Reserve address 0 by shifting ITCM start address

The runtime previously allowed function placement at address 0 in ITCM.
However, if you ever formed a pointer to the function placed there, it
would look like a null pointer. And you would never be able to call that
function if you relied on null pointer optimization. Also, most MCU
reference manuals (RM) recommend against this placement.

This commit reduces the total capacity of ITCM by 32 bytes, the smallest
possible size of a MPU region. Note that this is greater than the RM's
recommendation of a four byte reservation. It affects all supported
MCUs, except the 1180. If you're so inclined, your MPU could disallow
loads, stores, and execution from this reservation.

Revised unit tests should cover this change. Additionally, you can
manually verify that the ITCM region lengths are reduced by 32 bytes by
opening the linker scripts generated by the ELF test suite.

3 files changed, 20 insertions, 5 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a43b8e5..7e59f3a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+Establish a 32 byte reservation at the start of ITCM where instructions are
+never placed. This reduces the total capacity of ITCM by 32 bytes on nearly
+all MCUs, except the 1180.
+
 ## [0.1.7] 2025-06-14
 
 Introduce `RuntimeBuilder::in_flash` for creating images that can be launched
diff --git a/src/host.rs b/src/host.rs
index 43472fe..ba299dd 100644
--- a/src/host.rs
+++ b/src/host.rs
@@ -946,7 +946,7 @@ impl Family {
 
     /// Returns the start and size of the ITCM memory region.
     const fn itcm_start_size(self, itcm_banks: u32) -> (u32, u32) {
-        let itcm_size = itcm_banks * self.flexram_bank_size();
+        let mut itcm_size = itcm_banks * self.flexram_bank_size();
         let itcm_start = match self {
             Family::Imxrt1010
             | Family::Imxrt1015
@@ -956,7 +956,13 @@ impl Family {
             | Family::Imxrt1060
             | Family::Imxrt1064
             | Family::Imxrt1160
-            | Family::Imxrt1170 => 0x00000000,
+            | Family::Imxrt1170 => {
+                // Establish a reservation for null pointers.
+                // Note that this reservation is the minimum
+                // size of an MPU region.
+                itcm_size = itcm_size.saturating_sub(32);
+                32
+            }
             Family::Imxrt1180 => 0x10000000 - itcm_size,
         };
         (itcm_start, itcm_size)
@@ -1252,11 +1258,16 @@ mod tests {
     #[test]
     fn itcm_start_size() {
         // Most parts have an ITCM that could touch address 0.
+        // However, the implementation reserves an MPU region
+        // at address 0.
         for family in MOST_FAMILIES {
             for itcm_banks in 0..=family.flexram_bank_count() {
                 let (start, size) = family.itcm_start_size(itcm_banks);
-                assert_eq!(start, 0);
-                assert_eq!(size, family.flexram_bank_size() * itcm_banks);
+                assert_eq!(start, 32);
+                assert_eq!(
+                    size,
+                    (family.flexram_bank_size() * itcm_banks).saturating_sub(32)
+                );
             }
         }
 
diff --git a/tests/inspect_elf.rs b/tests/inspect_elf.rs
index 8108f3f..e9a8f73 100644
--- a/tests/inspect_elf.rs
+++ b/tests/inspect_elf.rs
@@ -198,7 +198,7 @@ struct Section {
 }
 
 const DTCM: u64 = 0x2000_0000;
-const ITCM: u64 = 0x0000_0000;
+const ITCM: u64 = 0x0000_0020;
 
 const fn aligned(value: u64, alignment: u64) -> u64 {
     (value + (alignment - 1)) & !(alignment - 1)