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zero when you initialise the system. A simple BASIC program of the type listed below would let you set the clock and read it. Because of the disabling of interrupts by NAS-DOS this clock will lose time during disc access. However, for many timing applications this does not present any problems, and it is a cheap and easy way of getting a clock/​timer on the computer. Naturally this type of clock will not run when the computer is turned off, and the time data is lost. The next memory location (CCA4) contains the buffer length, normally the maximum permitted value of 48 (decimal). You can reduce this if you wish – down to one to prevent typing ahead.

Note that there is a jump vector at C80C. This currently points to a RET instruction, which will be executed every 1/50 second. You could put in this vector a jump to your own routine to be executed every 1/50 of a second. You can try using this to display the time on the top line of the screen, and update it every second. Note that in adding such a routine you should be careful not to change any registers, or to make excessive use of the stack.

4. Clock read/​set from BASIC

The current time can be displayed by typing RUN. To set the clock you should type RUN 100.