Updates to interrupt details

Following some questions about the interrupt details posted, i updated the end of the document to include the following paragraphs.

RET / RETI

You may have noticed that the interrupt routine ends with RETI rather than the standard RET, this is for completeness and not specifically required, you can end with RET if you prefer.  The main reason RETI was added to the instruction set is to allow hardware to detect when the end of the interrupt routine and to pass control onto the next interrupt in a dasiy chained system.  The hardware could specifically check the opcode for RETI.  If it checked RET it could get confused when coming to the end of a function rather than the entire interrupt routine.  To my knowledge no devices for the spectrum use this feature.

EI but not DI?

There is another oddity in the interrupt routine, we have an EI at the end but we never do a DI.  This is due to the fact the Z80 will do an internal DI when it starts processing the interrupts.  The reason for this is to stop multiple interrupts triggering on top of each other and overrunning the system stack.  You can do an EI before the end of your interrupt routine if you wish but general practice is to wait until you have finished then enable them, that way there is no chance of you starting a new interrupt routine before the old one has finished.