Posted By
JamesC
on 2016-12-26
14:25:34
|  Re: PC RS232 Mouse project for Plus/4 User port_a cheap and working one :-)
If you only seek 3 bytes in ZP, consider the odds of someone RENUMBERing a program while using a mouse. Practically zero, since entering or modifying a program is a keyboard activity. This frees $03-$06 for your mouse pointer data. 
Will your interface only support one RS232 device at a time? If so, we cannot use a mouse and a modem at the same time. If we give up tape support too, this frees $0332-$0472 (320 bytes) for your mouse driver. (The RS232 buffer is in this area, which is why I ask about modem and mouse together.)
We don't have a list of programs known to use the application ZP area at $D8-E8. You would not be happy to write your code to use that area, then later discover that your graphics program (or flight simulator, or word processor) is overwriting your mouse data.
Similarly, we don't know with certainty that someone hasn't already appropriated the Speech areas for their own program(s).
At least we know what uses a modem or a tape drive. 
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