diff --git a/arch/z80/include/z80/irq.h b/arch/z80/include/z80/irq.h
index a187fd4c34..12dd502c12 100644
--- a/arch/z80/include/z80/irq.h
+++ b/arch/z80/include/z80/irq.h
@@ -51,7 +51,7 @@
 
 /* Z80 Interrupts */
 
-#define Z80_IRQ_SYSTIMER (1)
+#define Z80_IRQ_SYSTIMER (0)
 #define NR_IRQS          (1)
 
 /* IRQ Stack Frame Format
diff --git a/arch/z80/src/common/up_head.asm b/arch/z80/src/common/up_head.asm
index 36a145be7f..fd3bf20a10 100644
--- a/arch/z80/src/common/up_head.asm
+++ b/arch/z80/src/common/up_head.asm
@@ -111,15 +111,22 @@ forever:
 	push	af			; Offset 0: I with interrupt state in carry
 	di
 
-	 ; Call the interrupt decode logic. SP points to the beggining of the reg structure
+	; Call the interrupt decode logic. SP points to the beggining of the reg structure
+
 	ld	hl, #0			; Argument is the beginning of the reg structure
 	add	hl, sp			;
-	push	hl			;
+	push	hl			; Place argument at the top of thest
 	call	_up_decodeirq		; Decode the IRQ
 
+	; On return, HL points to the beginning of the reg structure to restore
+	; Note that (1) the argument pushed on the stack is not popped, and (2) the
+	; original stack pointer is lost.  In the normal case (no context switch),
+	; HL will contain the value of the SP before the argument was pushed.
+
+	ld	sp, hl			; Use the new stack pointer
+
 	; Restore registers.  HL points to the beginning of the reg structure to restore
 
-	ld	sp, hl			; Use the temp stack pointer
 	ex	af, af'			; Select alternate AF
 	pop	af			; Offset 0: AF' = I with interrupt state in carry
 	pop	bc			; Offset 1: BC
diff --git a/arch/z80/src/common/up_irq.c b/arch/z80/src/common/up_irq.c
index 5f8897a416..ef7c8c1c61 100644
--- a/arch/z80/src/common/up_irq.c
+++ b/arch/z80/src/common/up_irq.c
@@ -92,13 +92,14 @@ irqstate_t irqsave(void) __naked
 void irqrestore(irqstate_t flags) __naked
 {
   _asm
+	di			; Assume disabled
 	pop	hl		; HL = return address
 	pop	af		; AF Carry bit hold interrupt state
 	jr	nc, statedisable
 	ei
-	ret
 statedisable:
-	di
-	ret
+	push	af		; Restore stack
+	push	hl		;
+	ret			; and return
   _endasm;
 }
diff --git a/arch/z80/src/z80/chip.h b/arch/z80/src/z80/chip.h
index 9f85f1475c..d5c36bd19f 100644
--- a/arch/z80/src/z80/chip.h
+++ b/arch/z80/src/z80/chip.h
@@ -45,6 +45,15 @@
  * Definitions
  ************************************************************************************/
 
+/* Bits in the Z80 FLAGS register */
+
+#define Z80_C_FLAG      0x01       /* Bit 0: Carry flag */
+#define Z80_N_FLAG      0x02       /* Bit 1: Add/Subtract flag  */
+#define Z80_PV_FLAG     0x04       /* Bit 2: Parity/Overflow flag */
+#define Z80_H_FLAG      0x10       /* Bit 4: Half carry flag */
+#define Z80_Z_FLAG      0x40       /* Bit 5: Zero flag */
+#define Z80_S_FLAG      0x80       /* Bit 7: Sign flag */
+
 /************************************************************************************
  * Public Function Prototypes
  ************************************************************************************/