Nascom News­let­ter

1. How it works

A 50Hz clock signal derived from TP22 on the Nascom 2 board is connected to port A bit 7 of the PIO. The PIO is configured by the software to generate an interrupt when the edge of this clock pulse is received. The Z80 is set to use interrupt mode 2, in which mode an interrupt results in transfer of control to a location the address of which is located in memory at an address given by the contents of the interrupt register and data placed on the bus by the interrupting device.

At initialisation the PIO is configured for output on port B and bits 1 to 4 of port A, and input on port A bits 0 and 7. This allows a clock input to be used on port A bit 7, and also prepares the ports for parallel printer use, thus eliminating any need to configure ports in parallel printer drivers, which might otherwise upset the clock input during their configuration process. Control signals are also sent to the PIO so that it will generate an appropriate address and an interrupt when an edge occurs on the clock input of port A bit 7.

The interrupt routine and its workspace and buffer area are located at the top of memory – C800 in this case. When an interrupt occurs the interrupt routine scans the keyboard via the normal NAS-SYS routines, and places any data found in the buffer.

To insert the new keyboard input routing into NAS-BYS the entire SCAL table is copied into RAM, and the vector at 0C71 which points to the table is modified accordingly. The keyboard input entry in the table is then changed to point to the new input routine, which inspects the keyboard buffer to see if there has been any input.

Some systems programs use a similar method of moving the SCAL table to RAM and then perform their own changes to it. Such programs (eg NAS-SEMBLER) should therefore not be run with the keyboard buffering in operation.

Any program which disables interrupt will inhibit use of the keyboard, so be very careful! In particular you need to avoid cold starting BASIC, which for some incomprehensible reason disables interrupt. You can conveniently circumvent this by loading an empty BASIC program from tape or disc and then using the normal Z to warm start. This method has the added advantage that you can reserve the necessary space at the top of memory when you save this empty file, and you will then not need to remember the memory size when starting BASIC in the future. You should first J BASIC in the usual way (without loading these routines, of course), and specify 51200 memory available. Then use the CSAVE"B" command to save the workspace, which can then be loaded via an R command in future. BASIC can the be ‘cold’ started by the Z command. As only one block needs to be stored on tape this is still a quick operation. Next month we will mention an even faster solution! NAS-DOS users can of course save this empty file with a command ]B:BASIC, and then simply type ]E:BASIC to ‘cold’ start BASIC.

To avoid the danger of accidentally cold starting BASIC we have eliminated the J command in the SCAL table in RAM. NAS-DOS users