Did you miss me? Who am I kidding no one reads this, I will be lucky if Weebly is around when I am old and losing it. (When I will need to look back at these entries and remind myself of this time in my life.) I write for the enjoyment of it, like a graffiti artist spraying on a subway wall. This is me marking the world in some way.
So what have I been doing, well more Plato PLATO PLATO stuff on the speccy. Having refurbished the old machine, checked the Interface1 is "safe", made an RS232 lead up. What do you think I did. I loaded the PLATOTERM up and connected it to IRATA.ONLINE. Bloody worked first time, got the lead correct the code worked every thing went well. Up until I decided to write a note to claim dibs on 48K on-line. Pressed space and the program stopped. After a few repeated attempts, the pattern of the problem wasn't a coincidence. You have some room for doubts with a machine over 30 years old, an interface of similar age with no knowledge of history. Touching a Sinclair Spectrum and having it randomly crash is not unusual behaviour many people have been known to tape EVERY thing down on the table to prevent crashes.
Space on the old rubbery key 48K is a special key, it doubles as the BREAK key. Usually in combination with the Caps Shift key, to invoke the BREAK function. Looking at my modern keyboard its labelled as Pause, if you have one on your keyboard its probably dusty but in pristine condition. Its rarely used by most people, there is probably one person that will read this and say "I use it all the time". Great, what for?! Traditionally Break was the big red button for stopping programs that are executing. In today's multitasking multi threaded applications its a little difficult to know what you want to break! So knowing a little history, spotting the blanking (black ink on black paper) at the bottom of the screen it eventually clicked. Hitting space which I didn't need to do signing on, selecting menu items etc was causing the Break to fire.
OK so why do we get a break event on a 48K and not on the +3. Thom's cool head got me in to line, back to basics he sent over 2 programs. One a key code print program, easy press key get 0x## hex code for the key. The second program was a tiny terminal, basically read a key send the key check for RS232 inbound data. Nothing exciting about the key codes. Really didn't take long to find the fault was still present with the tiny terminal. In a way its nice to know the main PLATO application wasn't at fault. After some playing about with tiny terminal it became apparent the break key was detected if Space/Break was pressed during and RS232 operation. Having looked at the fairly empty library files and Z88DK dev team confirming what was suspected. The library files are just wrappers for the ROM code routines. Z88DK developers gave Thom a bunch of tricks to try and clear the keyboard buffers etc to avoid the problem. I suspected the problem would inevitably be the ROM code itself, based on gut/experience. (Sods law, Murphy's law its that one place we don't want the problem to be!) The tricks worked well enough that I managed to post a note. It went from 100% failure on pressing space to 1%, you have to try to break out but it can still happen.
Never expected to ever have a reason to look through the ROM disassembly of the spectrum. Surprising how the simplicity and almost beauty of assembly comes back to you. Nice to see comments in code to! Slapped boldly just before the bit banging of the line for data transmission, a call to check for "Break key". ...Sigh... Not a full check for break and Caps Shift, no just a quick cheeky check on the one key. Why would you do that?! From what I can tell, the RS232 port was only really intended for use with printers (back in the day serial printers were a thing). The ROM routines had been written with this in mind, hitting space/break would let you quickly stop a print job. The CPU bit bashing the port would lock the machine in to a fairly tight loop, this was the quick safety feature to stop printing. A design feature that would have stopped any one using basic for writing even a simple terminal application. If Clive Sinclair really wanted his machines to hit the Business market terminal applications would have been a big selling point. Granted there were modems devices sold, but a basic serial port enabled machine opens doors. Its impossible to know what was going through someones head at the time. May be the code was rushed known to be buggy so they just sold it as to be used for printing.
Looking at the disassembly of the IF1 ROM thankfully its well documented with comments. Finding the serial routines was not difficult, commenting them out wont be difficult. Getting the entry and exit conditions well that might take a little work.
One of the fun things about doing this sort of thing is leaving little Easter egg comments in the code. Some day it might snake its way upstream and sit in code that someone reads through looking for answers. It might just make someone smile at a time when they really need it! (P.S. Sorry if this code has screwed your project up!)
; OUT ($FE),A ; Change the border colour. - No Thanks Jetset Willy change the borders in program if wanted
Hard part is trying to get the ASM code to start and end as required so not to crash. I have a lot of crashing going on, which is nice because I am changing things and it is having an effect on the system. Keep at it and we will see how long this takes...
Me, slightly crazy engineer type. Generalist in nature, hardware or software with nonspecialist skill set.