I don't have all details, but seems like lots of work: * Mach APIs instead of ptrace (probably a lot of changes). * Mach-O instead of ELF. * Some other APIs

I don't have all details, but seems like lots of work: * Mach APIs instead of ptrace (probably a lot of changes). * Mach-O instead of ELF. * Some other APIs instead of /proc/<pid>/{maps,stat,...} * Probably arm in addition to x86? * Dealing with security stuff. * Probably lots of other small differences everywhere. Limiting the scope to one OS and CPU arhitecture was a big part of how I was able to make a usable debugger in a reasonable time.