This is a bit of a strange issue that came up during some recent testing.
It only happens if you load the PRG version of Rodman from a 1541 disk drive onto a VIC20 and run it immediately before the disk spins down.
When you do that, the disk activity light goes out, but the drive keeps spinning. The game runs fine, it is just the disk drive that behaves oddly.
It only if the drive is still spinning, there is not time to type R U N and press RETURN, so it is only automatic starts like the Penultimate Cartridge file browser. I also check it does the same thing if you have the BASIC 4 ROM if you do SHIFT + RUN/STOP and have Rodman as the first program on the disk. It both cases, the game starts immediately after loading.
I decided to wire up the logic analyser and have a look what is going on. Here I have the IEC bus side of Data, Clock and ATN, and the IO chip select line for the VIA chips.
This is the end of loading Rodman.
Looks OK to me.
For comparison, this is the end of loading Bertie the Ball, which does not leave the disk spinning.
I zoomed in a little closer for the screenshot, but it is doing the same thing. The three ATN pulses at the end to ask the drive to stop talking, close the file and release the bus.
Hmm. I was thinking it might be some weird edge case if the file was a multiple of some block size or off by 1 etc.
It was only when I zoomed out that I noticed a difference.
At some point, there is some IO activity and ATN gets asserted again and the drive thinks it is going to be asked to do something so keeps the rust spinning.
This is Bertie the Ball, you can see there is some IO activity, but the IEC lines are unchanged.
I think this might be a similar issue to that of the autofire on Bandits I saw previously.
The problem was with the port A output register on the userport VIA. The pins on there control IEC ATN line output, and are inputs for datasette sense, IEC clock and data and 4 of the 5 joystick directions (don't ask).
The PA7 output drives a 7406 inverter, which will assert the signal (pull the bus low) when PA7 is logic 1.
In Bandit's case, it was writing to that port as part of the initialisation and changing the value of bit 7 to 1. This asserted the ATN signal and because of a previous change to the other signals caused the drive to react, which meant the signals read from port A were different if a drive was connected.
I asked Misfit (the creator of Rodman) if he was writing to $9111, and he was good enough to check and found no writes to that address in his code (or the alternate version at $911F).
Odd.
He sent me the code, which consisted of 7 POKEs, setting up various registers on the two VIAs, but not including $9111.
POKE(0x911C,0xFE);
POKE(0x911E,0x7F);
POKE(0x912E,0x7F);
POKE(0x9112,0x00);
POKE(0x9113,0x00);
POKE(0x9122,0x00);
POKE(0x9123,0x00);
I zoomed in to the point where ATN was asserted, and I could see those 7 POKEs (it's quite neat when you can actually visualise code happening)
You will see the spacing of those is not quite even. This again is visualisation of code. Due to clever ordering the statements, the compiler is able to optimise out several of the LDA statements where the same value is written to several addresses, and has already been loaded.
Looking at the trace, you can see it is the fifth POKE which causes the problem.
POKE(0x9113,0x00);
Ah, of course. That is writing to the data direction register for port A, and setting all the pins as inputs.
The ATN out pin, controlled by bit 7, should be an output, as it directly driving the input of a 7406 inverter.
When bit 7 is set as an input, the input of the 7406 buffer will be floating, most lightly read as a 1. In practice, this depends on make, type and age of the VIA, how strong the pullups on the input pins are, and also on the make, type and age of the 7406 if it has an input pullup or what it will consider a logic 1.
This was first spotted by a colleague using Vice with "true drive sounds" on and it continued the "disk spinning" sound after the game had started.
I was testing here with a VIC20-CR and a Mini VIC and a 1541 drive.
The internal pullup in the 6522 most likely means that the ATN signal will be asserted. If this happens before the drive has spun down, it will keep the disk spinning as it thinks it is about to receive a new command.
It probably does this on an SD2IEC as well, but you wouldn't notice as there is no sound.
I tried a few more games, and noticed the same thing on Bolder Dan.
Here there are 20 pokes this time, the 4th one seems to assert clock and / or data, and the 13th asserts ATN.
How to fix this?
Ideally you would set bits 7 as an output and write 0 to those bit 7 in $9111. Both of which should already be the case, so the easiest fix is simply to remove the fifth POKE.
I am not sure this actually needs to be fixed, since it only affects someone using a Penultimate Cartridge and a 1541 disk drive to load a game which was never released on disk, and is already in the Penultimate Cartridge. Normal loads on any device are fine as the drive normally spins down before you have had time to type in R U N and press enter.
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