I haven't yet found any source code for any of the monarch software, if you know where there are copies or even print outs of the Main Software or cards or the PDP 11 code please get in touch !
There are description of the software in this technical description, that can be read in full here
It was published in October 1982. (when the compact and 120 existed but probably not the 250c, im not sure if the 8088 COU card set existed at the time)
3.1.11 The CPU itself consists of a micro-processor contained in a single integrated circuit. The information which it requires to fulfil its control functions is contained in 3 different stores:
A Main Program store which contains the information on which the basic operation of the exchange depends. This is recorded on Erasable Programmable Read Only Memory (EPROM). 11
A Customer's Data Base store which contains information relating to the particular installation (extension numbers, class of service information, short code dialling data, etc). This is recorded on EPROM and copied into Random Access Memory (RAM) to enable the customer to make changes, as described in Paragraph 2.7.1 Page 19 ( III 11
A Call Record Store which contains all the information relating to each call in progress at any particular time. This is recorded on a RAM store.
In addition to the main bus connections, referred to in Paragraph 3.1.10, three data input/output channels are provided for connection to external items of equipment:
To a portable teletype, or similar machine for use in interrogating the exchange, either directly or over a modem link.
To data logging equipment.
To provide specialist maintenance information.
Diagram below is PART 1, FIGURE 26. BLOCK SCHEMATIC DIAGRAM OF CENTRAL PROCESSING UNIT AND ASSOCIATED STORES.