I maintain a little diff patch for MAME and MESS that I call emubrk. This is an opcode that will cause the debugger to pop up when it is encountered in the instruction stream. Attached you’ll find the patches for this useful little bit of functionality. If you find the patches no longer work, please email me and I’ll fix them up.

Newer, LWASM approved, $113e style:
emubrk_diff 0.261.diff

Older, $11fc style:
emubrk_diff 0.126
emubrk_diff 0.118
emubrk_diff 0.113
emubrk_diff 0.110
emubrk_diff 0.109