An interface between the UNIX and CP/M operating systems

The purpose of this work was to design and implement the software required to utilize a hardware interface between a PDP-11/34 mini-computer and an Imsai 8080 micro-computer. The interface was designed and built by Andrew Wang at The University of Tennessee (Wang, 1978). The computers used were located in the University's mini-lab in the South College building on the Knoxville campus.

The interface built by Wang was parallel and asynchronous A pair of control and data registers on each computer afforded the data link. On each system, transmit and receive operations were done through this same register pair. This put some limitations on the software capabilities. Software to make use of this interface through the computers' operating systems had not been developed previous to the current work.

In order to produce the needed software, it was necessary to first understand the two operating systems involved, Unix on the PDP-11 and CP/M on the Imsai. Once the author had become familiar with both systems and the interface, the operating systems were modified to accommodate user programs which might use the capabilities of the cross-computer interface. Finally, a virtual floppy disk system was designed and implemented. This system uses the interface to send floppy disk format data from CP/M to a Unix user program. The Unix program maintains a set of disk files which simulate the CP/M format disks. Use of this system is relatively transparent to the CP/M user. Such a system was suggested in Wang's write-up of the interface. The software was also designed to give future users the ability to create new applications for the interface with a minimum of trouble.