EmulateHeap: The Windows 95 Heap Manager Still Shipping Inside Windows 11

Windows 11 still carries a copy of Windows 95's heap manager. It sits inside the application compatibility framework as a shim called EmulateHeap, and Windows Latest reported on October 19 that it is present on a Windows 11 26H2 PC.

What EmulateHeap does

A heap manager hands out memory to programs. Raymond Chen, writing in a Windows Confidential column that Microsoft Learn now archives, said that when the fix is enabled for a process, all the OS-level heap functions are redirected to replacements that "emulate the Windows 95 heap down to the very last detail". Windows Latest dates the column to 2010.

The point is bugs. Programs written for Windows 95 got away with mistakes because that allocator happened to be forgiving, and they crash on any other allocator. Chen listed three habits:

Habit in the old program	What it needs from Windows 95
Allocate two blocks, free both, allocate a third	The third pointer equals the first
Overflow a heap buffer	The memory after the buffer is unused
Read memory after freeing it	The old values are still there

How Microsoft built it

Chen said the compatibility team "simply took a copy of the Windows 95 Heap Manager source code, recompiled it, and added it to the compatibility infrastructure". In his words, there is "a copy of a significant chunk of the Windows 95 kernel inside the Windows application compatibility framework". The code loads from disk only for a process with the fix enabled, and it does real work then.

Turning it on has a price. Chen wrote that features such as the low fragmentation heap must be switched off for that process, because they would shift the coincidences the old program depends on. He also pointed to the Fault Tolerant Heap in Windows 7, which detects double-frees, small heap overruns and short-lived use after free, and hoped a general fix would remove the need for another copy.

The SimCity story

Windows Latest ties the approach to SimCity. The original Windows 3.x version, written by Jon Ross, read memory after freeing it, which worked because the memory stayed put. Windows Latest says it broke on Windows 95 betas, and Microsoft added code that detects SimCity and runs the allocator in a mode that does not free memory right away. That account comes from Joel Spolsky's 2000 essay as Windows Latest quotes it, and Chen's column does not mention SimCity, so treat it as the origin of the practice rather than of EmulateHeap itself.

Why old software ran on Windows 95

Chen cautioned against reading the shim as praise for the old allocator. The Windows 95 heap manager was not better than the others, he wrote, only different. The effect is self-selection: a 1995 program with a bug that crashed on Windows 95 got noticed and fixed, because Windows 95 was the target platform. What survived to ship were programs whose flaws happened to be harmless on that one allocator.

That is why the fix is a per-process switch and not a system default. A game from the Windows 95 era that runs fine on a modern heap needs nothing. One that crashes at launch, or loses its sound, is a candidate. The administrator sets the fix by program, so the cost of losing the low fragmentation heap lands only on the title that needs the old behaviour.

Seeing it yourself

Windows Latest's steps are short. Install the Windows ADK with only Application Compatibility Tools selected, and note that the September 2026 ADK release officially supports Windows 11 26H2. Open the 32-bit Compatibility Administrator, create a new compatibility mode database under Custom Databases, and find EmulateHeap in the fix list. That is about five clicks after the install.

The shim is not guaranteed to help. Windows Latest says that after the Windows 11 22H2 update, users reported GTA Vice City losing sound and Need for Speed: Most Wanted crashing with it enabled, citing a Microsoft Q&A thread. A programmer chasing the same class of fault in current code will recognise it from the PHP out-of-memory fixes in 8.5.12, and the compatibility rules for a new OS release also show up in the iPadOS 27 AirPlay bug.

Neither source says how many shipping programs still depend on EmulateHeap, and Microsoft has not published a count. The next date to watch is the 26H2 servicing cycle, when the ADK and its fix list get their next update.