Retrofitting the WM_COPYDATA message onto Windows 3.1
Raymond Chen explains how the WM_COPYDATA message was designed to be simple to retrofit onto Windows 3.1, allowing seamless data transfer between 16-bit and 32-bit applications without breaking compatibility.
Background
Raymond Chen's "The Old New Thing" is a long-running Microsoft blog that chronicles Windows history, often revealing design decisions and constraints faced by the Windows team in the 1990s.
WM_COPYDATA is a Windows message that lets one running program safely pass data to another. It was introduced in Windows 3.1 (1992) — a 16-bit operating system that ran on top of MS-DOS, with no memory protection between applications and very limited address space (64 KB segments). The challenge was to retrofit this inter-process communication mechanism into a system where processes could crash each other and memory was extremely scarce. Chen's point is that the design looks simple in retrospect because the team worked within brutal constraints — not despite them.