diff --git a/libavr b/libavr
index c291f07..afebe8e 160000
--- a/libavr
+++ b/libavr
@@ -1 +1 @@
-Subproject commit c291f070d939aabc46d36fbd0d06d8ac82430436
+Subproject commit afebe8eb8f51b7d1674a9210bd0d698fbc3abd31
diff --git a/pureboot/README.md b/pureboot/README.md
index 8732c7a..4f71304 100644
--- a/pureboot/README.md
+++ b/pureboot/README.md
@@ -429,6 +429,21 @@ SRAM, and through the same address space the register file and every I/O
 register. Reading an I/O register can have side effects (reading UDR clears its
 flags), which is the caller's business to know.
 
+Reads are safe anywhere; **two small regions cannot be written without ending the
+session,** because they are what the loader is standing on:
+
+- the **top of SRAM**, where its stack lives — a handful of bytes below RAMEND;
+- on an **autobaud** build, the **two bytes at RAMSTART**: the measured bit
+  period, in `.noinit`, which is the whole of that loader's static RAM. Overwrite
+  it and its next reply is timed against garbage. On an ATtiny13A that is
+  `0x60..0x61`, and the symptom is a mangled prompt byte rather than any error —
+  the loader is fine, it simply is no longer speaking the agreed rate.
+
+Both are self-inflicted rather than defects, and a reset clears them. Note also
+that `--poke` can write OSCCAL, which does take effect — but a session can only
+survive a step or two of it before the clock walks the link out of the rate
+autobaud locked to, and OSCCAL reverts on reset regardless.
+
 Readouts come one fact per line: `--info` prints the device's version and
 signature and the geometry that follows from them, `--fuses` each fuse byte
 plus, on a boot-sectioned mega, its decoded meaning. Transfers that take wire time draw a transient progress bar on stderr