IF	(PLATFORM == PLT_SBC)
+	.DW	UART_PREINIT
+;	.DW	CALLDUMMY
+#ENDIF
+PC_INITRLEN	.EQU	(($ - PC_INIT_REC) / 2)
+;
+HB_INIT_REC:
+#IF	(PLATFORM == PLT_SBC)
+	.DW	UART_INIT
+	.DW	MD_INIT
+#ENDIF
+HB_INITRLEN	.EQU	(($ - HB_INIT_REC) / 2)
+;
+#ENDIF
+;==================================================================================================
 ;   TABLE OF PRE-CONSOLE INITIALIZATION ENTRY POINTS
 ;==================================================================================================
 
@@ -3926,6 +4002,10 @@ RTCVAL		.DB	0		; SHADOW VALUE FOR RTC LATCH PORT
 ;
 HB_BATCOND	.DB	0		; BATTERY CONDITION (0=LOW, 1=OK)
 ;
+#IF	(BT_REC_TYPE!=BT_REC_NONE)
+HB_BOOT_REC	.DB	0		; BOOT MODE (0=NORMAL, 1=RECOVERY MODE)
+#ENDIF
+;
 STR_BANNER	.DB	"RetroBrew HBIOS v", BIOSVER, ", ", TIMESTAMP, "$"
 STR_PLATFORM	.DB	PLATFORM_NAME, "$"
 STR_SWITCH	.DB	"*** Activating CRT Console ***$"

Source/HBIOS/std.asm

@@ -23,7 +23,7 @@
 FALSE		.EQU 	0
 TRUE		.EQU 	~FALSE
 ;
-; DEBUGGNG OPTIONS
+; DEBUGGING OPTIONS
 ;
 USENONE		.EQU	0		; NO DEBUG
 USEXIO		.EQU	1		; BASIC SERIAL DRIVER
@@ -74,6 +74,15 @@ MM_Z180		.EQU	4		; Z180 NATIVE MEMORY MANAGER
 BT_MENU		.EQU	1		; WAIT FOR MENU SELECTION AT LOADER PROMPT
 BT_AUTO		.EQU	2		; AUTO SELECT BOOT_DEFAULT AFTER BOOT_TIMEOUT
 ;
+; BOOT RECOVERY METHODS
+;
+BT_REC_NONE	.EQU	0		; NO RECOVERY MODE
+BT_REC_FORCE	.EQU	1		; FORCE BOOT RECOVERY MODE
+BT_REC_SBC01	.EQU	2		; ECB-SBCV2 - BIT 1 RTC HIGH
+BT_REC_SBC02	.EQU	3		; ECB-SBCV2 - 1-BIT IO PORT
+;
+BT_REC_TYPE	.EQU	BT_REC_NONE	; BOOT RECOVERY METHOD TO USE
+;
 ; FLOPPY DISK MEDIA SELECTIONS (ID'S MUST BE INDEX OF ENTRY IN FCD_TBL)
 ;
 FDM720		.EQU	0		; 3.5" FLOPPY, 720KB, 2 SIDES, 80 TRKS, 9 SECTORS