Monday, January 15, 2018

Bringing the internal 3.5" floppy drive to life - part 2

Again, a longer post documenting the process of making the real 3.5" floppy drive work.  What I thought would be hard, the MFM decoding sector reading from the real disk, turned out to be quite easy -- taking only a day or so.  But what I thought would be easy, plumbing this into my existing F011 implementation, turned out to be a long string of strange bugs to track down, and took a week or so to work through.  Anyway, here it is.

So, yesterday, I managed to successfully decode MFM pulses from the 3.5" floppy drive, but only in a little C test program on my laptop.  Today, I want to move that into VHDL, make sure I can decode MFM data there, and then correctly parse the sector markers etc, to find a requested sector, and push the sector bytes out, and do all the other little bits and pieces, like CRC checking, required to plumb it into my F011 floppy controller implementation in the MEGA65. The result should be that the MEGA65 can read data from a real 1581 floppy disk in the 3.5" floppy drive.

Yesterday I had broken down the MFM parsing into a set of clearly defined steps: measure pulse gaps, quantise pulse gaps, turn pulse gaps into bits/sync markers, turn bits into bytes.  Turning those things into VHDL was rather easy, give or take the odd spot of debugging.  Similarly, implementing a parser that looks for sync marks and captures the track/sector numbers and compares them with requested track/sector, and works out whether the following sector data should be read out was also fairly easy.  Then came the CRC checks.

The 1581 uses a CRC check that is similar to, but not exactly like the CCITT CRC16 algorithm.  The C65 specifications manual provides an explanation and example routine:


Generating the CRC

     The  CRC  is a sixteen bit value that must be generated serially,
one  bit  at  a  time.  Think of it as a 16 bit shift register that is
broken in two places. To CRC a byte of data, you must do the following
eight  times,  (once  for each bit) beginning with the MSB or bit 7 of
the input byte.

     1. Take the exclusive OR of the MSB of the input byte and CRC
        bit 15. Call this INBIT.
     2. Shift the entire 16 bit CRC left (toward MSB) 1 bit position,
        shifting a 0 into CRC bit 0.
     3. If INBIT is a 1, toggle CRC bits 0, 5, and 12.

     To  Generate a CRC value for a header,  or for a data field,  you
must  first  initialize the CRC to all 1's (FFFF hex).  Be sure to CRC
all bytes of the header or data field, beginning with the first of the
three  A1  marks,  and ending with the before the two CRC bytes.  Then
output  the  most  significant CRC byte (bits 8-15) and then the least
significant CRC byte  (bits 7-0).  You may also CRC the two CRC bytes.
If you do, the final CRC value should be 0.

     Shown below is an example of code required to CRC bytes of data.

;
; CRC a byte. Assuming byte to CRC in accumulator and cumulative
;             CRC value in CRC (lsb) and CRC+1 (msb).

        CRCBYTE LDX  #8          ; CRC eight bits
                STA  TEMP
        CRCLOOP ASL  TEMP        ; shift bit into carry
                JSR  CRCBIT      ; CRC it
                DEX
                BNE  CRCLOOP
                RTS

;
; CRC a bit. Assuming bit to CRC in carry, and cumulative CRC
;            value in CRC (lsb) and CRC+1 (msb).

       CRCBIT   ROR
                EOR CRC+1       ; MSB contains INBIT
                PHP
                ASL CRC
                ROL CRC+1       ; shift CRC word
                PLP
                BPL RTS
                LDA CRC         ; toggle bits 0, 5, and 12 if INBIT is 1.
                EOR #$21
                STA CRC
                LDA CRC+1
                EOR #$10
                STA CRC+1
       RTS      RTS

It is super helpful to have an example implementation, as well as explanation. Nonetheless, it took me about 2 hours to actually get the CRC calculating correctly, as CRC routines are notorious to get exactly right, as they require considerable attention to detail and very sound comprehension of the algorithm.

Another interesting problem was how to test this in simulation.  I already have a debug register on the MEGA65 that allows me to read the FDC data read line. However, as some signals can be as narrow as 120ns, this requires sampling at at least 5MHz.  Using DMA I could sample it at around 20MHz, however, this meant being able to capture only a part of a sector.  And even at 50MHz, the CPU is fractionally too slow, unless I completely unrolled the data capture loop, in which case I would still only be able to capture a relatively few samples, as 64KB of data capture loop would only be able to cover about 10K samples.  What I realised after, was that I should just write a C program that MFM encodes a set of sectors, and feed that in.  If my existing C program for decoding MFM data can read it, then it should make fine test input data for the VHDL. Writing such a program would also be the first step towards being able to write data to floppies, so it is work that needs to happen, anyway.  If I have problems with the current work, I will certainly follow this path.

Then it was a case of debugging some out-by-one errors and the like on sector read lengths, and making sure the right flags are asserted at the right times.  Finally, the whole new MFM assembly had to be plumbed into the existing VHDL, and some new registers added to the SDcard (and now, floppy drive) controller, so that it is possible to select the SD card or the floppy drive as the source for C65 F011 floppy controller accesses.

At this stage, all the machinery is now in place for this to work, assuming that there are no bugs in my VHDL.  Because synthesis takes a long time, I have added some debug registers that will allow me to interrogate more closely what the floppy drive and MFM decoder are doing.  While it would be nice to think that I won't need them, and the test-benching of the MFM decoder with real captured data helps reduce the risks, I suspect I will be getting familiar with them.  We will see how that pans out in a few hours.

So, synthesis has finished, and the selection register to switch to using the real floppy drive seems to work, and the read request line gets asserted to the MFM decoder, but there is no sign of bytes being read from the drive.  So, I will do what I should have done before synthesising the first time, and debug registers to see right down to the lowest level of the MFM decoder.

Okay, next resynthesis, and I can see that it is finding gaps, and decoding bytes, but it never finds a sync byte. This is likely due to the register I setup for setting the number of cycles per MFM bit to be limited to too low a value, thus preventing the gaps from being properly detected.  I rather confused myself here for a while, because I couldn't remember definitively the data rate for a 720K drive. Was it 250K, 500K or 1Mbit?  It took a long time to actually find a list. It is 250K, so 4 usec per bit. At 50MHz, this means we should be using 200 as the cycles per interval. So I should be seeing gaps of around 200, 300 and 400 cycles.

So, in terms of gaps, I am seeing plenty of them around 200 cycles, corresponding to runs of identical bits. This makes sense.  I am also seeing a reasonable number around 400, which also makes sense, as well as some around 300. However, I am also seeing a lot of seemingly randomly distributed gaps, anywhere upto at least 1000 cycles, which is more than 2x the maximum we should see.  There is something clearly wrong here.  However, looking at the source code for the gap collecting code, it is outrageously simple, and of course, it works fine in simulation.  What is particularly curious is that the failure mode appears to be the missing of pulses, not seeing spurious pulses, for example, if there was high frequency glitching on the floppy read-data line.  Yet to be missing pulses is very strange, since the debug register that allows direct reading of the floppy read-data line shows no sign of these random-length pulse intervals. This makes me a little worried about intermittent glitches I have seen, where a couple of registers in particular are read with the wrong values.  As much as I would like to blame the synthesis tools, it is quite possible I have a subtle timing bug that might just be tickling things here.  My immediate next step is to resynthesise with the mfm_gaps module outputting a history of the values it has seen on the f_rdata line, so that I can see if it is indeed missing pulses.

And things get stranger.  While waiting for synthesis of the above debug register, I tried again, this time simply having the M65 try to boot the C65 ROM using the real floppy drive.  In this situation, the C65 ROM will try to load an auto-start file from the floppy.  And all of a sudden, I am seeing better distribution of gaps. I think that what is happening here is the ROM steps the head, which results in it being definitively on a track, where as if the drive was powered off, the head might have been able to move off track a little (or outside tracks 0 - 79).  I'm still not really sure, but it seems to be something mechanical.

To avoid the long resynthesis times,  I have imported my MFM controller into my joystick controlled test bench.  That way I can iterate after only a few minutes. The down side is I have very limited input and output, and can't capture and stream signal values.  This is what I used to work out the stepping the head helps.  However, even after stepping, while I see quantised gaps being detected, with essentially no invalid gap lengths, it is still not detecting any sync bytes.
So, I added yet another debug option (and had another 8,000 second synthesis) to log the quantised pulse gaps, and wrote a little program to interpret a capture of those signals.  That is, I tested the function of the pulse detection and gap quantisation.  And the results are good. Here is what I saw from my captured trace after decoding:

 $42 $42 $42 $42 $42 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72
 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72 $72
 $72 $72 $72 $70 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $05
Sync $A1
Sync $A1
Sync $A1
 $fe $00 $01 $01 $02 $fd $5f $4e $4e $4e $4e $4e $4e $4e $4e $4e
 $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $00 $00 $00 $00
 $00 $00 $00 $00 $00 $00 $00 $00
Sync $A1
Sync $A1
Sync $A1
 $fb $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
...

$00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
 $00 $00 $00 $00 $da $6e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e
 $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e
 $4e $4e $4e $4e $4e $4e $4e $00 $00 $00 $00 $00 $00 $00 $00 $00 $00
 $00 $00
Sync $A1
Sync $A1
Sync $A1
 $fe $00 $01 $02 $02 $a8 $0c $4e $4e $4e $4e $4e $4e $4e $4e $4e
 $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e $4e


First, we can see that Sync $A1 bytes are correctly detected.  Second, we can see that a complete sector is observed, with the $FE header ID byte and track, side, sector and sector size bytes. Then there is a $FB ID byte following the next burst of Sync bytes indicating the sector, and a completely empty sector, followed by two CRC bytes s($da and $6e), and the $4e and $00 gap bytes, before the header of another sector is visible. I did notice an error in checking this: I was pulling the header bytes out as Track, Sector, then Side, not Track, Side and then Sector. That's easy enough to fix.

So now we know that the intervals are being correctly quantised, and the stream of intervals that should be detected as a Sync mark are being generated.  Attention thus turns to the mfm_gaps_to_bits module, where the sync bytes are generated. Assuming that the problem must be in the collection and assembly of the strings of gaps into bytes, I am modifying my VHDL test rig so that I can sample the most recent four gaps at any point in time, and see that sample held.  I am wondering if the gap_valid signal is being asserted for more than one clock cycle, for example, which is causing a gap to be registered more than once.  It is also possible that gaps are being missed, but that will be harder to determine.

The logged sets of recent gaps seem to be okay, and I even managed to see a sync mark. So I added a counter for the number of times the sync marks are found. I also added a safety catch in case the strobes to indicate a new pulse has been seen were sticking around for more than one clock cycle.  Whether that was the issue or not, I am seeing of the order of 256 sync marks per second.  Given that there should be 5 rotations per second, and 10 physical sectors each with 2 x 3 sync marks per second, this means I should be seeing 5 x 10 x 2 x 3 = 300 sync marks per second.  This basically matches my eye-ball investigation of ~256 per second, watching the highest order bit of an 8-bit counter blinking about twice per second.  In short, it seems reasonable to conclude that sync marks are now being reliably recognised.  The question is whether that tweak to the strobe line handling fixed anything, or not. This is at least easy to test: Change one line of VHDL to remove the extra check I put in place, and then resynthesise the VHDL test rig. Hmm. Seems that it wasn't necessary.

So, it seems that the problem must be higher up the processing chain.  So I took a closer look at the top level mfm_decoder module, and noticed a couple of errors in CRC checking of sectors, and when the found_track/sector/side variables are set.  I then realised that my test bench tested that the sectors were found on the disk, but not that this information was communicated back up to the caller.  Needless to say I have fixed those now, and am resynthesising.

While waiting for synthesis of the whole MEGA65, I also made the same change to my VHDL test rig.  Finally, that is now finding sectors! I can also see which track I am on, from the reported track number, as well as watching the sector numbers cycle through.  Now I am impatiently waiting for synthesis to complete, so that I can test it in the MEGA65 again...

Resynthesis has finished, so now trying it out.  I have hit the problem that sometimes happens where some registers for the SD card interface and the non-existent input switches misbehave. This mean booting up without an SD card, and hence, without the F011 disk present flag being set by mounting a D81 image.  It turns out that this is the only source of the disk present flag at the moment. So I need to plumb the disk ready line. 

However, I have just hit the exact same problem that causes Amiga 600 and 1200 disk drives to tick constantly when there is no disk inserted: The /DISKCHANGE line does double duty as the disk present signal.  As it doesn't seem right for an 8-bit computer that is likely to only sometimes have a disk in the drive to tick like a bomb, I'll just lie and tell the F011 that it always has a disk inserted when it is talking to a real floppy drive.

I also found a similar problem in the logic that checks if a disk is available before dispatching a sector read or write job via the F011.  I have now fixed that logic ready for next synthesis. This was again because the existing F011 code assumed that the SD card was the only source of floppy disks.  It also meant that I could work around it by setting the disk image enable flag for floppy drive 0, so that it would look like a disk is available to the internal logic.

With those two fixes, the C65 BASIC no longer reports DRIVE NOT READY when attempting to run a DIR command.  Instead it hangs... because I don't assert the Request Not Found (RNF) flag if a requested sector is not found.  Reading the C65 specifications, the RNF flag should be set if the requested sector has not been found before six index pulses, i.e., within about a second.
This is implemented now, ready for the next synthesis run.

The hanging, I think, was because I had stepped the disk drive to a different track, which confused it, as the sector headers would not have matched, and so it would have kept on searching.  Now with the drive on the correct track, the DIR command returns a broken directory, that looks to me as though it is reading all zeroes from disk, or some similar problem.

Taking a look at the sector buffer for the F011, it looks like it is finding all zeroes.  Ah, that would be because I set the "match any sector" flag.  This is actually encouraging, as it means that it is reading the empty sectors on the disk.  With that flag turned off, DOS is back to hanging. Perhaps I have the side 1/2 head select flag wrong?  The request that it is currently stuck on is track 39 (this is floppy track numbering, the C65 is talking about track 40, the directory track on a 1581 disk), sector 1, side 0.  Checking the sectors that the MFM decoder is seeing (via the debug registers I added at $D6A3-5), I can see that it is in fact correctly finding the sectors for that track and side.  More specifically, I can see that it finds track 39, sector 1, side 0.  Yet it never seems to indicate to the F011 that it has read the sector, and the busy flag stays asserted, which is why DOS is hanging.

At this point, I could try to work my way through the C65 DOS to figure out everything that is going on. However, as the regular SD card mode of operation works fine, I am instead going to write a little test program that tries to drive the F011 to read a sector, and see if that works. If not, then I will know where the process is failing.  If it does work, then I can take a look at the DOS after all.

This reveals that reading a sector doesn't seem to properly complete. So I finally got around to writing a program that produces a whole sector's worth of MFM data to input into simulation, to see if that sheds any light on the situation. Found one more bug: sector_found was being reset the same cycle as the sector_end flag was being asserted, and the MEGA65 was not handling that correctly.  So I have added a fix for that, which is now synthesising.  I have also added some extra debug registers so that I can see if the MEGA65 thinks it is accepting sector bytes from the MFM decoder.  Synthesis time again.

Following synthesis of the above, I have worked out that the SIDE flag from the F011 registers is being inverted in sense, so I have pushed a fix for that, which will require another synthesis.  The next mystery is that the Request Not Found flag is still being set, even when the track and side are correct. Setting the match any sector flag solves this, by accepting any sector. So this suggests that there is still something faulty with the track/sector/side match logic.  Indeed, I can see that the found_track/sector/side matches what is being asked for from the F011.
This problem was caused by comparing unsigned values, instead of first casting them to integers.  This is one of many really annoying "features" of VHDL.

After various other little fixes, I can now see the correct sector is being found, and the various signals are being asserted, that should be telling the MEGA65 that bytes are ready for writing to the sector buffer as they are read from the floppy drive. However, still no bytes are read.  The combinations of signals that I am seeing, using the debug register $D6A1 as the source of my information are recorded by incrementing a byte of RAM corresponding to each value read using a routine like LDX $D6A1, INC $0400,X. In this way the contents of $0400-$04FF form a histogram, which I can see on screen as it is gathered, and yet the sampling rate is still able to be ~1MHz.  This will miss some instances of short-lived signals, however, by repeatedly plotting in this way, those will still tend to show up over time.  So here are the combinations of signals I am seeing:

fdc_sector_end - Very frequent. This is pulsed each time the end of a sector is reached, whether or not it is the sector we are looking for.

fdc_sector_found - Is held for a period of time, ~5 times per second, i.e., each time the sector we are searching for passes under the head.  This is encouraging, as it means we are finding our sector quite reliably.

fdc_sector_found | fdc_sector_data_gap - As above. This indicates that we have passed the sector header, and are now in the gap between the sector header, and the data of the sector itself. Again, a very healthy sign.

fdc_sector_found | fdc_byte_valid - This also pulses a number of times each rotation, indicating that the MFM decoder has the bytes off the sector to serve.

fdc_sector_found | fdc_byte_valid | fdc_first_byte - This gets indicated only once per reading of the sector, to indicate the first byte of the sector is being presented. Again, a very healthy sign.

So, we have clear evidence that the sequence of signals is more or less correct when the MFM decoder is doing its job.  It should be noted that these are generated automatically, whether or not the F011 has asked for a sector to be read or not.  When a read sector command is given, there is an extra signal that is asserted by the F011 so that it knows that it should accept the bytes presented by the MFM decoder, fdc_read_request.  By running a bunch of read requests while my histogram is collecting, I can verify if all of these combinations also occur while fdc_read_request is asserted. 

Because the histogram is running continuously collecting, I have to manually issue the read command via the serial monitor, so the number of samples is much smaller.  As a result I didn't detect any instances of the rather rare combination of fdc_sector_found | fdc_byte_valid | fdc_first_byte, however the face that all of the others are showing up gives me confidence that it should be showing up.

For a byte to be written, fdc_read_request, at least one of fdc_sector_found and fdc_sector_end, and fdc_byte_valid must be simultaneously asserted. From my histogram, I can see that fdc_read_request | fdc_byte_valid | fdc_sector_found happens quite frequently, as one would expect. That is, the conditions are satisfied for writing the bytes read from the floppy into the buffer. However, there is not so much as a single byte changed in the buffer. The logic that makes this decision consists of a couple of nested if statements, so I might put something in one of the outer if statements, so that I can see if it is getting there.  I'll also go through the synthesis warnings to see if there is anything fishy there that looks like it could be causing this problem.

Nothing fishy turned up in the synthesis warnings, however instrumenting those if statements also showed that it never gets to the appropriate test.  A little further searching, and it looks like the fdc_sector_end signal was not being handled correctly: Whenever it occurred -- whether it was the end of the sector that was being searched or not -- it would cancel the current read request.  I have now added this extra check, and am resynthesising. 

As I was thinking about this bug, I was initially thinking that this should mean I have a 1 in (sectors per track) chance of reading a sector. However, it instead required that the read request be initiated in the gap after the end of the sector just before the one desired, and the sector to be read.  This was quite a comforting revelation, as it explained why I was never seeing a sector being read -- because the probability was probably less than one in a thousand that this condition would be met. Even when set to accept any sector, the inter-sector gap is only a small percentage of the length of a sector.  Now we await synthesis to see if this theory is correct.
 
Indeed, with that fix, the sector data thinks it is being read into the sector buffer. I say thinks, because the sector buffer data doesn't change.  I lost several hours to this problem, until Falk reminded me that we have two copies of the sector buffer, one where the CPU controls access to it, and the other where the SD card writes directly into it, because the SD card can't be held off while the CPU is looking at the sector buffer, and vice-versa, when the CPU is reading from the buffer, there isn't a mechanism to add an extra wait-state if the SD card (or now also the floppy controller) is writing to the buffer.  There is special logic that allows each side to cause writes to happen to the other copy of the sector buffer, and I had forgotten to implement that for floppy reads. That is now synthesising.

Although the data itself is not being read into the sector buffer, in theory, the status signalling should be mostly complete, and I should be able to boot the C65 ROM with the real floppy drive enabled, so I tried doing this. This revealed that there is a problem with the track stepping: The DOS gets stuck looking on boot trying to read 1581 track 40, sector 0, because no data ever arrives, which is because the floppy drive has stepped only to track 38, not track 40. I'm not quite sure what the cause for this is.  If I manually step the drive forward to the correct track, the read can then complete, although I think I am seeing a problem with the buffer empty/full flag being erroneously set to indicate the buffer is empty, rather than full. This is used by the C65 ROM to work out if a sector has been read or not.  This might just be because of how I am probing the registers to debug, as the EQ flag inhibit when reading gets cleared if $D087 (the port register to the sector buffer) is read. Since I am reading 16 registers at $D080-$D08F as a batch, this would cause that problem.  So I expect that this is a non-problem. So it really just leaves the track stepping problem.

The C65 DOS steps tracks assuming that this works without problem, and always lands on the correct track. The routine for seeking to a track is at $9B98, and looks something like:

findtrack:
   phy
   ldy #$00
   ldx $1FE8    ; presumably the current drive ID
   lda $d084    ; The target track
waitforready:
   jsr $9A88    ; Wait for F011 busy flag to clear
   cmp $010F,x ; compare with the track we think we are on?
   beq foundtrack
   bcc lowertrack

   inc $010f,x   ; increase the track we think we are on?
   ldy #$18
   bra issuestepcommand

lowertrack:
   dec $010f,x      ; decrease the track we think we are on?
   ldy #$10

issuestepcommand:
   sty $D081     ; Tell F011 to step
   bra waitforready

foundtrack:
   tya
   beq done      ; if we didn't step, skip head settling time
   jsr waitforheadsettle

done:
   ply
   rts

Single-stepping through this routine, it seems to do what it should. However, after that $14 gets written to the command register in the waitforheadsettle routine. And that's where the problem is: I was interpreting that as a head step command, because the high-nybl is $1.  Again, a couple of lines to fix it, and probably a couple of hours of waiting for synthesis to run.  Then I found that the F011 disk-side select line was inverted, and I wasn't updating the pointer to the location in the second copy of the sector buffer, i.e., the one that the CPU sees.  So, it's off to synthesis again, but very much with the feeling now that it is the last few barriers this time.

It turns out there were still a number of other little niggly bugs to track down with filling the sector buffer, which once solved frustratingly still haven't quite got it working.  Sectors do now get read, and loading a directory goes through the motions of working -- but it is reading the wrong sectors for some reason.  Or perhaps looking at the wrong halves of the sectors, since the 1581 uses standard 512 byte sectors as the on-disk format, which each contain two logical 256 byte sectors, presumably because the PC-style floppy controller IC it uses doesn't support 256 byte sectors, or because using 256 byte sectors would have resulted in a slightly lower disk capacity.  Whatever the cause, the question is how SD card and floppy can behave differently, when all the buffer pointer management is the same regardless of whether a sector is read from the SD card or from the floppy drive -- and yet the problem only occurs when reading from the floppy drive.  Thus there must still be some subtle difference. 

To try to figure out what the difference might be, I tried loading the same sector from both SD card and from the real floppy drive.  Lo and behold, the sector from the floppy drive was rotated by one byte through the sector buffer.  Looking through the source code, I can see that the buffer pointer in question was not being zeroed when the read sector job was accepted, i.e., the part common to both SD card and floppy drive access, but instead in the setup stage for SD card access. Satisfying that I can see that this makes sense. In retrospect, I saw that the buffer write addresses were out by one in simulation, but the consequences didn't fully occur to me at the time.  Anyway, time for synthesis again...

Okay, so now the sector buffer rotation bug is fixed, and yet DIR still shows gibberish, as though it is reading the sector data incorrectly.  The C65 DOS uses unbuffered reading, where it accepts each byte as it is supplied by the F011 floppy controller, rather than waiting for it all to arrive. So I wondered if there was some sort of bug in that handling -- although, again, for SD card reads, it works without problem.  So I wrote a test program to make sure that unbuffered reading works correctly, and the correct data is read out, and in order. All seems to be correct there.  However, when trying to load the directory of a disk, it still looks very much like it is accessing the wrong half of the sector.

Unlike for the sector write offset, I can see no difference in the way the read pointer into the sector buffer is handled between the FDC and SD card data paths. Yet, like the last time I said this, there must be some difference, or else it would be working.

The F011 has a SWAP bit, that allows the halves of the sector buffer to be switched from the CPU's perspective, and the handling of this is the prime suspect as the root cause for this bug.  Single stepping through the C65 DOS, it turns out that this is not the problem.  Rather, the problem is that the floppy drive reads data at a slower rate than the CPU can read it, and the EQ flag that indicates if the buffer is empty claims not-empty as soon as the sector starts being read, regardless of whether the data has been read far enough or not.  As a result, the problem is actually that the C65 DOS is reading the contents of the buffer, before it has been filled from the floppy drive.  So the SWAP flag can stay as it is, but the EQ flag requires correction.

In the end, I looked at the whole handling of the EQ flag, and with the distance of time from when I first implemented, it seemed rather over complicated to me.  So I re-worked the entire sector buffer handling code, so that it now works much more like the real F011, and the duplicate copies of the sector buffer to solve bus contention have been reduced to a single buffer, with a bit of clever pre-fetching and write-buffer. The result is a whole lot simpler, and, it works.  FINALLY I can stick a 1581 disk in the real floppy drive, type DIR from C65 BASIC, and get a directory listing. Here is one I specially formatted the other day using my resurrected 1581:



And here is an old disk from the mid-90s when I was using the C65 I had at the time as my main computer:


I tried a bunch of my other old 1581 disks, but only a couple would work.  What I don't know is whether that means those disks have simply rotted away, which is entirely possible after a 20+ years, or that my error correction for MFM decoding could do with a bit more work.  I guess I will pull them out again at some point, and do some data captures from them, and from there work out whether my error correction could be improved. But adding write-support to the floppy drive is a higher priority than that.

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