diff --git a/Source/Apps/RTC.asm b/Source/Apps/RTC.asm
index 4d91681f..2d72482b 100644
--- a/Source/Apps/RTC.asm
+++ b/Source/Apps/RTC.asm
@@ -1331,9 +1331,6 @@ DELAY_LOOP:				; LOOP IS 26TS
 	OR	L			; 4TS
 	JR	NZ,DELAY_LOOP		; 12TS
 	; RESTART SYSTEM FROM ROM BANK 0, ADDRESS $0000
-	;LD	A,BID_BOOT		; BOOT BANK
-	;LD	HL,0			; ADDRESS ZERO
-	;CALL	HB_BNKCALL		; DOES NOT RETURN
 	LD	B,BF_SYSRESET		; SYSTEM RESTART
 	LD	C,BF_SYSRES_COLD	; COLD START
 	CALL	$FFF0			; CALL HBIOS
diff --git a/Source/HBIOS/Config/RCZ280_nat.asm b/Source/HBIOS/Config/RCZ280_nat.asm
index 5c81f316..5a3a7555 100644
--- a/Source/HBIOS/Config/RCZ280_nat.asm
+++ b/Source/HBIOS/Config/RCZ280_nat.asm
@@ -32,14 +32,14 @@ CPUOSC		.SET	24000000	; CPU OSC FREQ IN MHZ
 ;
 MEMMGR		.SET	MM_Z280		; MEMORY MANAGER: MM_[SBC|Z2|N8|Z180|Z280]
 ;
-INTMODE		.SET	1
+INTMODE		.SET	3
 ;
 Z280_MEMWAIT	.SET	0		; Z280: MEMORY WAIT STATES (0-3)
 Z280_IOWAIT	.SET	1		; Z280: I/O WAIT STATES TO ADD ABOVE 1 W/S BUILT-IN (0-3)
 Z280_INTWAIT	.SET	0		; Z280: INT ACK WAIT STATUS (0-3)
 ;
 UARTENABLE	.SET	TRUE		; UART: ENABLE 8250/16550-LIKE SERIAL DRIVER (UART.ASM)
-ACIAENABLE	.SET	TRUE		; ACIA: ENABLE MOTOROLA 6850 ACIA DRIVER (ACIA.ASM)
+ACIAENABLE	.SET	FALSE		; ACIA: ENABLE MOTOROLA 6850 ACIA DRIVER (ACIA.ASM)
 SIOENABLE	.SET	TRUE		; SIO: ENABLE ZILOG SIO SERIAL DRIVER (SIO.ASM)
 ;
 IDEENABLE	.SET	TRUE		; IDE: ENABLE IDE DISK DRIVER (IDE.ASM)
diff --git a/Source/HBIOS/hbios.asm b/Source/HBIOS/hbios.asm
index 211afa24..3f90969d 100644
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -113,12 +113,12 @@ MODCNT	.SET	MODCNT + 1
 #DEFINE	HB_DI		;
 #DEFINE	HB_EI		;
 #ENDIF
-#IF ((INTMODE == 1) | (INTMODE == 2))
-; MODE 1 OR 2 INTERRUPT HANDLING
+#IF ((INTMODE == 1) | (INTMODE == 2) | (INTMODE == 3))
+; MODE 1 OR 2 OR 3 INTERRUPT HANDLING
 #DEFINE	HB_DI		DI
 #DEFINE	HB_EI		EI
 #ENDIF
-#IF (INTMODE > 2)
+#IF (INTMODE > 3)
 	.ECHO	"*** ERROR: INVALID INTMODE SETTING!!!\n"
 	!!!	; FORCE AN ASSEMBLY ERROR
 #ENDIF
@@ -377,6 +377,25 @@ HBX_INVOKE:
 	POP	HL			; RESTORE HL
 #ENDIF
 
+;#IF (MEMMGR == MM_Z280)
+#IF 0
+
+	LD	A,(HB_CURBNK)		; GET CURRENT BANK
+	LD	(HB_INVBNK),A		; SAVE INVOCATION BANK
+
+	LD	A,BID_BIOS		; HBIOS BANK
+	LD	(HB_CURBNK),A		; SET AS CURRENT BANK
+
+	.DB	$ED,$71			; SC
+	.DW	HB_DISPATCH		; SC PARAMETER
+
+	PUSH	AF			; SAVE AF (FUNCTION RETURN)
+	LD	A,(HB_INVBNK)		; LOAD ORIGINAL BANK
+	LD	(HB_CURBNK),A
+	POP	AF			; RESTORE AF
+
+#ELSE
+
 	LD	(HBX_INVSP),SP		; SAVE ORIGINAL STACK FRAME
 	LD	SP,HBX_BUF_END		; BORROW HBX_BUF FOR TEMP STACK
 
@@ -398,6 +417,8 @@ HBX_INVOKE:
 	LD	SP,0			; RESTORE ORIGINAL STACK FRAME
 HBX_INVSP	.EQU	$ - 2
 
+#ENDIF
+
 #IF (HBIOS_MUTEX == TRUE)
 	PUSH	HL			; SAVE HL
 	LD	HL,HB_LOCK		; POINT TO LOCK
@@ -496,7 +517,17 @@ HBX_BNKSEL1:
 	RET				; DONE
 #ENDIF
 #IF (MEMMGR == MM_Z280)
-	JP	Z280_BNKSEL		; DO THE REAL WORK
+	.DB	$ED,$71			; SC
+	.DW	Z280_BNKSEL		; SC PARAMETER
+	RET
+;
+Z280_SYSCALL:
+	EX	(SP),HL
+	LD	(Z280_SCADR),HL
+	POP	HL
+Z280_SCADR	.EQU	$ + 1
+	CALL	$FFFF
+	.DB	$ED,$55			; RETIL
 #ENDIF
 #IF (MEMMGR == MM_ZRC)
 	BIT	7,A			; BIT 7 SET REQUESTS RAM PAGE
@@ -524,19 +555,9 @@ HBX_ROM:
 ;::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
 ;
 HBX_BNKCPY:
-#IF (CPUFAM == CPU_Z280)
-	PUSH	IY
-	PUSH	BC
-	LD	C,Z280_MSR
-	.DB	$FD,$ED,$66		; LDCTL IY,(C)
-	POP	BC
-	PUSH	IY
-	DI
-#ELSE
 	LD	A,I
 	DI
 	PUSH	AF
-#ENDIF
 	LD	(HBX_BC_SP),SP		; PUT STACK
 	LD	SP,HBX_TMPSTK		; ... IN HI MEM
 
@@ -566,18 +587,9 @@ HBX_BC_LAST:
 
 	LD	SP,$FFFF		; RESTORE STACK
 HBX_BC_SP	.EQU	$ - 2		; ... TO ORIGINAL VALUE
-#IF (CPUFAM == CPU_Z280)
-	POP	IY
-	PUSH	BC
-	LD	C,Z280_MSR
-	.DB	$FD,$ED,$6E		; LDCTL (C),IY
-	POP	BC
-	POP	IY
-#ELSE
 	POP	AF
 	JP	PO,$+4
 	EI
-#ENDIF
 	RET
 ;
 HBX_BC_ITER:
@@ -678,7 +690,7 @@ HBX_INTSTK	.EQU	$
 	!!!	; FORCE AN ASSEMBLY ERROR
 #ENDIF
 ;
-#IF (INTMODE == 2)
+#IF ((INTMODE == 2) | (INTMODE == 3))
 ;
 ; HBIOS INTERRUPT SLOT ASSIGNMENTS
 ;
@@ -750,47 +762,83 @@ INT_IM1:
 ;
 #IF (INTMODE > 0)
 ;
-; COMMON INTERRUPT DISPATCHING CODE
-; SETUP AND CALL HANDLER IN BIOS BANK
-;
 HBX_INT:	; COMMON INTERRUPT ROUTING CODE
+;
+  #IF (MEMMGR == MM_Z280)
 ;
 	EX	(SP),HL			; SAVE HL AND GET INT JP TABLE OFFSET
+	
+	; SAVE STATE (HL SAVED PREVIOUSLY ON ORIGINAL STACK FRAME)
+	PUSH	AF			; SAVE AF
+	PUSH	BC			; SAVE BC
+	PUSH	DE			; SAVE DE
+	PUSH	IY			; SAVE IY
+;
+	; HANDLE INT VIA JP TABLE IN HBIOS
+	LD	L,(HL)			; OFFSET INTO JP TABLE FOR THIS INT
+	LD	H,HB_IVT >> 8		; MSB OF HBIOS INT JP TABLE
+	CALL	JPHL			; CALL HANDLER VIA INT JP TABLE
+;
+	; RESTORE STATE
+	POP	IY			; RESTORE IY
+	POP	DE			; RESTORE DE
+	POP	BC			; RESTORE BC
+	POP	AF			; RESTORE AF
+	POP	HL			; RESTORE HL
+;
+	; BURN THE REASON CODE
+	EX	(SP),HL			; HL TO STK, RC TO HL
+	POP	HL			; RESTORE HL
 
+	CALL	HBX_RETI		; RETI FOR Z80 PERIPHERALS
+	.DB	$ED,$55			; BACK TO USER LAND
+;
+HBX_RETI:	
+	RETI
+;
+  #ELSE
+;
+; COMMON INTERRUPT DISPATCHING CODE
+; SETUP AND CALL HANDLER IN BIOS BANK
+;
+	EX	(SP),HL			; SAVE HL AND GET INT JP TABLE OFFSET
+;
 	LD	(HBX_INT_SP),SP		; SAVE ORIGINAL STACK FRAME
 	LD	SP,HBX_INTSTK		; USE DEDICATED INT STACK FRAME IN HI MEM
-
+;
 	; SAVE STATE (HL SAVED PREVIOUSLY ON ORIGINAL STACK FRAME)
 	PUSH	AF			; SAVE AF
 	PUSH	BC			; SAVE BC
 	PUSH	DE			; SAVE DE
 	PUSH	IY			; SAVE IY
-
+;
 	LD	A,BID_BIOS		; HBIOS BANK
 	CALL	HBX_BNKSEL_INT		; SELECT IT
-
+;
 	LD	L,(HL)			; OFFSET INTO JP TABLE FOR THIS INT
 	LD	H,HB_IVT >> 8		; MSB OF HBIOS INT JP TABLE
-
+;
 	CALL	JPHL			; CALL HANDLER VIA INT JP TABLE
-
+;
 	LD	A,(HB_CURBNK)		; GET PRE-INT BANK
 	CALL	HBX_BNKSEL		; SELECT IT
-
+;
 	; RESTORE STATE
 	POP	IY			; RESTORE IY
 	POP	DE			; RESTORE DE
 	POP	BC			; RESTORE BC
 	POP	AF			; RESTORE AF
-
+;
 	LD	SP,$FFFF		; RESTORE ORIGINAL STACK FRAME
 HBX_INT_SP	.EQU	$ - 2
-
+;
 	POP	HL			; RESTORE HL
-
+;
 	HB_EI				; ENABLE INTERRUPTS
 	RETI				; AND RETURN
 ;
+  #ENDIF
+  
 #ENDIF
 ;
 ; SMALL TEMPORARY STACK FOR USE BY HBX_BNKCPY
@@ -899,7 +947,7 @@ HB_IVT:
 ; NOTE THAT EACH ENTRY HAS A FILLER BYTE OF VALUE ZERO.  THIS BYTE
 ; HAS NO FUNCTION.  IT IS JUST USED TO MAKE ENTRIES AN EVEN 4 BYTES.
 ;
-#IF (INTMODE == 2)
+#IF ((INTMODE == 2) | (INTMODE == 3))
 ;
 HB_IVT:
 HB_IVT00:	JP	HB_BADINT \ .DB 0
@@ -982,8 +1030,21 @@ HB_START:
 ;	LD	BC,Z280_BNKSELZ-Z280_BNKSEL ; BYTE COUNT TO COPY
 ;	LDIR				; COPY
 ;
-	DIAG(%11111110)
-;	JP	XZ280_BNKSELZ		; JUMP TO CODE CONTINUATION
+
+	; CONFIGURE Z280 INT/TRAP VECTOR TABLE POINTER REGISTER
+	LD	C,Z280_VPR
+	LD	HL,$0010		; TOP 16 BITS OF PHYSICAL ADR OF IVT
+	.DB	$ED,$6E			; LDCTL	(C),HL
+
+	;; TEST SYSTEM CALL
+	;.DB	$ED,$71			; SC
+	;.DW	Z280_BNKSEL		; SC PARAMETER
+	;
+	;LD	A,%10101010
+	;OUT	(DIAGPORT),A
+	;DI
+	;HALT
+
 	JR	Z280_INITZ		; JUMP TO CODE CONTINUATION
 ;
 Z280_PDRTBL:
@@ -1006,6 +1067,58 @@ Z280_PDRTBL:
 	.DW	($0FE << 4) | $A
 	.DW	($0FF << 4) | $A
 ;
+Z280_PRIVINST:
+	EX	(SP),HL			; GET MSR, SAVE HL
+	LD	(HB_MSRSAV),HL		; SAVE IT
+	POP	HL			; RECOVER HL, POP STACK
+	EX	(SP),HL			; GET ADR, SAVE HL
+	
+	PUSH	AF
+	PUSH	BC
+	PUSH	DE
+	
+	LD	A,(HL)
+	INC	HL			; BUMP PAST PRIV INST
+	
+	; HANDLE DI
+	CP	$F3			; DI?
+	JR	NZ,Z280_PRIVINST2
+	DI				; DO THE DI
+	JR	Z280_PRIVINSTX
+
+Z280_PRIVINST2:
+	; HANDLE EI
+	CP	$FB			; EI?
+	JR	NZ,Z280_PRIVINST_HALT
+	EI				; DO THE EI
+	JR	Z280_PRIVINSTX
+
+Z280_PRIVINST_HALT:
+	; SOMETHING ELSE, DIAGNOSE & HALT SYSTEM
+	
+	CALL	NEWLINE
+	LD	DE,Z280_PRIVSTR
+	CALL	WRITESTR
+	CALL	PRTHEXWORDHL
+	
+	DI
+	HALT
+
+Z280_PRIVINSTX:	
+	POP	DE
+	POP	BC
+	POP	AF
+	
+	EX	(SP),HL			; RECOVER HL, ADR TO STK
+	PUSH	HL			; SAVE HL
+	LD	HL,(HB_MSRSAV)		; GET SAVED MSR
+	EX	(SP),HL			; MSR TO STK, RECOVER HL
+	.DB	$ED,$55			; RETIL
+;
+HB_MSRSAV	.DW	0
+;
+Z280_PRIVSTR	.TEXT	"\n\n*** Privileged Instruction @$"
+;
 Z280_INITZ:
 ;
   #ENDIF 
@@ -1443,6 +1556,17 @@ HB_CPU2:
 	IM	2			; SWITCH TO INT MODE 2
 #ENDIF
 ;
+#IF (INTMODE == 3)
+;
+	; SETUP Z280 INT A FOR VECTORED INTERRUPTS
+	LD	HL,%0010000000000000
+	LD	C,Z280_ISR
+	.DB	$ED,$6E			; LDCTL (C),HL
+;
+	.DB	$ED,$4E			; IM 3
+;
+#ENDIF
+;
 #IF (PLATFORM == PLT_SBC)
 ;
   #IF (HTIMENABLE)			; SIMH TIMER
@@ -1858,6 +1982,63 @@ IS_REC_M1:
 #ENDIF
 ;
 INITSYS3:
+;
+#IF (MEMMGR == MM_Z280)
+	; LEAVE SYSTEM MODE STACK POINTING TO AN OK PLACE
+	LD	SP,HB_STACK - $40	; *** FIX ***
+	
+	HB_DI
+
+	; FIXUP BNKSEL TO WORK ON USER MODE PDRS
+	XOR	A
+	LD	(W_MMU0+1),A
+	LD	(W_MMU1+1),A
+
+	;DIAG(%10000001)
+	;DI
+	;HALT
+
+	; MAKE USER MODE BANK CORRECT
+	LD	A,(HB_CURBNK)		; GET CURRENT BANK
+	CALL	HBX_BNKSEL
+	
+	;DIAG(%10000011)
+	;DI
+	;HALT
+	
+	;LD	C,Z280_TCR
+	;.DB	$ED,$66		; LDCTL HL,(C)
+	;LD	A,L
+	;CALL	PC_SPACE
+	;CALL	PRTHEXBYTE
+	
+	LD	HL,HBX_LOC
+	.DB	$ED,$8F		; LDCTL	USP,HL
+
+	;DIAG(%10000111)
+	;DI
+	;HALT
+
+	HB_EI
+
+	; SWITCH TO USER MODE NOW
+	LD	C,Z280_MSR
+	LD	HL,$407F
+	.DB	$ED,$6E			; LDCTL	(C),HL
+	;LD	A,%10001111
+	;OUT	(DIAGPORT),A
+	
+	;DI
+	;HALT
+	LD	SP,HBX_LOC		; SETUP INITIAL STACK JUST BELOW HBIOS PROXY
+	
+	;DIAG(%10001111)
+	;CALL	LDELAY
+	
+	;DIAG(%10011111)
+	;CALL	LDELAY
+	
+#ENDIF
 ;
 	CALL	PRTSUM			; PRINT UNIT/DEVICE SUMMARY TABLE
 ;
@@ -2107,6 +2288,11 @@ IDLE:
 ;
 HB_DISPATCH:
 ;
+#IF (MEMMGR == MM_Z280)
+	HB_EI
+#ENDIF
+;
+;
 #IF 0	; *DEBUG* START
 ;
 	CALL	HB_DISPATCH1	; DO THE WORK
@@ -3541,6 +3727,46 @@ HB_TMPSZ	.DW	0
 HB_TMPREF	.DW	0
 ;
 ;==================================================================================================
+;   Z280 INTERRUPT VECTOR TABLE
+;==================================================================================================
+;
+#IF (MEMMGR = MM_Z280)
+;
+	.FILL	$1000 - ($ & $FFF)	; MUST BE 4K ALIGNED!
+;
+Z280_IVT:
+	.DW	0, 0			; RESERVED
+	.DW	0			; NMI MSR
+	.DW	0			; NMI VECTOR
+	.DW	$0000			; INT A MSR
+	.DW	0			; INT A VECTOR
+	.FILL	$50 - ($ - Z280_IVT),0	; SKIP TO SYS CALL TRAP VECTOR
+	.DW	$0000			; SYS CALL, NEW MSR
+	.DW	Z280_SYSCALL		; SYS CALL, VECTOR ADR
+	.DW	$0000			; PRIV INST TRAP, NEW MSR
+	.DW	Z280_PRIVINST		; PRIV INST, VECTOR ADR
+	.FILL	$70 - ($ - Z280_IVT),0	; SKIP TO START OF NMI/INTA VECTORS
+	.DW	HBX_IV00
+	.DW	HBX_IV01
+	.DW	HBX_IV02
+	.DW	HBX_IV03
+	.DW	HBX_IV04
+	.DW	HBX_IV05
+	.DW	HBX_IV06
+	.DW	HBX_IV07
+	.DW	HBX_IV08
+	.DW	HBX_IV09
+	.DW	HBX_IV0A
+	.DW	HBX_IV0B
+	.DW	HBX_IV0C
+	.DW	HBX_IV0D
+	.DW	HBX_IV0E
+	.DW	HBX_IV0F
+	.FILL	$370 - ($ - Z280_IVT),0	; FILL REMAINDER OF Z280 IVT
+;
+#ENDIF
+;
+;==================================================================================================
 ;   DEVICE DRIVERS
 ;==================================================================================================
 ;
diff --git a/Source/HBIOS/romldr.asm b/Source/HBIOS/romldr.asm
index 38ff1a29..17783fa4 100644
--- a/Source/HBIOS/romldr.asm
+++ b/Source/HBIOS/romldr.asm
@@ -571,13 +571,9 @@ reboot:
 	call	DSKY_SHOWSEG		; display message
 #endif
 ;
-	;; switch to rom bank 0 and jump to address 0
-	;ld	a,BID_BOOT		; boot bank
-	;ld	ix,0			; address zero
-	;call	HB_BNKCALL		; does not return
 	; cold boot system
-	LD	B,BF_SYSRESET		; SYSTEM RESTART
-	LD	C,BF_SYSRES_COLD	; COLD START
+	ld	b,BF_SYSRESET		; system restart
+	ld	c,BF_SYSRES_COLD	; cold start
 	rst	08			; do it, no return
 #endif
 ;
diff --git a/Source/HBIOS/sio.asm b/Source/HBIOS/sio.asm
index a7c41062..aeb12f08 100644
--- a/Source/HBIOS/sio.asm
+++ b/Source/HBIOS/sio.asm
@@ -30,7 +30,7 @@ SIO_WR1VAL	.EQU	$00		; WR1 VALUE FOR NO INTS
 SIO_WR1VAL	.EQU	$18		; WR1 VALUE FOR INT ON RECEIVED CHARS
 #ENDIF
 ;
-#IF (INTMODE == 2)
+#IF ((INTMODE == 2) | (INTMODE == 3))
 ;
 SIO0_IVT	.EQU	IVT(INT_SIO0)
 SIO1_IVT	.EQU	IVT(INT_SIO1)
@@ -144,7 +144,7 @@ SIO_PREINIT2:
 	CALL	HB_ADDIM1		; ADD TO IM1 CALL LIST
   #ENDIF
 ;
-  #IF (INTMODE == 2)
+  #IF ((INTMODE == 2) | (INTMODE == 3))
 	; SETUP IM2 VECTORS
 	LD	HL,SIO_INT0
 	LD	(SIO0_IVT),HL		; IVT INDEX
@@ -831,7 +831,7 @@ SIO_INITGO:
 ;
 ; SET INTERRUPT VECTOR OFFSET WR2
 ;
-#IF (INTMODE == 2)
+#IF ((INTMODE == 2) | (INTMODE == 3))
 	LD	A,(IY+2)		; CHIP / CHANNEL
 	SRL	A			; SHIFT AWAY CHANNEL BIT
 	LD	L,SIO0_VEC		; ASSUME CHIP 0
diff --git a/Source/HBIOS/std.asm b/Source/HBIOS/std.asm
index 4643ce7b..b95339bc 100644
--- a/Source/HBIOS/std.asm
+++ b/Source/HBIOS/std.asm
@@ -501,9 +501,9 @@ MON_SERIAL	.EQU	MON_LOC + (1 * 3)	; MONITOR ENTRY (SERIAL PORT)
 ;
 ; INTERRUPT MODE 2 SLOT ASSIGNMENTS
 ;
-#IF (INTMODE == 2)
+#IF ((INTMODE == 2) | (INTMODE == 3))
 
-  #IF (CPUFAM == CPU_Z180)
+  #IF ((CPUFAM == CPU_Z180) | (CPUFAM == CPU_Z280))
 
 ; Z180-BASED SYSTEMS