shadow space
32 bytes a Windows caller must leave free on the stack just above the return address, where the called function may store its four register arguments.
Under Microsoft x64 the caller allocates 32 bytes (the home area for rcx, rdx, r8, r9) immediately above the return address before every call, even for functions with fewer than four arguments.
Examples
On Windows x64, a caller reserves 32 bytes just above the return address where the callee may store its four register arguments.