diff --git a/dwt/heapmap.py b/dwt/heapmap.py
new file mode 100644
index 0000000..8afc4fe
--- /dev/null
+++ b/dwt/heapmap.py
@@ -0,0 +1,139 @@
+#!/usr/bin/env python3
+"""Dump the VT1200 firmware heap free lists over rdebug `peek`.
+
+The firmware has a real bit-addressed allocator (malloc `0x07EE0246`, free
+`0x07EE0488`, realloc near `0x07EE0B84`).  It keeps:
+
+  * a free-bytes counter at bit `0x1E8BA0` (32-bit, in **bits**)
+  * a global free-list head at bit `0x1E8AC0`
+  * three arenas at `0x1E8930` / `0x1E89B0` / `0x1E8A30`, each with its free-list
+    head at `+0x60`
+
+Each free block is a 0x40-bit (8-byte) header:
+
+  +0x00  size in bits (includes the 8-byte header)
+  +0x20  next block, a **bit** address
+
+This tool walks them, so payload placement can be checked against what the OS
+may actually hand out.  Note that the firmware's payload scratch addresses
+(bit `0x01800000` = byte `0x00300000`) sit **inside** the largest free block on
+a default boot -- see `dwt/notes/HEAP.md`.
+
+The **global** list is the one that matters for payload placement and is
+reliable.  The per-arena lists are optional (`--arenas`); following one has
+been observed to kill the rdebug process, so each list is read on its own
+connection.
+
+    python3 dwt/heapmap.py --host 192.168.99.2
+    python3 dwt/heapmap.py --host 192.168.99.2 --at 0x01800000
+    python3 dwt/heapmap.py --host 192.168.99.2 --blocks --arenas
+"""
+from __future__ import annotations
+
+import argparse
+import struct
+import sys
+
+import exploit
+
+FREE_COUNTER = 0x1E8BA0
+GLOBAL = ("global", 0x1E8AC0)
+ARENAS = (
+    ("arena0", 0x1E8990),
+    ("arena1", 0x1E8A10),
+    ("arena2", 0x1E8A90),
+)
+RAM_BITS = 0x02000000          # 4 MiB, as bit addresses
+HDR_NEXT = 0x20                # bits
+HDR_BYTES = 8
+
+
+def u32(rd: exploit.RDebug, bit: int) -> int:
+    return struct.unpack("<I", rd.peek(bit, 4))[0]
+
+
+def walk(rd: exploit.RDebug, head: int, limit: int = 400):
+    p = u32(rd, head)
+    out, seen = [], set()
+    while p and len(out) < limit and p not in seen and 8 <= p < RAM_BITS:
+        seen.add(p)
+        sz = u32(rd, p)
+        nxt = u32(rd, p + HDR_NEXT)
+        out.append((p, sz, nxt))
+        if not (8 <= nxt < RAM_BITS):
+            break
+        p = nxt
+    return out
+
+
+def read_list(host, port, timeout, name, head):
+    """Read one list on its own connection; a bad list must not abort the rest."""
+    rd = exploit.RDebug(host, port, timeout, False)
+    rd.connect()
+    try:
+        return walk(rd, head)
+    finally:
+        rd.close()
+
+
+def main(argv):
+    ap = argparse.ArgumentParser(description=__doc__,
+                                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("--host", default="192.168.99.2")
+    ap.add_argument("--port", type=int, default=771)
+    ap.add_argument("--timeout", type=float, default=8.0)
+    ap.add_argument("--at", type=lambda s: int(s, 0), default=None,
+                    help="bit address to check (e.g. a payload scratch address)")
+    ap.add_argument("--blocks", action="store_true", help="list every block")
+    ap.add_argument("--arenas", action="store_true",
+                    help="also try the per-arena lists (can kill the rdebug process)")
+    a = ap.parse_args(argv[1:])
+
+    rd = exploit.RDebug(a.host, a.port, a.timeout, False)
+    rd.connect()
+    try:
+        free_bits = u32(rd, FREE_COUNTER)
+    finally:
+        rd.close()
+    print(f"free counter: {free_bits} bits = {free_bits // 8} bytes "
+          f"({free_bits // 8000} KB decimal)")
+
+    lists = (GLOBAL,) + (ARENAS if a.arenas else ())
+    all_blocks = []
+    for name, head in lists:
+        try:
+            blocks = read_list(a.host, a.port, a.timeout, name, head)
+        except Exception as exc:                      # noqa: BLE001
+            print(f"{name:6s}: unreadable ({type(exc).__name__}: {exc})")
+            continue
+        if not blocks:
+            print(f"{name:6s}: empty")
+            continue
+        lo = min(b for b, _, _ in blocks)
+        hi = max(b + s for b, s, _ in blocks)
+        usable = sum(s // 8 - HDR_BYTES for _, s, _ in blocks
+                     if s >= HDR_BYTES * 8)
+        print(f"{name:6s}: {len(blocks)} blocks, byte 0x{lo>>3:06X}..0x{hi>>3:06X}, "
+              f"{usable} bytes usable")
+        all_blocks += [(name, b, s) for b, s, _ in blocks]
+        if a.blocks:
+            for b, s, nxt in blocks:
+                print(f"        bit 0x{b:08X} byte 0x{b>>3:06X}  "
+                      f"size {s//8:>9} B  end 0x{(b+s)>>3:06X}  next 0x{nxt:08X}")
+
+    if a.at is not None:
+        hits = [(n, b, s) for n, b, s in all_blocks if b <= a.at < b + s]
+        print(f"check bit 0x{a.at:08X} (byte 0x{a.at>>3:06X}): "
+              + ("INSIDE a free block" if hits else "outside every free block"))
+        for n, b, s in hits:
+            print(f"        {n}: block bit 0x{b:08X}, "
+                  f"{(b+s-a.at)//8} bytes of headroom to its end")
+    return 0
+
+
+if __name__ == "__main__":
+    try:
+        sys.exit(main(sys.argv))
+    except (exploit.ProtocolError, OSError) as exc:
+        print(f"error: {exc}", file=sys.stderr)
+        sys.exit(1)
diff --git a/dwt/notes/HEAP.md b/dwt/notes/HEAP.md
new file mode 100644
index 0000000..67ba15b
--- /dev/null
+++ b/dwt/notes/HEAP.md
@@ -0,0 +1,87 @@
+# Payload memory and the firmware heap
+
+Where the injected payloads live, and what the firmware's allocator actually
+owns. Measured on a default `VT1200_SKIP_LEVEL1=1` boot, 2026-10-18.
+
+## Short answer
+
+* Payloads are poked to **fixed scratch addresses** at bit `0x01800000`
+  (byte `0x00300000`, 3 MiB) and above — the top of the 4 MiB RAM.
+* That region is **not allocated**. On a default boot it lies **inside the
+  largest free block** of the firmware's heap (`byte 0x12B772..0x400000`,
+  2,967,694 bytes). Nothing protects it; the OS may hand it out.
+* There **is** a real allocator — `malloc`/`free`/`realloc` — and it is
+  callable from a payload. `rdebug`'s own handlers already use it.
+
+## The allocator
+
+| what | address |
+|---|---|
+| `malloc` | Ghidra `0x07EE0246` / bit `0x3F701230` |
+| `free` | Ghidra `0x07EE0488` / bit `0x3F702440` |
+| `realloc` | near Ghidra `0x07EE0B84` (string `realloc: bad address %x not allocated`, bit `0x3F705C20`) |
+| `calloc` | present (used by `rdebug_gather_usage_stats` and `remote_debug`) |
+
+It is a **bit-addressed first-fit** allocator over free lists:
+
+* free-bytes counter, 32-bit, **in bits**: bit `0x1E8BA0`
+* global free-list head: bit `0x1E8AC0`
+* three arenas at bit `0x1E8930`, `0x1E89B0`, `0x1E8A30`; each free-list head at
+  `+0x60` (so `0x1E8990`, `0x1E8A10`, `0x1E8A90`)
+* a block is `{ u32 size_in_bits; ...; u32 next_bitaddr at +0x20 }`; the header
+  is `0x40` bits (8 bytes), so the **returned pointer is `block + 8 bytes`**
+* `malloc` returns `0` on failure
+
+`create_process` (`0x07EE0D48`) mallocs a 0x39-byte PCB and the stack;
+`rdebug_display_message` mallocs `len+2`; `jump` mallocs the argument block.
+So allocation from our payload's context is already proven by the firmware
+itself.
+
+## Measured boot
+
+```
+free counter @0x1E8BA0 = 24,795,312 bits = 3,099,414 bytes (3099 KB decimal)
+global free list: 9 blocks
+  [0] byte 0x0F5B5A   196 B
+  ...
+  [8] byte 0x12B772  2,967,694 B   end byte 0x400000
+```
+
+The last block runs to the top of RAM. `0x01800000` (byte `0x300000`) falls
+**inside** it — about 1.9 MB from the block's start and 1 MiB below its end.
+That is why the payloads work, and why they are not safe indefinitely: the
+allocator splits this block from its start, so roughly 1.9 MB of allocation
+from it would reach the payload.
+
+The Memory bar's `3099 KB` is this counter in decimal kilobytes, a useful
+cross-check.
+
+**The per-arena lists are not safe to follow.** Reading them has twice killed
+the terminal's network stack (`TimeoutError`, then `No route to host`), while
+the global list reads cleanly every time. `dwt/heapmap.py` therefore reads the
+global list by default and treats `--arenas` as an experiment; do the same, and
+treat `peek` of an unvalidated pointer as able to take the machine down.
+
+## Safe pattern for a payload
+
+1. **Bootstrap**: the code itself must live somewhere before `malloc` runs.
+   Poke it to a fixed scratch address as today.
+2. Call `malloc(size)` from the payload's own process (the `create_process`
+   child, or the launcher before it starts Xlib).
+3. Copy any long-lived buffers — or the payload itself — into the returned
+   block, keep the pointer in a **callee-saved register** or a fixed marker, and
+   never write before `ptr-8` (the header).
+4. One allocation at startup is safest. The allocator's lock is a per-process
+   byte at `[@0x1B3900]+0x1C1`; assume cooperative scheduling, do not allocate
+   from an interrupt handler, and do not race it from two payloads.
+5. Check for `0`, and remember sizes and addresses are **bits** internally even
+   though `malloc` takes bytes.
+
+## Tool
+
+`dwt/heapmap.py` walks the free lists over `peek`:
+
+```sh
+python3 dwt/heapmap.py --host 192.168.99.2 --blocks
+python3 dwt/heapmap.py --host 192.168.99.2 --at 0x01800000   # is my payload safe?
+```