The setup is using uVision3 with FS2 CAST 8051 driver for R8051XC. Associating a "_break_ = 1" debug statement causes uVision to hang immediately after the break point is hit. Error displayed is AGDI-Error: error halting emulation: Processor is already stopped. (error 84) FS2 Sysnav (version 2.6.13) says the target is not running.
My guess is that the target has been stopped immediately after the break point is hit (presumably by FS2) but the debugger is not aware of it. Any operation in the debug function that require the target to be halted return error as the debugger thinks the target is still running.
If the debug function does not have a "_break_ = 1" statement, execution continues after the break point is hit. This is inline with the documentation for debug functions.