diff --git a/src/cpu.rs b/src/cpu.rs
index f2fcf46..b203a1e 100644
--- a/src/cpu.rs
+++ b/src/cpu.rs
@@ -279,7 +279,7 @@ impl M68kMachine {
             dbg_crash_on: crate::envcfg::flag("COPPERLINE_DIAG_CRASH"),
             dbg_crash_dumped: false,
             dbg: crate::debugger::Debugger::from_env(),
-            ui_breaks: crate::debugger::InteractiveBreaks::default(),
+            ui_breaks: crate::debugger::InteractiveBreaks::new(address_mask_for_model(cpu_model)),
             ui_stop: None,
             ui_last_this_task: None,
             ui_pc_history: [0; UI_PC_HISTORY_CAP],
@@ -1241,6 +1241,13 @@ impl M68kMachine {
         &self.ui_breaks
     }
 
+    /// The CPU model's address-bus mask, for debugger surfaces that
+    /// normalize or display CPU addresses (A0-A23 on 24-bit models,
+    /// full 32 bits on 020+).
+    pub fn ui_addr_mask(&self) -> u32 {
+        self.cpu.address_mask
+    }
+
     /// Toggle a PC breakpoint carrying an optional condition and ignore count.
     /// Returns true when the breakpoint is now set.
     pub fn ui_set_breakpoint(
@@ -1277,7 +1284,7 @@ impl M68kMachine {
         addr: u32,
         filter: Option<crate::debugger::WatchSource>,
     ) -> bool {
-        let addr = addr & crate::debugger::UI_ADDR_MASK & !1;
+        let addr = addr & self.cpu.address_mask & !1;
         let current = self.bus.bus.peek_word_any(addr);
         let added = self.ui_breaks.toggle_watch(addr, current, filter);
         let addrs: Vec<u32> = self.ui_breaks.watches.iter().map(|w| w.addr).collect();
@@ -1443,7 +1450,7 @@ impl M68kMachine {
     /// is the instruction that just retired.
     fn ui_check_breaks_after_step(&mut self) {
         use crate::debugger::DebugStop;
-        let pc = self.cpu.pc & crate::debugger::UI_ADDR_MASK;
+        let pc = self.cpu.pc & self.cpu.address_mask;
         // Exception catchpoints: the core records every exception entry
         // (trap, fault, or interrupt) as it loads the handler vector;
         // drain it here so a hit stops at the handler's first
@@ -1467,7 +1474,7 @@ impl M68kMachine {
         if self.ui_stop.is_some() {
             return;
         }
-        let writer_pc = self.cpu.ppc & crate::debugger::UI_ADDR_MASK;
+        let writer_pc = self.cpu.ppc & self.cpu.address_mask;
         for i in 0..self.ui_breaks.watches.len() {
             let addr = self.ui_breaks.watches[i].addr;
             let new = self.bus.bus.peek_word_any(addr);
@@ -1548,7 +1555,7 @@ impl M68kMachine {
                 // a caught vector; stop before the handler's first
                 // instruction executes.
                 if self.ui_breaks.armed() {
-                    let pc = self.cpu.pc & crate::debugger::UI_ADDR_MASK;
+                    let pc = self.cpu.pc & self.cpu.address_mask;
                     if let Some(vector) = self.cpu.last_exception_vector.take() {
                         let vector = vector.min(u32::from(u16::MAX)) as u16;
                         if self.ui_breaks.catches.contains(&vector) {
diff --git a/src/debugger.rs b/src/debugger.rs
index 6ff15c6..1d0a7fc 100644
--- a/src/debugger.rs
+++ b/src/debugger.rs
@@ -61,8 +61,9 @@ pub struct Watch {
     pub len: u32,
 }
 
-/// The 68000 address-bus width the interactive debugger compares PCs and
-/// watch addresses through (A0-A23).
+/// The 68000/EC020 address-bus mask (A0-A23). Debugger surfaces compare
+/// and display through the machine's model mask (`ui_addr_mask()`); this
+/// constant remains for tests and 24-bit callers.
 pub const UI_ADDR_MASK: u32 = 0x00FF_FFFF;
 
 /// An interactive memory watchpoint: a 16-bit word and the value it held
@@ -687,8 +688,11 @@ pub struct Breakpoint {
 /// The debugger window's breakpoint/watchpoint set. Owned by the CPU
 /// machine so it stays armed while the window is closed; `armed` is the
 /// single per-instruction gate the hot loop checks.
-#[derive(Default)]
 pub struct InteractiveBreaks {
+    /// Address-bus mask breakpoint/watch addresses and PCs are compared
+    /// through: A0-A23 on 24-bit models, full 32 bits on 020+ (set from
+    /// the CPU model at machine construction).
+    pub addr_mask: u32,
     pub breakpoints: Vec<Breakpoint>,
     pub watches: Vec<UiWatch>,
     /// Watched custom-register word offsets into $DFF000 ($000-$1FE).
@@ -706,6 +710,20 @@ pub struct InteractiveBreaks {
 }
 
 impl InteractiveBreaks {
+    /// An empty break set comparing addresses through `addr_mask` (the
+    /// owning machine's address-bus mask; see `address_mask_for_model`).
+    pub fn new(addr_mask: u32) -> Self {
+        Self {
+            addr_mask,
+            breakpoints: Vec::new(),
+            watches: Vec::new(),
+            reg_watches: Vec::new(),
+            catches: Vec::new(),
+            task_catch: None,
+            armed: false,
+        }
+    }
+
     pub fn armed(&self) -> bool {
         self.armed
     }
@@ -721,7 +739,7 @@ impl InteractiveBreaks {
     /// Whether any breakpoint is set at `pc`, ignoring its condition. Used for
     /// display (marking the address) and the reverse-debug scan.
     pub fn is_breakpoint(&self, pc: u32) -> bool {
-        let pc = pc & UI_ADDR_MASK;
+        let pc = pc & self.addr_mask;
         self.breakpoints.iter().any(|bp| bp.addr == pc)
     }
 
@@ -734,7 +752,7 @@ impl InteractiveBreaks {
         cond: Option<BreakCond>,
         ignore: u32,
     ) -> bool {
-        let addr = addr & UI_ADDR_MASK;
+        let addr = addr & self.addr_mask;
         let added = match self.breakpoints.iter().position(|bp| bp.addr == addr) {
             Some(pos) => {
                 self.breakpoints.remove(pos);
@@ -759,7 +777,7 @@ impl InteractiveBreaks {
     /// ignore count has been exhausted -- each qualifying hit before that is
     /// counted and skipped.
     pub fn breakpoint_stops(&mut self, pc: u32, ctx: &dyn BreakContext) -> bool {
-        let pc = pc & UI_ADDR_MASK;
+        let pc = pc & self.addr_mask;
         let Some(bp) = self.breakpoints.iter_mut().find(|bp| bp.addr == pc) else {
             return false;
         };
@@ -1118,7 +1136,7 @@ mod tests {
 
     #[test]
     fn interactive_breakpoints_toggle_mask_and_arm() {
-        let mut breaks = InteractiveBreaks::default();
+        let mut breaks = InteractiveBreaks::new(UI_ADDR_MASK);
         assert!(!breaks.armed());
 
         // Adding masks the address to the 68000 bus width.
@@ -1132,6 +1150,16 @@ mod tests {
         assert!(!breaks.is_breakpoint(0x00C0_33C2));
     }
 
+    #[test]
+    fn full_mask_keeps_z3_breakpoints_distinct_from_chip_aliases() {
+        // On a 32-bit CPU a Zorro III breakpoint must not fire at the
+        // chip-RAM address it would alias through a 24-bit mask.
+        let mut breaks = InteractiveBreaks::new(0xFFFF_FFFF);
+        assert!(breaks.toggle_breakpoint_full(0x4000_1000, None, 0));
+        assert!(breaks.is_breakpoint(0x4000_1000));
+        assert!(!breaks.is_breakpoint(0x0000_1000));
+    }
+
     /// Fixed register/memory snapshot for exercising condition evaluation.
     #[derive(Default)]
     struct FakeCtx {
@@ -1162,7 +1190,7 @@ mod tests {
 
     #[test]
     fn conditional_breakpoint_stops_only_when_condition_holds() {
-        let mut breaks = InteractiveBreaks::default();
+        let mut breaks = InteractiveBreaks::new(UI_ADDR_MASK);
         breaks.toggle_breakpoint_full(
             0x1000,
             Some(BreakCond {
@@ -1186,7 +1214,7 @@ mod tests {
 
     #[test]
     fn ignore_count_skips_the_first_qualifying_hits() {
-        let mut breaks = InteractiveBreaks::default();
+        let mut breaks = InteractiveBreaks::new(UI_ADDR_MASK);
         // Stop on the 4th qualifying hit (ignore the first 3).
         breaks.toggle_breakpoint_full(0x1000, None, 3);
         let ctx = FakeCtx::default();
@@ -1200,7 +1228,7 @@ mod tests {
 
     #[test]
     fn bit_test_condition_uses_memory_word() {
-        let mut breaks = InteractiveBreaks::default();
+        let mut breaks = InteractiveBreaks::new(UI_ADDR_MASK);
         breaks.toggle_breakpoint_full(
             0x40,
             Some(BreakCond {
@@ -1219,7 +1247,7 @@ mod tests {
 
     #[test]
     fn interactive_watches_record_baselines_and_clear() {
-        let mut breaks = InteractiveBreaks::default();
+        let mut breaks = InteractiveBreaks::new(UI_ADDR_MASK);
         assert!(breaks.toggle_watch(0x1000, 0xABCD, None));
         assert_eq!(breaks.watches[0].last, 0xABCD);
         // The register watch normalizes a full $DFFxxx address to the
diff --git a/src/gdbstub.rs b/src/gdbstub.rs
index 1c2e9ae..23fe005 100644
--- a/src/gdbstub.rs
+++ b/src/gdbstub.rs
@@ -7,7 +7,7 @@
 //! memory-mapped devices; Amiga custom-chip state is exposed through `monitor`
 //! commands so inspection remains side-effect-free.
 
-use crate::debugger::{custom_reg_name, UI_ADDR_MASK};
+use crate::debugger::custom_reg_name;
 use crate::emulator::Emulator;
 use crate::timetravel::ReverseOutcome;
 use anyhow::{anyhow, bail, Context, Result};
@@ -391,7 +391,7 @@ impl Session {
 
     fn add_breakpoint(&mut self, packet: &str) -> Result<String> {
         let (addr, _) = parse_z_packet(packet)?;
-        let addr = addr & UI_ADDR_MASK;
+        let addr = addr & self.emu.machine.ui_addr_mask();
         if !self.breakpoints.contains(&addr) {
             self.breakpoints.push(addr);
         }
@@ -400,7 +400,7 @@ impl Session {
 
     fn remove_breakpoint(&mut self, packet: &str) -> Result<String> {
         let (addr, _) = parse_z_packet(packet)?;
-        let addr = addr & UI_ADDR_MASK;
+        let addr = addr & self.emu.machine.ui_addr_mask();
         self.breakpoints.retain(|&candidate| candidate != addr);
         Ok("OK".to_string())
     }
@@ -485,7 +485,7 @@ impl Session {
         if self.emu.bus_mut().take_ui_copper_hit().is_some() {
             return Ok(Some(StopReason::CopperBreak));
         }
-        let pc = self.emu.machine.pc() & UI_ADDR_MASK;
+        let pc = self.emu.machine.pc() & self.emu.machine.ui_addr_mask();
         if self.breakpoints.contains(&pc) {
             return Ok(Some(StopReason::Breakpoint));
         }
diff --git a/src/video/window.rs b/src/video/window.rs
index 6cdafc2..7a0f8ba 100644
--- a/src/video/window.rs
+++ b/src/video/window.rs
@@ -3206,7 +3206,7 @@ impl App {
             let mut panel = ui::DebuggerPanel::new();
             // Start the memory view at the current program counter's
             // neighbourhood; it is usually what you came to look at.
-            panel.mem_addr = self.emu.machine.pc() & 0x00FF_FFF0;
+            panel.mem_addr = self.emu.machine.pc() & self.emu.machine.ui_addr_mask() & !0xF;
             self.debugger_panel = Some(panel);
             self.emu.machine.ui_set_pc_history_enabled(true);
             // Arm reverse debugging so the < Step / < Run controls work. A
@@ -4453,7 +4453,7 @@ impl App {
                 self.reported_double_fault = true;
                 let message = format!(
                     "CPU halted: double fault at pc ${:06X} (bus/address error during exception)",
-                    self.emu.machine.pc() & 0x00FF_FFFF
+                    self.emu.machine.pc() & self.emu.machine.ui_addr_mask()
                 );
                 warn!("{message}");
                 self.last_debug_stop = Some(message.clone());
@@ -4498,7 +4498,7 @@ impl App {
         let set = self.emu.machine.ui_set_breakpoint(addr, cond, ignore);
         let mut msg = format!(
             "Breakpoint ${:06X} {}",
-            addr & 0x00FF_FFFF,
+            addr & self.emu.machine.ui_addr_mask(),
             if set { "set" } else { "removed" }
         );
         if set {
@@ -4517,7 +4517,7 @@ impl App {
         let Some(addr) = self.debugger_entry_addr("Watch") else {
             return;
         };
-        let addr = addr & 0x00FF_FFFE;
+        let addr = addr & self.emu.machine.ui_addr_mask() & !1;
         let set = self.emu.machine.ui_toggle_watch(addr);
         self.show_osd(format!(
             "Watchpoint ${addr:06X} {}",
@@ -4564,7 +4564,7 @@ impl App {
                     panel.mem_addr.wrapping_sub(delta)
                 } else {
                     panel.mem_addr.wrapping_add(delta)
-                } & 0x00FF_FFFF;
+                } & self.emu.machine.ui_addr_mask();
                 if !panel.mem_view_bits {
                     panel.mem_addr &= !0xF;
                 }
@@ -4587,7 +4587,7 @@ impl App {
                     panel.mem_addr.wrapping_sub(delta)
                 } else {
                     panel.mem_addr.wrapping_add(delta)
-                } & 0x00FF_FFFF;
+                } & self.emu.machine.ui_addr_mask();
                 self.request_redraw();
             }
         }
@@ -4608,7 +4608,7 @@ impl App {
             .mem_last_find
             .map(|addr| addr.wrapping_add(1))
             .unwrap_or(panel.mem_addr)
-            & 0x00FF_FFFF;
+            & self.emu.machine.ui_addr_mask();
         const SPACE: u64 = 0x0100_0000;
         const CHUNK: usize = 4096;
         let mut offset = 0u64;
@@ -4689,7 +4689,7 @@ impl App {
             self.show_osd("Writer: type a hex address first");
             return;
         };
-        let addr = addr & 0x00FF_FFFE;
+        let addr = addr & self.emu.machine.ui_addr_mask() & !1;
         let before = self.emu.retired_instructions();
         match self.emu.tt_last_writer(addr, before) {
             Ok(ReverseOutcome::Found(rec)) => {
@@ -4698,7 +4698,7 @@ impl App {
                     rec.addr,
                     rec.old,
                     rec.new,
-                    rec.pc & 0x00FF_FFFF,
+                    rec.pc & self.emu.machine.ui_addr_mask(),
                     rec.frame
                 );
                 info!("last-writer {message}");
@@ -5367,7 +5367,7 @@ impl App {
                 if panel.mem_view_bits {
                     let stride = panel.mem_bitmap_stride.max(1) as usize;
                     let rows = ui::mem_bitmap_rows();
-                    let base = panel.mem_addr & 0x00FF_FFFF;
+                    let base = panel.mem_addr & machine.ui_addr_mask();
                     lines.push(ui::DbgLine::plain(format!(
                         "bitplane at ${base:06X}, stride {stride} bytes ({} px), {rows} rows",
                         stride * 8
@@ -5383,9 +5383,9 @@ impl App {
                         "$ box: jump / \"ADDR VALUE\" poke / \"ADDR LEN\" save / hex bytes find",
                     ));
                     lines.push(ui::DbgLine::plain(""));
-                    let base = panel.mem_addr & 0x00FF_FFF0;
+                    let base = panel.mem_addr & machine.ui_addr_mask() & !0xF;
                     for row in 0..16u32 {
-                        let addr = base.wrapping_add(row * 16) & 0x00FF_FFFF;
+                        let addr = base.wrapping_add(row * 16) & machine.ui_addr_mask();
                         let mut bytes = [0u8; 16];
                         for word in 0..8u32 {
                             let value = bus.peek_word_any(addr.wrapping_add(word * 2));
diff --git a/src/video/window/console.rs b/src/video/window/console.rs
index 9f5e899..77bcfad 100644
--- a/src/video/window/console.rs
+++ b/src/video/window/console.rs
@@ -378,7 +378,7 @@ impl App {
                 let set = self.emu.machine.ui_set_breakpoint(addr, cond, ignore);
                 ConsoleOutcome::one(format!(
                     "breakpoint ${:06X} {}",
-                    addr & 0x00FF_FFFF,
+                    addr & self.emu.machine.ui_addr_mask(),
                     if set { "set" } else { "removed" }
                 ))
             }
@@ -398,7 +398,7 @@ impl App {
                 let set = self.emu.machine.ui_toggle_watch_filtered(addr, filter);
                 ConsoleOutcome::one(format!(
                     "watchpoint ${:06X}{} {}",
-                    addr & 0x00FF_FFFE,
+                    addr & self.emu.machine.ui_addr_mask() & !1,
                     filter
                         .map(|f| format!(" ({} writes only)", f.label()))
                         .unwrap_or_default(),
@@ -482,7 +482,10 @@ impl App {
                     // exec's Alert() lives at LVO -108; the jump-table
                     // entry itself executes, so a PC breakpoint there
                     // fires on every alert with D7 = the guru code.
-                    vec![format!("{:06X}", base.wrapping_sub(108) & 0x00FF_FFFF)]
+                    vec![format!(
+                        "{:06X}",
+                        base.wrapping_sub(108) & self.emu.machine.ui_addr_mask()
+                    )]
                 });
                 let Some(addr) = lvo
                     .first()
@@ -805,7 +808,7 @@ impl App {
                 let Some(addr) = args.first().and_then(|t| hex32(t)) else {
                     return ConsoleOutcome::error("usage: WRITER ADDR (hex, word)");
                 };
-                let addr = addr & 0x00FF_FFFE;
+                let addr = addr & self.emu.machine.ui_addr_mask() & !1;
                 let before = self.emu.retired_instructions();
                 let outcome = match self.emu.tt_last_writer(addr, before) {
                     Ok(crate::timetravel::ReverseOutcome::Found(rec)) => {
@@ -814,7 +817,7 @@ impl App {
                             rec.addr,
                             rec.old,
                             rec.new,
-                            rec.pc & 0x00FF_FFFF,
+                            rec.pc & self.emu.machine.ui_addr_mask(),
                             rec.frame
                         ))
                     }
@@ -1284,8 +1287,8 @@ impl App {
         let cpu_type = machine.cpu_type();
         let mut lines = vec![format!(
             "#0 pc ${:06X}  sp ${:06X}",
-            machine.pc() & 0x00FF_FFFF,
-            sp & 0x00FF_FFFF
+            machine.pc() & machine.ui_addr_mask(),
+            sp & machine.ui_addr_mask()
         )];
         let mut frame = 1usize;
         for slot in 0..SLOTS {
@@ -1293,7 +1296,7 @@ impl App {
                 break;
             }
             let slot_addr = sp.wrapping_add(slot * 4);
-            let value = peek32(slot_addr) & 0x00FF_FFFF;
+            let value = peek32(slot_addr) & machine.ui_addr_mask();
             if !looks_like_return(value) {
                 continue;
             }