Z80 NMOS Int Status Bug, Issue #620

Add a config setting to enable code that works around the Z80 interrupt status (LD A,I) bug. Currently enabled only for MSX platform.
2026-02-06 14:11:48 -06:00 · 2025-10-10 11:18:18 -07:00
parent 6684a3e039
commit f164ffdc76
28 changed files with 83 additions and 30 deletions
--- a/Source/HBIOS/cfg_DUO.asm
+++ b/Source/HBIOS/cfg_DUO.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_DUO		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_DYNO.asm
+++ b/Source/HBIOS/cfg_DYNO.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_DYNO	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_EPITX.asm
+++ b/Source/HBIOS/cfg_EPITX.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_EPITX	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_EZZ80.asm
+++ b/Source/HBIOS/cfg_EZZ80.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_EZZ80	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_GMZ180.asm
+++ b/Source/HBIOS/cfg_GMZ180.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_GMZ180	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_HEATH.asm
+++ b/Source/HBIOS/cfg_HEATH.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_HEATH	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_MASTER.asm
+++ b/Source/HBIOS/cfg_MASTER.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.EQU	PLT_NONE	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.EQU	CPU_NONE	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.EQU	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.EQU	BIOS_NONE	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.EQU	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.EQU	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_MBC.asm
+++ b/Source/HBIOS/cfg_MBC.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_MBC		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_MK4.asm
+++ b/Source/HBIOS/cfg_MK4.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_MK4		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_MON.asm
+++ b/Source/HBIOS/cfg_MON.asm
@@ -53,6 +53,7 @@
 ;
 PLATFORM	.SET	PLT_MON		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_MSX.asm
+++ b/Source/HBIOS/cfg_MSX.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_MSX		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|FZ80|MSX]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	TRUE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_N8.asm
+++ b/Source/HBIOS/cfg_N8.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_N8		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_NABU.asm
+++ b/Source/HBIOS/cfg_NABU.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_NABU	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_RCEZ80.asm
+++ b/Source/HBIOS/cfg_RCEZ80.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_RCEZ80	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_EZ80	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_RCZ180.asm
+++ b/Source/HBIOS/cfg_RCZ180.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_RCZ180	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_RCZ280.asm
+++ b/Source/HBIOS/cfg_RCZ280.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_RCZ280	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z280	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_RCZ80.asm
+++ b/Source/HBIOS/cfg_RCZ80.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_RCZ80	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_RPH.asm
+++ b/Source/HBIOS/cfg_RPH.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_RPH		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	TRUE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_S100.asm
+++ b/Source/HBIOS/cfg_S100.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_S100	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_SBC.asm
+++ b/Source/HBIOS/cfg_SBC.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_SBC		; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_SCZ180.asm
+++ b/Source/HBIOS/cfg_SCZ180.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_SCZ180	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z180	; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_SZ80.asm
+++ b/Source/HBIOS/cfg_SZ80.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_SZ80	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_Z80RETRO.asm
+++ b/Source/HBIOS/cfg_Z80RETRO.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_Z80RETRO	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_ZETA.asm
+++ b/Source/HBIOS/cfg_ZETA.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_ZETA	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/cfg_ZETA2.asm
+++ b/Source/HBIOS/cfg_ZETA2.asm
@@ -51,6 +51,7 @@
 ;
 PLATFORM	.SET	PLT_ZETA2	; PLT_[SBC|ZETA|ZETA2|N8|MK4|UNA|RCZ80|RCEZ80|RCZ180|EZZ80|SCZ180|GMZ180|DYNO|RCZ280|MBC|RPH|Z80RETRO|S100|DUO|HEATH|EPITX|MON|STDZ180|NABU|SZ80]
 CPUFAM		.SET	CPU_Z80		; CPU FAMILY: CPU_[Z80|Z180|Z280|EZ80]
+NMOSCPU		.SET	FALSE		; NMOS CPU (ENABLES INT STATUS BUG WORKAROUND)
 BIOS		.SET	BIOS_WBW	; HARDWARE BIOS: BIOS_[WBW|UNA]
 BATCOND		.SET	FALSE		; ENABLE LOW BATTERY WARNING MESSAGE
 HBIOS_MUTEX	.SET	FALSE		; ENABLE REENTRANT CALLS TO HBIOS (ADDS OVERHEAD)
--- a/Source/HBIOS/hbios.asm
+++ b/Source/HBIOS/hbios.asm
@@ -256,6 +256,46 @@ SUPCTS	.EQU	FALSE		; SUPPRESS CTS DURING HBIOS BOOT
  #ENDIF
 #ENDIF
 ;
+; PUSH/POP INTERRUPT STATE
+; AF IS DESTROYED IN ALL CASES
+;
+; IF NMOSCPU IS SET, THE INT STATUS BUG WORKAROUND IS USED
+; NMOS Z80 BUG PREVENTS USE OF "LD A,I" TO GATHER IFF2 INTO P/V FLAG
+; SEE http://www.z80.info/zip/ZilogProductSpecsDatabook129-143.pdf
+;
+#IF NMOSCPU
+  #DEFINE HB_PUSHINT \
+  #DEFCONT \	XOR	A
+  #DEFCONT \	PUSH	AF
+  #DEFCONT \	POP	AF
+  #DEFCONT \	LD	A,I
+  #DEFCONT \	JP	PE,$+10
+  #DEFCONT \	DEC	SP
+  #DEFCONT \	DEC	SP
+  #DEFCONT \	POP	AF
+  #DEFCONT \	AND	A
+  #DEFCONT \	JR	NZ,$+3
+  #DEFCONT \	SCF
+  #DEFCONT \	PUSH	AF
+#ELSE
+  #DEFINE HB_PUSHINT \
+  #DEFCONT \	LD	A,I
+  #DEFCONT \	HB_DI
+  #DEFCONT \	PUSH	AF
+#ENDIF
+
+#IF NMOSCPU
+  #DEFINE HB_POPINT \
+  #DEFCONT \	POP	AF
+  #DEFCONT \	JR	NC,$+3
+  #DEFCONT \	EI
+#ELSE
+  #DEFINE HB_POPINT \
+  #DEFCONT \	POP	AF
+  #DEFCONT \	JP	PO,$+4
+  #DEFCONT \	EI
+#ENDIF
+;
 ; CONSISTENCY CHECKS
 ;
 #IF (INTMODE > 3)
@@ -774,9 +814,7 @@ MM_EZ512_BANK0TO7:
 ;
  #IF (INTMODE == 1)
 	LD	(HBX_MMA),A		; SAVE ACCUM
-	LD	A,I			; GET INT CTL REG
-	HB_DI				; DISABLE INTS
-	PUSH	AF			; SAVE INT CTL REG
+	HB_PUSHINT
 	LD	A,(HBX_MMA)		; RESTORE ACCUM
  #ENDIF
 ;
@@ -795,9 +833,7 @@ HBX_ROM:
 HBX_RAMX:
 ;
  #IF (INTMODE == 1)
-	POP	AF			; RESTORE INT CTL REG
-	JP	PO,$+4			; WERE INTS DISABLED AT ENTRY?
-	EI				; *** DO NOT USE HB_EI HERE ***
+	HB_POPINT
 	LD	A,(HBX_MMA)		; RESTORE INCOMING ACCUM
  #ENDIF
 ;
@@ -1009,7 +1045,8 @@ HBX_BNKCPY:
 IOPRSAV	.DW	0			; TEMP STORAGE FOR IOPR
 ;
 #ELSE
-  #IF (CPUFAM == CPU_Z280)
+  #IF (INTMODE == 1)
+    #IF (CPUFAM == CPU_Z280)
 	PUSH	HL
 	PUSH	BC
 	LD	C,Z280_MSR
@@ -1017,10 +1054,9 @@ IOPRSAV	.DW	0			; TEMP STORAGE FOR IOPR
 	POP	BC
 	EX	(SP),HL
 	HB_DI
-  #ELSE
-	LD	A,I
-	HB_DI
-	PUSH	AF
+    #ELSE
+	HB_PUSHINT
+    #ENDIF
  #ENDIF
 	LD	(HBX_BC_SP),SP		; PUT STACK
 	LD	SP,HBX_TMPSTK		; ... IN HI MEM
@@ -1051,17 +1087,17 @@ HBX_BC_LAST:

 	LD	SP,$FFFF		; RESTORE STACK
 HBX_BC_SP	.EQU	$ - 2		; ... TO ORIGINAL VALUE
-  #IF (CPUFAM == CPU_Z280)
+  #IF (INTMODE == 1)
+    #IF (CPUFAM == CPU_Z280)
 	EX	(SP),HL			; SAVE HL, RECOVER MSR
 	PUSH	BC			; SAVE BC
 	LD	C,Z280_MSR
 	LDCTL	(C),HL
 	POP	BC			; RECOVER BC
 	POP	HL			; RECOVER HL
-  #ELSE
-	POP	AF
-	JP	PO,$+4
-	EI				; *** DO NOT USE HB_EI HERE ***
+    #ELSE
+	HB_POPINT
+    #ENDIF
  #ENDIF
 	RET
 ;
@@ -6631,9 +6667,7 @@ SYS_PEEK:
 	PUSH	IY
 	HB_DI
  #ELSE
-	LD	A,I			; SAVE THE INTERRUPT STATUS
-	DI         			;  COPY IFF2 TO P/V FLAG
-	PUSH	AF
+	HB_PUSHINT
  #ENDIF
 #ENDIF
 	CALL	HBX_PEEK		; IMPLEMENTED IN PROXY
@@ -6644,9 +6678,7 @@ SYS_PEEK:
 	LDCTL	(C),IY
 	POP	IY
  #ELSE
-	POP	AF			; RECALL INITIAL INTERRUPT STATUS
-	JP	PO,$+4			; RETURN TO INITIAL STATE
-	EI				; *** DO NOT USE HB_EI HERE ***
+	HB_POPINT
  #ENDIF
 #ENDIF
 	XOR	A
@@ -6672,9 +6704,7 @@ SYS_POKE:
 	PUSH	IY
 	HB_DI
  #ELSE
-	LD	A,I			; SAVE THE INTERRUPT STATUS
-	HB_DI      			;  COPY IFF2 TO P/V FLAG
-	PUSH	AF
+	HB_PUSHINT
  #ENDIF
 #ENDIF
 	CALL	HBX_POKE		; IMPLEMENTED IN PROXY
@@ -6685,9 +6715,7 @@ SYS_POKE:
 	LDCTL	(C),IY
 	POP	IY
  #ELSE
-	POP	AF			; RECALL INITIAL INTERRUPT STATUS
-	JP	PO,$+4			; RETURN TO INITIAL STATE
-	EI				; *** DO NOT USE HB_EI HERE ***
+	HB_POPINT
  #ENDIF
 #ENDIF
 	XOR	A