diff --git a/vcs.l65 b/vcs.l65
index 61e38a6..fb8dd8f 100644
--- a/vcs.l65
+++ b/vcs.l65
@@ -207,58 +207,58 @@ end
 
 mappers = {}
 
-mappers['2K'] = function(irq)
+mappers['2K'] = function()
     rom0 = location(0xf800, 0xffff)
-    section{"vectors", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 mappers.CV = mappers['2K']
 
-mappers['4K'] = function(irq)
+mappers['4K'] = function()
     rom0 = location(0xf000, 0xffff)
-    section{"vectors", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 
-local bank_stubs = function(irq, count, hotspot, entry)
-    function switchrom(i) assert(i>=0 and i<count) bit 0x1000+hotspot+i end
+local bank_stubs = function(count, hotspot, entry)
+    function switchrom(i) assert(i>=0 and i<count) if mappers.noillegal then bit 0x1000+hotspot+i else nop 0x1000+hotspot+i end end
     local base = 0x10000 - (count << 12)
     for bi=0,count-1 do
         local o = base+(bi<<12)
         _ENV['rom' .. bi] = location{o, o+0xfff, rorg=0xf000, rom=bi}
         local start=section{"entry"..bi, org=o+(entry or hotspot-6)} switchrom(0) if bi==0 then jmp main end
         section{"switchtab"..bi, org=o+hotspot} for i=1,count do byte(0) end
-        section{"vectors"..bi, org=o+0xffc} dc.w start if irq then dc.w start end
+        section{"vectors"..bi, org=o+0xffc} dc.w start if mappers.irq then dc.w start end
     end
 end
-mappers.FA = function(irq) bank_stubs(irq, 3, 0xff8) end
-mappers.F8 = function(irq) bank_stubs(irq, 2, 0xff8) end
-mappers.F6 = function(irq) bank_stubs(irq, 4, 0xff6) end
-mappers.F4 = function(irq) bank_stubs(irq, 8, 0xff4) end
-mappers.EF = function(irq) bank_stubs(irq, 16, 0xfe0, 0xffc-6) end
+mappers.FA = function() bank_stubs(3, 0xff8) end
+mappers.F8 = function() bank_stubs(2, 0xff8) end
+mappers.F6 = function() bank_stubs(4, 0xff6) end
+mappers.F4 = function() bank_stubs(8, 0xff4) end
+mappers.EF = function() bank_stubs(16, 0xfe0, 0xffc-6) end
 
-mappers.FE = function(irq)
+mappers.FE = function()
     rom1 = location(0xd000, 0xdfff)
-    section{"vectors1", org=0xdffc} dc.w main if irq then dc.w main end
+    section{"vectors1", org=0xdffc} dc.w main if mappers.irq then dc.w main end
     location{0xe000, 0xefff, nofill=true}
     rom0 = location(0xf000, 0xffff)
-    section{"vectors0", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors0", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 
-mappers.E0 = function(map, irq)
-    function switchrom0(i) assert(map[i]==0) bit 0x1fe0+i end
-    function switchrom1(i) assert(map[i]==1) bit 0x1fe8+i end
-    function switchrom2(i) assert(map[i]==2) bit 0x1ff0+i end
+mappers.E0 = function(map)
+    function switchrom0(i) assert(map[i]==0) if mappers.noillegal then bit 0x1fe0+i else nop 0x1fe0+i end end
+    function switchrom1(i) assert(map[i]==1) if mappers.noillegal then bit 0x1fe8+i else nop 0x1fe8+i end end
+    function switchrom2(i) assert(map[i]==2) if mappers.noillegal then bit 0x1ff0+i else nop 0x1ff0+i end end
     map[7]=3
     for bi=0,7 do
         local o = bi*0x400
         _ENV["rom"..bi] = location{0xe000+o, 0xe3ff+o, rorg=0xf000+map[bi]*0x400}
     end
     section{"switchtab", org=0xffe0} for i=1,8*3 do byte(0) end
-    section{"vectors", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 
 -- rom0 refers to the last one in ROM, ie the one always mapped to 0xF800-0xFFFF,
 -- such that changing the rom count does not change its index
-mappers['3F'] = function(count, irq)
+mappers['3F'] = function(count)
     function switchrom(i) assert(i>=0 and i<count-1) lda#i sta 0x3f end
     local symbols=cpu.symbols for k,v in pairs(vcs) do -- remap TIA to 0x40-0x7F
         if v<0x40 then v=v+0x40 vcs[k]=v symbols[k]=v end
@@ -269,23 +269,23 @@ mappers['3F'] = function(count, irq)
     end
     local o = 0x800 * (count-1) + 0x10000
     rom0 = location{o, 0x7ff+o, rorg=0xf800}
-    section{"vectors", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 
-mappers.E7 = function(irq)
-    function switchrom(i) assert(i>=0 and i<7) bit 0x1fe0+i end
-    function enableram() bit 0x1fe7 end
-    function switchram(i) assert(i>=0 and i<4) bit 0x1fe8+i end
+mappers.E7 = function()
+    function switchrom(i) assert(i>=0 and i<7) if mappers.noillegal then bit 0x1fe0+i else nop 0x1fe0+i end end
+    function enableram() if mappers.noillegal then bit 0x1fe7 else nop 0x1fe7 end end
+    function switchram(i) assert(i>=0 and i<4) if mappers.noillegal then bit 0x1fe8+i else nop 0x1fe8+i end end
     for bi=0,6 do
         local o = bi*0x800
         _ENV['rom' .. bi] = location{o+0xc000, o+0xc7ff, rorg=0xf000}
     end
     rom7 = location{0xf800, 0xffff, rorg=0xf800}
     section{"switchtab", org=0xffe0} for i=1,12 do byte(0) end
-    section{"vectors", org=0xfffc} dc.w main if irq then dc.w main end
+    section{"vectors", org=0xfffc} dc.w main if mappers.irq then dc.w main end
 end
 
-mappers.F0 = function(irq)
+mappers.F0 = function()
     function switchrom() lda 0x1ff0 end
     for bi=0,15 do
         local o = bi*0x1000 + 0x1000
@@ -294,14 +294,14 @@ mappers.F0 = function(irq)
             local l=label() switchrom() bne l
             if bi==0 then jmp main end
         section{"switchtab"..bi, org=o+0xff0} byte(bi)
-        section{"vectors"..bi, org=o+0xffc} dc.w start if irq then dc.w start end
+        section{"vectors"..bi, org=o+0xffc} dc.w start if mappers.irq then dc.w start end
     end
 end
 
-local bank_stubs2 = function(irq, hotspot0, hotspot1)
+local bank_stubs2 = function(hotspot0, hotspot1)
     function switchrom(i)
-        if i==0 then bit hotspot0
-        elseif i==1 then bit hotspot1
+        if i==0 then if mappers.noillegal then bit hotspot0 else nop hotspot0 end
+        elseif i==1 then if mappers.noillegal then bit hotspot1 else nop hotspot1 end
         else error("invalid rom index: " .. i) end
     end
     local base = 0xe000
@@ -309,29 +309,29 @@ local bank_stubs2 = function(irq, hotspot0, hotspot1)
         local o = base+(bi<<12)
         _ENV['rom' .. bi] = location{o, o+0xfff, rorg=0xf000}
         local start=section{"entry"..bi, org=o+0xffc-6} switchrom(0) if bi==0 then jmp main end
-        section{"vectors"..bi, org=o+0xffc} dc.w start if irq then dc.w start end
+        section{"vectors"..bi, org=o+0xffc} dc.w start if mappers.irq then dc.w start end
     end
 end
-mappers.UA = function(irq) bank_stubs2(irq, 0x220, 0x240) end
-mappers['0840'] = function(irq) bank_stubs2(irq, 0x800, 0x840) end
-mappers.SB = function(count, irq)
-    function switchrom(i) assert(i>=0 and i<count) bit 0x800+i end
+mappers.UA = function() bank_stubs2(0x220, 0x240) end
+mappers['0840'] = function() bank_stubs2(0x800, 0x840) end
+mappers.SB = function(count)
+    function switchrom(i) assert(i>=0 and i<count) if mappers.noillegal then bit 0x800+i else nop 0x800+i end end
     for bi=0,count-1 do
         local o = bi*0x1000 + 0x1000
         _ENV['rom' .. bi] = location{o, o+0xfff, rorg=0xf000}
         local start=section{"entry"..bi, org=o+0xffc-6} switchrom(0) if bi==0 then jmp main end
-        section{"vectors"..bi, org=o+0xffc} dc.w start if irq then dc.w start end
+        section{"vectors"..bi, org=o+0xffc} dc.w start if mappers.irq then dc.w start end
     end
 end
 
-mappers['3E'] = function(rom_count, ram_count, irq)
-    mappers['3F'](rom_count, irq)
+mappers['3E'] = function(rom_count, ram_count)
+    mappers['3F'](rom_count)
     function switchram(i) assert(i>=0 and i<ram_count-1) lda#i sta 0x3E end
 end
 
 -- rom0 refers to the last 1k in ROM, which is mapped automatically on reset,
 -- such that changing the rom count does not change its index
-mappers.MC = function(rom_count, ram_count, irq)
+mappers.MC = function(rom_count, ram_count)
     for i=0,3 do
         _ENV['switchrom'..i] = function(i) assert(i>=0 and i<128) lda#i sta 0x3c+i end
         _ENV['switchram'..i] = function(i) assert(i>=0 and i<128) lda#i+0x80 sta 0x3c+i end
@@ -341,17 +341,17 @@ mappers.MC = function(rom_count, ram_count, irq)
         _ENV["rom"..(count-1-bi)] = location{o, 0x3ff+o, rorg=0xf000+(o&0xfff)}
     end
     local start=section("entry") switchrom3(0) jmp main
-    section{"vectors", org=0xfffc} dc.w start if irq then dc.w start end
+    section{"vectors", org=0xfffc} dc.w start if mappers.irq then dc.w start end
 end
 
-mappers.X07 = function(irq)
-    function switchrom(i) assert(i>=0 and i<16) bit 0x80b+(i<<4) end
+mappers.X07 = function()
+    function switchrom(i) assert(i>=0 and i<16) if mappers.noillegal then bit 0x80b+(i<<4) else nop 0x80b+(i<<4) end end
     -- map TIA also to 0x40-0x7F: use VSYNC for bank 14, and VSYNC2 for bank 15
     vcs.VSYNC2=0x40 cpu.symbols.VSYNC2=0x40
     for bi=0,count-1 do
         local o = bi*0x1000 + 0x1000
         _ENV['rom' .. bi] = location{o, o+0xfff, rorg=0xf000}
         local start=section{"entry"..bi, org=o+0xffc-6} switchrom(0) if bi==0 then jmp main end
-        section{"vectors"..bi, org=o+0xffc} dc.w start if irq then dc.w start end
+        section{"vectors"..bi, org=o+0xffc} dc.w start if mappers.irq then dc.w start end
     end
 end