STM32 machine language monitor

Allows a few rudimentary commands to be entered on the STM32 Minimum System Development Board Module ARM STM32F103C8T6 available from various online outlets.
The code was originally forged in C but limitations in that approach led me to ARM assembler, and fortunately GNU compiler collection helped in that transition.
Small assembler code experiments can be typed into the module's read write memory, displayed and executed.
One strange, annoying feature is that the code running in the STM32 can't execute any subroutines in the flash memory.
This project requires a serial terminal e.g. on Linux, minicom
on a USB serial TTL adapter running at 9600 baud, 8bits, no parity. The usart port on the STM32 is at the other end of this hardware. Using TTL levels is easier that introducing RS-232 levels.
The code experiments in read write memory should be subroutines and will be called by this monitor and a return in the user's code will return to this monitor program.