I asked this a while back on the old Keil forum, but got no reply.
https://community.arm.com/developer/tools-software/tools/f/keil-forum/35230/is-monitor-mode-debug-mmd-supported-with-ulink
So perhaps the Cortex-M experts can help?
Segger support Monitor-Mode Debug (as opposed to Halting Debug) for Cortex-M using their J-Link:
https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging/
I have used it with Nordic devices - where it is invaluable for application debugging while the BLE stack is running:
https://devzone.nordicsemi.com/f/nordic-q-a/49990/debugging-ble-uart-causes-reset-when-breakpoint-set-in-sdk/199327#199327
However, as far as I can find, Segger is the only one to have an implementation that uses it.
So the question is: Is anyone aware of any other implementation?
Found come interesting references:
ARM Cortex Core microcontrollers8th DebuggingBalázs SchererBudapest University of Technology and Economics © BME-MIT 2017Department of Measurement and Information Systems
https://www.mit.bme.hu/eng/system/files/oktatas/targyak/11072/ARM_Cortex_08_Debug_v04.pdf
MRI - Monitor for Remote Inspection. The gdb compatible debug monitor for Cortex-M devicesAdam Green
https://github.com/adamgreen/mri
No specific debug probe implementations, though.