IBM DISK FUNCTION DIAGNOSTIC 309 LOADED INTO CORE
IBM wrote a diagnostic to shake down a 13SD disk drive (the drive that is inside the IBM 1130 as well as separately mounted in an enclosure as the 2310).
The diagnostic identified two flaws in the software simulation of disk drives in the IBM 1130 simulators that run on my laptop. These are the last two tests of the diagnostic, checking for unusual cases either writing a single word sector or writing too many words until the write fails. This will be a great test of my emulation logic in the FPGA.
Both of these behaviors are produced by the disk controller logic inside the IBM 1130, not by the disk drive. Thus, I have every reason to believe that my logic is going to breeze through those two tests. However, it was worth testing just to be certain that I don't do anything to confuse the disk controller.
LOADED THE DIMAL CARTRIDGE FILE IN THE V2315CF
I inserted the microSD card into the V2315CF main box, switched the V2315CF interface board to virtual mode, then powered up the 1130 system. Flipping the Load/Unload switch to load resulted in this cartridge being loaded into the virtual disk drive and after 90 seconds becoming ready for testing.
I used my core memory loader feature to load memory in the 1130 with the contents of the 309 Disk Functional Test diagnostic program. It restarts when you push Prog Start with the machine reset, then setting the console entry switches to 0181 and pushing the interrupt request button on the keyboard restarts and pauses test 309.
Issuing two more interrupt requests with the console entry switches set to 8000 and then 01C0 fired up the diagnostic, which typed out each test as it completed as well as a summary at the end.
DIAGNOSTIC BEGINS AND MOVES INTO ROUTINE 1 - GETTING ERRORS
The diagnostic first seeks backwards and confirms the Home cylinder is detected. It verifies that it receives the four sector numbers 0, 1, 2 and 3 in the correct sequence. It times the speed of the machine and prints out the 1130 cycle time. Lastly it seeks to cylinder 199 and reads sector 3 which contains a specific data word if the cartridge was initialized as a CE testing pack. After this initialization testing is complete, it begins running the test routines starting with routine 1.
I began to see messages printing out reporting seek errors. Routine 1 starts by seeking to cylinder 0 (Home) and verifying by reading all eight sectors. It then moves around in various patterns, reading to check the achieved location. For each seek error, it flagged the movement (from location X to target Y) along with the cylinder that was actually read.
My first error was from 199 to 000 but the data read in was from cylinder 14. The second error shows that the software tried to move from cylinder 14 to zero but now found itself at cylinder 12. This continued until I stopped running the diagnostic.
THE V2315CF TRACKS THE CURRENT CYLINDER AND RETURNS DATA FROM THERE
My logic is supposed to monitor every seek command and simulate the achieved position of the disk arm. That also determines which data is returned for a read operation. What I can't tell is what cylinder the 13SD disk arm reached. All we know is that the diagnostics code, the 1130 disk controller logic and the V2315CF were not in sync.
The diagnostic subroutine that does a seek is given a target cylinder but is tracking the previous location, so that it can request a seek of a given number of cylinders from the 1130 disk controller logic. The controller logic converts that into a number of two cylinder seeks with a final one cylinder seek if the count is an odd number. The 13SD disk drive and the V2315CF should respond to those one and two cylinder seeks by moving the arm that amount.
The source of the problem could lie in various places. The diagnostic code could have miscalculated the seek distance to request, thus moving both the 13SD arm and the V2315CF simulated arm to the wrong position. The 1130 disk controller logic could have failed to convert the seek count into the proper number of one and two cylinder movements Finally, the V2315CF might have missed movement requests or spuriously detected additional requests.
The failure appears to be moving smaller distances that was intended - so I don't believe there are spurious movement detections in the V2315CF. It is possible that we are not correctly seeing all of the movements from the 1130 disk controller.
However, I also have to consider that the 1130 disk controller might have bad logic, as it has not been fully shaken out prior to running this diagnostic. The diagnostic code might have been corrupted on the card decks I used to load it into core memory, thus miscalculating the target movement.
NEXT STEPS DEBUGGING THIS
I have a few ways I can identify the failing component - diagnostic, disk controller, or V2315CF - in order to dive deeper into debugging. The debug output of the V2315CF should record every seek command detected by my logic - if that shows that, for example, the seek from 14 to 0 really only did a seek of 2 cylinders, then the flaw is more likely to be in the diagnostics or disk controller logic.
I can try to manually recreate the seek pattern and read a sector with hand entered code. That is, I can first seek to cylinder 199, the back 199 positions and do a read. If I am at cylinder 0 after this, I will become suspicious of corruption to the diagnostic. If I am at cylinder 14, this confirms the issue is in disk controller or V2315CF. Comparing what my seek attempted to the seeks detected on the debug output of the V2315CF will shed more light on the failing.
Signal integrity issues might lead the V2315CF to see a two cylinder movement as a single cylinder movement, or do a few forward seeks when they should be in reverse. That would be immediately obvious from the debug output.
Ultimately I may need to use a logic analyzer to capture the activity of the disk controller performing a seek of 199 cylinders in reverse, identifying whether it counted correctly and issued 99 two cylinder and a single one cylinder seek in reverse.
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