Everything I know about the obscure "linked patch" system programming/binary interface in System 7.1-??

The idea was to provide a macro-based interface that a 68k assembly programmer could use to create RAM-based "patches" for a newly booted MacOS system. Similar patches were needed even in the original Macintosh System Software to fix bugs in the shipping ROM. Runtime patching also came to be used to add new software features to old Mac models. But writing a patch in pure 68k assembly, especially a "come-from" patch, was very tedious and usually required self-modifying code.

The linked patches incrementally improved that situation. Here is a summary of the design decisions made:

• A library of 68k macros provided a nearly-declarative way to describe to installation process for a given patch.
• The runtime RAM usage was minimised by separating installation from runtime code, and by segmenting each code module at build time according to its target ROM releases.
• Advantage was taken of the huge commonality carefully between Macintosh ROM releases, which was painstakingly maintained by binary patching and "overpatching".
• Object files containing patches woulod be linked into resources by a full linker, making dead code elimination available and allowing the direct inclusion of code originally meant for a ROM build.
• Runtime self-modification of code was done almost entirely by a generic runtime loader, instead of allowing each patch writer to come up with a unique and uniquely buggy solution.
• Special assembly facilities were made available to ease the writing of come-from patches (patches that wrested control from a buggy segment of ROM by hijacking a possibly unrelated trap in the vicinity). Specifically, it was easy to describe the address of the target trap and to produce code that would test that the trap was indeed being called from that address.