Update compiler-error-c3859.md

Author: Rastaban

docs/error-messages/compiler-errors-2/compiler-error-c3859.md

@@ -10,19 +10,25 @@ helpviewer_keywords: ["C3859"]
 > Failed to create virtual memory for PCH
 
 The message will have one of the following notes:
->PCH: Address is not a multiple of the system's allocation granularity
->PCH: The chunk has not been previously reserved
->PCH: Commit size too large
->PCH: Unable to commit memory across file map
->PCH: Exhausted chunk list before committing all bytes
->PCH: Unexpected end of chunk list while trying to free
->PCH: Shouldn't be hitting a file map in the decommit case
->PCH: Invalid chunk
->PCH: Map size too large
->PCH: Unable to map file: memory already committed
->PCH: File map already in place
->PCH: Unable to get the requested block of memory
->consider using /Fp to allow the compiler to reserve the memory early
+>the system returned code *error code*: *OS error message*\
+>PCH: Address is not a multiple of the system's allocation granularity\
+>PCH: The chunk has not been previously reserved\
+>PCH: Commit size too large\
+>PCH: Unable to commit memory across file map\
+>PCH: Exhausted chunk list before committing all bytes\
+>PCH: Unexpected end of chunk list while trying to free\
+>PCH: Shouldn't be hitting a file map in the decommit case\
+>PCH: Invalid chunk\
+>PCH: Map size too large\
+>PCH: Unable to map file: memory already committed\
+>PCH: File map already in place\
+>PCH: Unable to get the requested block of memory\
+>consider using /Fp to allow the compiler to reserve the memory early\
 
 There isn't enough virtual memory allocated for your precompiled header. If your precompiled header uses an explicit `#pragma hdrstop` directive, use the **`/Zm`** compiler flag to specify a larger value for the precompiled header file. Otherwise, consider reducing the number of parallel compilation processes in your build. For more information, see [`/Zm` (Specify precompiled header memory allocation limit)](../../build/reference/zm-specify-precompiled-header-memory-allocation-limit.md).
 
+This disganotic primarily in two scenarios. 
+
+The first scenario is that the system is overloaded with multiple `/Yu` compile requests at the same time. In this scenario setting the maximum starting virtual memory size typically resolves the issue.
+
+The second scenario is when the the Windows loader injects a DLL into the process at startup. That injected DLL can allocate memory which conflicts with where the PCH must reside. For example, `msbuild.exe` will inject `FileTracker.dll` into every `CL.exe` process at startup. In this scenario, using the [`/Fp` (Name .pch file)](../../build/reference/fp-name-dot-pch-file.md) flag will ensure that the memory for the PCH is allocated as early as possible in the `CL.exe` process, before any injected DLL. These failures can be intermittent because Windows ASLR will allocate memory at different address across different process invocations. Without `/Fp`, memory for the PCH cannot be allocated until the compiler has compiled enough of the program to see the header file specified in the `/Yu` command line option or the `#pragma hdrstop`. By this time, it is much more likely that the memory required by the PCH will already be reserved.