diff --git a/313index-bounds-check.subx b/313index-bounds-check.subx
new file mode 100644
index 00000000..1349e3d4
--- /dev/null
+++ b/313index-bounds-check.subx
@@ -0,0 +1,84 @@
+# Helper to check an array's bounds, and to abort if they're violated.
+# Really only intended to be called from code generated by mu.subx.
+
+== code
+
+__check-mu-array-bounds:  # index: int, elem-size: int, arr-size: int, function-name: (addr array byte), array-name: (addr array byte)
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    # . not bothering saving ebx; it's only clobbered if we're going to abort
+    # ecx = arr-size
+    8b/-> *(ebp+0x10) 1/r32/ecx
+    # var overflow/edx: int = 0
+    ba/copy-to-edx 0/imm32
+    # var offset/eax: int = index * elem-size
+    8b/-> *(ebp+8) 0/r32/eax
+    f7 4/subop/multiply-eax-with *(ebp+0xc)
+    # check for overflow
+    81 7/subop/compare %edx 0/imm32
+    0f 85/jump-if-!= __check-mu-array-bounds:overflow/disp32
+    # check bounds
+    39/compare %eax 1/r32/ecx
+    0f 82/jump-if-unsigned< $__check-mu-array-bounds:end/disp32  # negative index should always abort
+    # abort if necessary
+    (write-buffered Stderr "fn ")
+    (write-buffered Stderr *(ebp+0x14))
+    (write-buffered Stderr ": offset ")
+    (write-int32-hex-buffered Stderr %eax)
+    (write-buffered Stderr " is too large for array '")
+    (write-buffered Stderr *(ebp+0x18))
+    (write-buffered Stderr "'\n")
+    (flush Stderr)
+    # exit(1)
+    bb/copy-to-ebx 1/imm32
+    e8/call syscall_exit/disp32
+$__check-mu-array-bounds:end:
+    # . restore registers
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
+__check-mu-array-bounds:overflow:
+    # "fn " function-name ": offset to array '" array-name "' overflowed 32 bits\n"
+    (write-buffered Stderr "fn ")
+    (write-buffered Stderr *(ebp+0x14))
+    (write-buffered Stderr ": offset to array '")
+    (write-buffered Stderr *(ebp+0x18))
+    (write-buffered Stderr "' overflowed 32 bits\n")
+    (flush Stderr)
+    # exit(1)
+    bb/copy-to-ebx 1/imm32
+    e8/call syscall_exit/disp32
+
+# potential alternative
+
+#? __bounds-check:  # msg: (addr array byte)
+#?   (write-buffered Stderr "abort: array bounds exceeded in fn ")
+#?   8b/-> *(esp+4) 0/r32/eax  # we're going to abort, so just clobber away
+#?   (write-buffered Stderr %eax)
+#?   (write-buffered Stderr Newline)
+#?   # exit(1)
+#?   bb/copy-to-ebx 1/imm32
+#?   e8/call syscall_exit/disp32
+
+# to be called as follows:
+#   var/reg <- index arr/rega: (addr array T), idx/regi: int
+#     | if size-of(T) is 1, 2, 4 or 8
+#         => # temporarily save array size to reg to check bounds
+#            "8b/-> *" rega " " reg "/r32"
+#            "c1/shift 5/subop/right %" reg " " log2(size-of(T)) "/imm32"
+#            "3b/compare " reg "/r32 *" rega
+#            "68/push \"" function "\"/imm32"  # pass function name to error message
+#            "0f 8d/jump-if->= __bounds_check/disp32"
+#            "81 0/subop/add %esp 4/imm32"  # drop function name
+#            # actually save the index addr in reg
+#            "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"
diff --git a/apps/mu b/apps/mu
index 62632aa7..4437dab7 100755
Binary files a/apps/mu and b/apps/mu differ
diff --git a/apps/mu.subx b/apps/mu.subx
index 139e04fa..3ceb5f29 100644
--- a/apps/mu.subx
+++ b/apps/mu.subx
@@ -5997,15 +5997,16 @@ test-convert-index-into-array:
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 0/imm32"                  "F - test-convert-index-into-array/7")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array/8")
     (check-next-stream-line-equal _test-output-stream "    b9/copy-to-ecx 3/imm32"                  "F - test-convert-index-into-array/9")
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx<<0x00000002 + 4) 0x00000000/r32"  "F - test-convert-index-into-array/10")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array/12")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array/13")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array/14")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array/15")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array/16")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array/17")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array/18")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 0x00000004 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array/10")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx<<0x00000002 + 4) 0x00000000/r32"  "F - test-convert-index-into-array/11")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array/13")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array/14")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array/15")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array/16")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array/17")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array/18")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array/19")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6046,15 +6047,16 @@ test-convert-index-into-array-of-bytes:
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 0/imm32"                  "F - test-convert-index-into-array-of-bytes/7")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array-of-bytes/8")
     (check-next-stream-line-equal _test-output-stream "    b9/copy-to-ecx 3/imm32"                  "F - test-convert-index-into-array-of-bytes/9")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 0x00000001 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-of-bytes/10")
     (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx<<0x00000000 + 4) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes/13")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes/14")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes/15")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes/16")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes/17")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes/18")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes/19")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes/20")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes/13")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes/14")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes/15")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes/16")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes/17")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes/18")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes/19")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6092,15 +6094,16 @@ test-convert-index-into-array-with-literal:
     (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:loop:"                       "F - test-convert-index-into-array-with-literal/5")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %eax"                    "F - test-convert-index-into-array-with-literal/6")
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 0/imm32"                  "F - test-convert-index-into-array-with-literal/7")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds 2 0x00000004 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-with-literal/8")
                                                                                  # 2 * 4 bytes/elem + 4 bytes for size = offset 12
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + 0x0000000c) 0x00000000/r32"  "F - test-convert-index-into-array-with-literal/8")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-with-literal/9")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-with-literal/10")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-with-literal/11")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-with-literal/12")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-with-literal/13")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-with-literal/14")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-with-literal/15")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + 0x0000000c) 0x00000000/r32"  "F - test-convert-index-into-array-with-literal/9")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-with-literal/10")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-with-literal/11")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-with-literal/12")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-with-literal/13")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-with-literal/14")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-with-literal/15")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-with-literal/16")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6138,6 +6141,7 @@ test-convert-index-into-array-of-bytes-with-literal:
     (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:loop:"                       "F - test-convert-index-into-array-of-bytes-with-literal/5")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %eax"                    "F - test-convert-index-into-array-of-bytes-with-literal/6")
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 0/imm32"                  "F - test-convert-index-into-array-of-bytes-with-literal/7")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds 2 0x00000001 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-of-bytes-with-literal/8")
                                                                                  # 2 * 1 byte/elem + 4 bytes for size = offset 6
     (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + 0x00000006) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-with-literal/8")
     (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-with-literal/9")
@@ -6189,19 +6193,20 @@ test-convert-index-into-array-on-stack:
     # var idx
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %eax"                    "F - test-convert-index-into-array-on-stack/8")
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 2/imm32"                  "F - test-convert-index-into-array-on-stack/9")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %eax 0x00000004 *(ebp+0xfffffff0) \"foo\" \"arr\")"  "F - test-convert-index-into-array-on-stack/10")
     # var x is at (ebp-0x10) + idx<<2 + 4 = ebp + idx<<2 - 0xc
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + eax<<0x00000002 + 0xfffffff4) 0x00000000/r32"  "F - test-convert-index-into-array-on-stack/10")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + eax<<0x00000002 + 0xfffffff4) 0x00000000/r32"  "F - test-convert-index-into-array-on-stack/11")
     # reclaim idx
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-on-stack/11")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-on-stack/12")
     # reclaim arr
-    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000010/imm32"    "F - test-convert-index-into-array-on-stack/12")
+    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000010/imm32"    "F - test-convert-index-into-array-on-stack/13")
     #
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-on-stack/13")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-on-stack/14")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-on-stack/15")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-on-stack/16")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-on-stack/17")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-on-stack/18")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-on-stack/14")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-on-stack/15")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-on-stack/16")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-on-stack/17")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-on-stack/18")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-on-stack/19")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6242,19 +6247,20 @@ test-convert-index-into-array-on-stack-with-literal:
     (check-next-stream-line-equal _test-output-stream "    68/push 0x0000000c/imm32"                "F - test-convert-index-into-array-on-stack-with-literal/7")
     # var x
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %eax"                    "F - test-convert-index-into-array-on-stack-with-literal/8")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds 2 0x00000004 *(ebp+0xfffffff0) \"foo\" \"arr\")"  "F - test-convert-index-into-array-on-stack-with-literal/9")
     # x is at (ebp-0x10) + 4 + 2*4 = ebp-4
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + 0xfffffffc) 0x00000000/r32"  "F - test-convert-index-into-array-on-stack-with-literal/9")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + 0xfffffffc) 0x00000000/r32"  "F - test-convert-index-into-array-on-stack-with-literal/10")
     # reclaim x
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-on-stack-with-literal/10")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-on-stack-with-literal/11")
     # reclaim arr
-    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000010/imm32"    "F - test-convert-index-into-array-on-stack-with-literal/11")
+    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000010/imm32"    "F - test-convert-index-into-array-on-stack-with-literal/12")
     #
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-on-stack-with-literal/12")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-on-stack-with-literal/13")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-on-stack-with-literal/14")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-on-stack-with-literal/15")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-on-stack-with-literal/16")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-on-stack-with-literal/17")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-on-stack-with-literal/13")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-on-stack-with-literal/14")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-on-stack-with-literal/15")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-on-stack-with-literal/16")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-on-stack-with-literal/17")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-on-stack-with-literal/18")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6295,19 +6301,20 @@ test-convert-index-into-array-of-bytes-on-stack-with-literal:
     (check-next-stream-line-equal _test-output-stream "    68/push 0x00000003/imm32"                "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/7")
     # var x
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %eax"                    "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/8")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds 2 0x00000001 *(ebp+0xfffffff9) \"foo\" \"arr\")"  "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/9")
     # x is at (ebp-7) + 4 + 2 = ebp-1
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + 0xffffffff) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/9")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(ebp + 0xffffffff) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/10")
     # reclaim x
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/10")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/11")
     # reclaim arr
-    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000007/imm32"    "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/11")
+    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000007/imm32"    "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/12")
     #
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/12")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/13")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/14")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/15")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/16")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/17")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/13")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/14")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/15")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/16")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/17")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-on-stack-with-literal/18")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6350,15 +6357,16 @@ test-convert-index-into-array-using-offset:
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array-using-offset/8")
     (check-next-stream-line-equal _test-output-stream "    b9/copy-to-ecx 3/imm32"                  "F - test-convert-index-into-array-using-offset/9")
     (check-next-stream-line-equal _test-output-stream "    69/multiply %ecx 0x00000004/imm32 0x00000001/r32"  "F - test-convert-index-into-array-using-offset/10")
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-using-offset/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-using-offset/12")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-using-offset/13")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-using-offset/14")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-using-offset/15")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-using-offset/16")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-using-offset/17")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-using-offset/18")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-using-offset/19")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 1 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-using-offset/11")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-using-offset/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-using-offset/13")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-using-offset/14")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-using-offset/15")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-using-offset/16")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-using-offset/17")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-using-offset/18")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-using-offset/19")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-using-offset/20")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6401,15 +6409,16 @@ test-convert-index-into-array-of-bytes-using-offset:
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array-of-bytes-using-offset/8")
     (check-next-stream-line-equal _test-output-stream "    b9/copy-to-ecx 3/imm32"                  "F - test-convert-index-into-array-of-bytes-using-offset/9")
     (check-next-stream-line-equal _test-output-stream "    69/multiply %ecx 0x00000001/imm32 0x00000001/r32"  "F - test-convert-index-into-array-of-bytes-using-offset/10")
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-using-offset/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes-using-offset/12")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-using-offset/13")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-using-offset/14")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-using-offset/15")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-using-offset/16")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-using-offset/17")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-using-offset/18")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-using-offset/19")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 1 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-of-bytes-using-offset/11")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-using-offset/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes-using-offset/13")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-using-offset/14")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-using-offset/15")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-using-offset/16")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-using-offset/17")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-using-offset/18")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-using-offset/19")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-using-offset/20")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6452,16 +6461,17 @@ test-convert-index-into-array-using-offset-on-stack:
     (check-next-stream-line-equal _test-output-stream "    68/push 0/imm32"                         "F - test-convert-index-into-array-using-offset-on-stack/8")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array-using-offset-on-stack/9")
     (check-next-stream-line-equal _test-output-stream "    69/multiply *(ebp+0xfffffff8) 0x00000004/imm32 0x00000001/r32"  "F - test-convert-index-into-array-using-offset-on-stack/10")
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-using-offset-on-stack/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-using-offset-on-stack/12")
-    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000004/imm32"    "F - test-convert-index-into-array-using-offset-on-stack/13")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-using-offset-on-stack/14")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-using-offset-on-stack/15")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-using-offset-on-stack/16")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-using-offset-on-stack/17")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-using-offset-on-stack/18")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-using-offset-on-stack/19")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-using-offset-on-stack/20")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 1 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-using-offset-on-stack/11")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-using-offset-on-stack/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-using-offset-on-stack/13")
+    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000004/imm32"    "F - test-convert-index-into-array-using-offset-on-stack/14")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-using-offset-on-stack/15")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-using-offset-on-stack/16")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-using-offset-on-stack/17")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-using-offset-on-stack/18")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-using-offset-on-stack/19")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-using-offset-on-stack/20")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-using-offset-on-stack/21")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -6504,16 +6514,17 @@ test-convert-index-into-array-of-bytes-using-offset-on-stack:
     (check-next-stream-line-equal _test-output-stream "    68/push 0/imm32"                         "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/8")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/9")
     (check-next-stream-line-equal _test-output-stream "    69/multiply *(ebp+0xfffffff8) 0x00000001/imm32 0x00000001/r32"  "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/10")
-    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/12")
-    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000004/imm32"    "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/13")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/14")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/15")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/16")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/17")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/18")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/19")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/20")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 1 *eax \"foo\" \"arr\")"  "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/11")
+    (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx + 4) 0x00000000/r32"  "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/13")
+    (check-next-stream-line-equal _test-output-stream "    81 0/subop/add %esp 0x00000004/imm32"    "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/14")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/15")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/16")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/17")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/18")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/19")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/20")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-index-into-array-of-bytes-using-offset-on-stack/21")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -9534,15 +9545,16 @@ test-convert-array-of-user-defined-types:
     (check-next-stream-line-equal _test-output-stream "    b8/copy-to-eax 0/imm32"                  "F - test-convert-array-of-user-defined-types/7")
     (check-next-stream-line-equal _test-output-stream "    ff 6/subop/push %ecx"                    "F - test-convert-array-of-user-defined-types/8")
     (check-next-stream-line-equal _test-output-stream "    b9/copy-to-ecx 3/imm32"                  "F - test-convert-array-of-user-defined-types/9")
+    (check-next-stream-line-equal _test-output-stream "    (__check-mu-array-bounds %ecx 0x00000008 *eax \"foo\" \"arr\")"  "F - test-convert-array-of-user-defined-types/10")
     (check-next-stream-line-equal _test-output-stream "    8d/copy-address *(eax + ecx<<0x00000003 + 4) 0x00000000/r32"  "F - test-convert-array-of-user-defined-types/11")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-array-of-user-defined-types/13")
-    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-array-of-user-defined-types/14")
-    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-array-of-user-defined-types/15")
-    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-array-of-user-defined-types/16")
-    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-array-of-user-defined-types/17")
-    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-array-of-user-defined-types/18")
-    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-array-of-user-defined-types/19")
-    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-array-of-user-defined-types/20")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %ecx"                     "F - test-convert-array-of-user-defined-types/12")
+    (check-next-stream-line-equal _test-output-stream "    8f 0/subop/pop %eax"                     "F - test-convert-array-of-user-defined-types/13")
+    (check-next-stream-line-equal _test-output-stream "  }"                                         "F - test-convert-array-of-user-defined-types/14")
+    (check-next-stream-line-equal _test-output-stream "$foo:0x00000001:break:"                      "F - test-convert-array-of-user-defined-types/15")
+    (check-next-stream-line-equal _test-output-stream "  # . epilogue"                              "F - test-convert-array-of-user-defined-types/16")
+    (check-next-stream-line-equal _test-output-stream "  89/<- %esp 5/r32/ebp"                      "F - test-convert-array-of-user-defined-types/17")
+    (check-next-stream-line-equal _test-output-stream "  5d/pop-to-ebp"                             "F - test-convert-array-of-user-defined-types/18")
+    (check-next-stream-line-equal _test-output-stream "  c3/return"                                 "F - test-convert-array-of-user-defined-types/19")
     # . epilogue
     89/<- %esp 5/r32/ebp
     5d/pop-to-ebp
@@ -24774,6 +24786,11 @@ Curr-local-stack-offset:  # (addr int)
 
 == code
 
+# We may not need to pass err/ed everywhere here. I think they're relics of Mu
+# getting type checks later in life.
+# But we do need them for runtime checks, particularly array index bounds checks.
+# So perhaps it's not worth taking them out. They're a safety net.
+
 emit-subx:  # out: (addr buffered-file), err: (addr buffered-file), ed: (addr exit-descriptor)
     # . prologue
     55/push-ebp
@@ -26844,6 +26861,8 @@ translate-mu-index-stmt:  # out: (addr buffered-file), stmt: (addr stmt), fn: (a
     (lookup *(ebx+0xc) *(ebx+0x10))  # Stmt1-inouts Stmt1-inouts => eax
     (lookup *eax *(eax+4))  # Stmt-var-value Stmt-var-value => eax
     89/<- %ebx 0/r32/eax
+    # emit bounds-check
+    (emit-mu-index-bounds-check *(ebp+8) *(ebp+0xc) *(ebp+0x10) *(ebp+0x14) *(ebp+0x18))
     # if (var->register) do one thing
     {
       81 7/subop/compare *(ebx+0x18) 0/imm32  # Var-register
@@ -26880,6 +26899,125 @@ $translate-mu-index-stmt:error2:
     (stop *(ebp+0x18) 1)
     # never gets here
 
+emit-mu-index-bounds-check:  # out: (addr buffered-file), stmt: (addr stmt), err: (addr buffered-file), ed: (addr exit-descriptor)
+    # . prologue
+    55/push-ebp
+    89/<- %ebp 4/r32/esp
+    # . save registers
+    50/push-eax
+    51/push-ecx
+    52/push-edx
+    53/push-ebx
+    # ecx = stmt
+    8b/-> *(ebp+0xc) 1/r32/ecx
+    #
+    (emit-indent *(ebp+8) *Curr-block-depth)
+    (write-buffered *(ebp+8) "(__check-mu-array-bounds ")
+$emit-mu-index-bounds-check:compute-base:
+    # var base/ebx: (addr var) = inouts[0]
+    (lookup *(ecx+0xc) *(ecx+0x10))  # Stmt1-inouts Stmt1-inouts => eax
+    (lookup *eax *(eax+4))  # Stmt-var-value Stmt-var-value => eax
+    89/<- %ebx 0/r32/eax
+$emit-mu-index-bounds-check:emit-index:
+    # var index/edx: (addr var) = inouts[1]
+    (lookup *(ecx+0xc) *(ecx+0x10))  # Stmt1-inouts Stmt1-inouts => eax
+    (lookup *(eax+8) *(eax+0xc))  # Stmt-var-next Stmt-var-next => eax
+    (lookup *eax *(eax+4))  # Stmt-var-value Stmt-var-value => eax
+    89/<- %edx 0/r32/eax
+    # if index->register, print its code
+    81 7/subop/compare *(edx+0x18) 0/imm32  # Var-register
+    {
+      0f 84/jump-if-= break/disp32
+$emit-mu-index-bounds-check:emit-register-index:
+      (write-buffered *(ebp+8) "%")
+      (lookup *(edx+0x18) *(edx+0x1c))  # Var-register Var-register => eax
+      (write-buffered *(ebp+8) %eax)
+      eb/jump $emit-mu-index-bounds-check:index-done/disp8
+    }
+    # otherwise if index is a literal, print it
+$emit-mu-index-bounds-check:emit-literal-index:
+    (lookup *(edx+8) *(edx+0xc))  # Var-type Var-type => eax
+    (is-simple-mu-type? %eax 0)  # => eax
+    3d/compare-eax-and 0/imm32/false
+    {
+      0f 84/jump-if-= break/disp32
+      (lookup *edx *(edx+4))  # Var-name Var-name => eax
+      (write-buffered *(ebp+8) %eax)
+    }
+$emit-mu-index-bounds-check:index-done:
+    (write-buffered *(ebp+8) " ")
+$emit-mu-index-bounds-check:emit-element-size:
+    # if index is a literal or int, print size of array element
+    {
+      {
+        (lookup *(edx+8) *(edx+0xc))  # Var-type Var-type => eax
+        (is-simple-mu-type? %eax 0)  # literal => eax
+        3d/compare-eax-and 0/imm32/false
+        75/jump-if-!= break/disp8
+        (lookup *(edx+8) *(edx+0xc))  # Var-type Var-type => eax
+        (is-simple-mu-type? %eax 1)  # int => eax
+        3d/compare-eax-and 0/imm32/false
+        75/jump-if-!= break/disp8
+        eb/jump $emit-mu-index-bounds-check:emit-element-size-offset/disp8
+      }
+$emit-mu-index-bounds-check:emit-int-register-index:
+      (array-element-size %ebx *(ebp+0x14) *(ebp+0x18))  # => eax
+      (write-int32-hex-buffered *(ebp+8) %eax)
+      e9/jump $emit-mu-index-bounds-check:emit-base/disp32
+    }
+$emit-mu-index-bounds-check:emit-element-size-offset:
+    # if index has type (offset ...), print "1"
+    (lookup *(edx+8) *(edx+0xc))  # Var-type Var-type => eax
+    81 7/subop/compare *eax 0/imm32/false  # Type-tree-is-atom
+    {
+      75/jump-if-!= break/disp8
+      (lookup *(eax+4) *(eax+8))  # Type-tree-left Type-tree-left => eax
+      (is-simple-mu-type? %eax 7)  # => eax
+      3d/compare-eax-and 0/imm32/false
+      {
+        0f 84/jump-if-= break/disp32
+$emit-mu-index-bounds-check:emit-offset-register-index:
+        (write-buffered *(ebp+8) "1")
+      }
+    }
+$emit-mu-index-bounds-check:emit-base:
+    # if base is in a register, print " *" base->register
+    81 7/subop/compare *(ebx+0x18) 0/imm32  # Var-register
+    {
+      74/jump-if-= break/disp8
+      (write-buffered *(ebp+8) " *")
+      (lookup *(ebx+0x18) *(ebx+0x1c))  # Var-register Var-register => eax
+      (write-buffered *(ebp+8) %eax)
+      e9/jump $emit-mu-index-bounds-check:emit-function-name/disp32
+    }
+    # otherwise print " *(ebp+" base->offset ")"
+    (write-buffered *(ebp+8) " *(ebp+")
+    (write-int32-hex-buffered *(ebp+8) *(ebx+0x14))  # Var-offset
+    (write-buffered *(ebp+8) ")")
+$emit-mu-index-bounds-check:emit-function-name:
+    # " \"" function-name "\""
+    (write-buffered *(ebp+8) " \"")
+    8b/-> *(ebp+0x10) 1/r32/ecx
+    (lookup *ecx *(ecx+4))  # Function-name Function-name => eax
+    (write-buffered *(ebp+8) %eax)
+    (write-buffered *(ebp+8) "\"")
+$emit-mu-index-bounds-check:emit-array-name:
+    # " \"" base->name "\""
+    (write-buffered *(ebp+8) " \"")
+    (lookup *ebx *(ebx+4))  # Var-name Var-name => eax
+    (write-buffered *(ebp+8) %eax)
+    (write-buffered *(ebp+8) "\")\n")
+$emit-mu-index-bounds-check:end:
+    # . restore registers
+    5b/pop-to-ebx
+    5a/pop-to-edx
+    59/pop-to-ecx
+    58/pop-to-eax
+    # . epilogue
+    89/<- %esp 5/r32/ebp
+    5d/pop-to-ebp
+    c3/return
+
 translate-mu-index-stmt-with-array-in-register:  # out: (addr buffered-file), stmt: (addr stmt), err: (addr buffered-file), ed: (addr exit-descriptor)
     # . prologue
     55/push-ebp
diff --git a/mu.md b/mu.md
index 8c5975e7..ba0c8b66 100644
--- a/mu.md
+++ b/mu.md
@@ -681,7 +681,7 @@ populate x, 3  # array of 3 T's
 I said at the start that most instructions map 1:1 to x86 machine code. To
 enforce type- and memory-safety, I was forced to carve out a few exceptions:
 
-* the `index` instruction on arrays, for bounds-checking (not yet implemented)
+* the `index` instruction on arrays, for bounds-checking
 * the `length` instruction on arrays, for translating the array size in bytes
   into the number of elements.
 * the `lookup` instruction on handles, for validating fat-pointer metadata
diff --git a/mu_instructions b/mu_instructions
index f487d29a..bd00e7a4 100644
--- a/mu_instructions
+++ b/mu_instructions
@@ -312,24 +312,28 @@ var/reg: (addr T) <- address var2: T
   => "8d/copy-address *(ebp+" var2.stack-offset ") " reg "/r32"
 
 # Array operations
-(TODO: bounds-checking)
 
 var/reg <- index arr/rega: (addr array T), idx/regi: int
-  | if size-of(T) is 4 or 8
-      => "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"
+  | if size-of(T) is 1, 2, 4 or 8
+      => "(__check-mu-array-bounds *" rega " %" regi " " size-of(T) ")"
+         "8d/copy-address *(" rega "+" regi "<<" log2(size-of(T)) "+4) " reg "/r32"
 var/reg <- index arr: (array T len), idx/regi: int
-  => "8d/copy-address *(ebp+" regi "<<" log2(size-of(T)) "+" (arr.stack-offset + 4) ") " reg "/r32"
+  => "(__check-mu-array-bounds *(ebp+" arr.stack-offset ") %" regi " " size-of(T) ")"
+     "8d/copy-address *(ebp+" regi "<<" log2(size-of(T)) "+" (arr.stack-offset + 4) ") " reg "/r32"
 var/reg <- index arr/rega: (addr array T), n
-  => "8d/copy-address *(" rega "+" (n*size-of(T)+4) ") " reg "/r32"
+  => "(__check-mu-array-bounds *" rega " " n " " size-of(T) ")"
+     "8d/copy-address *(" rega "+" (n*size-of(T)+4) ") " reg "/r32"
 var/reg <- index arr: (array T len), n
-  => "8d/copy-address *(ebp+" (arr.stack-offset+4+n*size-of(T)) ") " reg "/r32"
+  => "(__check-mu-array-bounds *(ebp+" arr.stack-offset ") " n " " size-of(T) ")"
+     "8d/copy-address *(ebp+" (arr.stack-offset+4+n*size-of(T)) ") " reg "/r32"
 
 var/reg: (offset T) <- compute-offset arr: (addr array T), idx/regi: int  # arr can be in reg or mem
   => "69/multiply %" regi " " size-of(T) "/imm32 " reg "/r32"
 var/reg: (offset T) <- compute-offset arr: (addr array T), idx: int       # arr can be in reg or mem
   => "69/multiply *(ebp+" idx.stack-offset ") " size-of(T) "/imm32 " reg "/r32"
-var/reg <- index arr/rega: (addr array T), o/rego: offset
-  => "8d/copy-address *(" rega "+" rego "+4) " reg "/r32"
+var/reg <- index arr/rega: (addr array T), o/rego: (offset T)
+  => "(__check-mu-array-bounds %" rega " %" rego " 1 \"" function-name "\")"
+     "8d/copy-address *(" rega "+" rego "+4) " reg "/r32"
 
 Computing the length of an array is complex.