Differences

This shows you the differences between two versions of the page.

--- doc:cbm:disk:image:g64 [2020/06/01 01:27] – [File Format] eek
+++ doc:cbm:disk:image:g64 [2020/06/01 01:47] (current) – [Analysing the GCR data stream] eek
@@ Line 20: / Line 20: @@
 ===== File Format =====
-The intention behind G64 is not to replace the widely used D64 format, as
+The intention behind G64 is not to replace the widely used [[d64|D64]] format, as
-D64 works fine with the vast majority of disks in existence. It is intended
+[[d64|D64]] works fine with the vast majority of disks in existence. It is intended
 for those small percentage of programs which demand to work with  the  1541
 drive in a non-standard way, such as reading or writing data  in  a  custom
@@ Line 28: / Line 28: @@
 another format other than  standard  GCR.  The  other  obvious  example  is
 copy-protected software which looks for some specific data on a track, like
-the disk ID, which is not stored in a standard D64 image.
+the disk ID, which is not stored in a standard [[d64|D64]] image.
 One protection method that G64 has trouble emulating  is  data  alignment
@@ Line 41: / Line 41: @@
 are also hard to emulate.
-G64 has a deceptively simply layout for what it is capable of  doing.  We
+G64 has a deceptively simple layout for what it is capable of  doing.  We
 have a signature, version byte, some predefined size values, and  a  series
 of offsets to the track data and speed zones. It is what's contained in the
@@ Line 62: / Line 62: @@
 FIXME
+Now, why are there 84 tracks defined when a normal [[d64|D64]] disk only  has  35
+tracks? By definition, an image of a 1541 must include all the tracks  that
+a real 1541 can access, which is at most 42 tracks and 42 half tracks. Even
+though using more than 35 tracks is not typical, it was important to define
+this format from the start with what the 1541 is capable of doing, and  not
+just what it typically does. Some 1541 drives  may  have  problems  reading
+past track 40, and pushing  the  head  past  track  42  might  be  somewhat
+hazardous to the health of the drive as the head could get stuck.
+The typical value seen for the maximum track size is 7928.  This  is  the
+value used for 1541 images which use standard GCR encoding. This  value  is
+determined by the fastest write speed possible (speed zone 0), coupled with
+the average rotation speed of the  disk  (300  rpm),  and  assuming  normal
+Commodore GCR data formatting. After some math, the  answer  that  actually
+comes up is 7692 bytes. Allowing for a slower disk rotation of -3%, which
+would allow more data to be written, and  some  rounding,  7928  bytes  per
+track was arrived at.
+Even though it might appear so, it is very important to  know  that  this
+maximum track size value is not a fixed or  hard-coded  value.  This  value
+depends on the what the original  disk  was  and  the  GCR  encoding  used.
+Non-1541 images such as SFD1001 or 8050 will result  in  different,  likely
+larger, track sizes. Also, disks with non-standard GCR encoding like  those
+using V-MAX can result in tracks exceeding 8000 bytes.
+Since it is a flexible format in both track count and  track  byte  size,
+file sizes can vary greatly. However, given a few constants like 42  tracks
+with no halftracks, a consistent track size of  7928  bytes  and  no  speed
+offset entries, the typical file size will be 333744 bytes.
+In my investigation using MNIB (a utility by Markus Brenner  that  allows
+you to nibble a 1541 diskette to  the  PC  in  G64  format)  on  a  cleanly
+formatted 1541 disk (using the built-in 1541 format  command),  I  saw  the
+following numbers, compared with the defaults that MNIB uses:
+FIXME
+Note that  the  first  size  number  (7720)  is  larger  than  previously
+mentioned track size of 7692. Why? Likely the drive that I used  to  create
+and nibble the clean disk was rotating a little bit  slower  than  300  RPM
+(~299 RPM), so more data than normal was stored on each track. I calculated
+the percentage difference between my numbers and the established  benchmark
+of 7692, multiplied all my numbers by  this  factor,  and  arrived  at  the
+following chart:
+FIXME
+See how close the real numbers come to what MNIB uses?  I  can  attribute
+the differences of a few bytes to  my  own  rounding  errors.  Therefore  I
+conclude that the numbers MNIB uses can be taken as the standard  that  all
+-compatible G64 tracks should be created with.
+All of the  above  calculations  are  shown  here  to  establish  a  safe
+benchmark to create G64 images in the event that someday we can  copy  them
+back to a real 1541 disk. If the G64 track size was  too  large,  it  might
+happen that the track cannot be written back out. By using the  above  MNIB
+track size numbers, this problem should be alleviated.
+Below is a dump of the first section of a G64 file, showing  the  offsets
+to the data portion for each track and half-track entry.
+FIXME
+The track offsets require some explanation. When one is set to all 0's, no
+track data exists for this entry. If there is a value, it  is  an  absolute
+reference into the file (starting from the beginning of the file).
+If an image stored here only contains 35 tracks  (e.g.  a  standard  1541
+disk), then all the offset values for track 35.5 and higher will be set  to
+. This can be used to detect the maximum track count when converting to  a
+[[d64|D64]] image. Since [[d64|D64]]'s cannot hold over 40 tracks, and typically only  have
+, some information will be lost when converting a G64.
+From the track 1.0 entry we see it is set for $000002AC.  Going  to  that
+file offset, here is what we see...
+FIXME
+Following the track data is filler bytes. In this  case,  there  are  368
+bytes of unused space. This space can contain anything, but for the sake of
+those wishing to compress these images for storage, they should all be  set
+to the same value. In the sample I used, these are all set to $FF.
+Below is a dump of the end of the track 1.0 data area.  Note  the  actual
+track data ends at address $20B9, with the rest of the block being  unused,
+and set to $FF.
+FIXME
+Now we can look at the speed zone area. Below is a dump of the speed zone
+offsets.
+FIXME
+Starting at $02AC is the first track entry (from above, it is  the  first
+entry for track 1.0)
+The speed offset entries can be a little more complex. The 1541 has  four
+speed zones defined, which means the drive can write data at four  distinct
+speeds. On a normal 1541 disk, these zones are as follows:
+FIXME
+Note that you can, through custom programming of  the  1541,  change  the
+speed zone of any track to something different (change the 3 to  a  0)  and
+write data differently.
+From the above speed zone sample, all the zones  use  4-byte  entries  in
+lo-hi format. If the value of the entry is less than 4, then  there  is  no
+speed offset block for the track and the value  is  applied  to  the  whole
+track. If the value is greater than 4 then we have an  actual  file  offset
+referencing a speed zone block for the track.
+In the above example shown, there were no offsets defined,  so  no  speed
+zone block dump can be shown. However, I can define what should  be  there.
+You will have a block of data, 1982 bytes long. Each  byte  is  encoded  to
+represent the speed of 4 bytes in the track offset area, and is broken down
+as follows:
+FIXME
+It was very smart of the designers of the G64 format  to  allow  for  two
+speed zone settings, one in the offset block and another defining the speed
+on a per-byte basis. If you are working with  a  normal  disk,  where  each
+track is one constant speed, then  you  don't  need  the  extra  blocks  of
+information hanging around the image, wasting space.
+What may not be obvious is the flexibility of this format to  add  tracks
+and speed offset zones at will. If a program decides to write a  track  out
+with varying speeds, and no speed offset exist, a new block will be created
+by appending it to the end of the image, and the offset  pointer  for  that
+track set to point to the new block. If a track has no offset yet,  meaning
+it doesn't exist (like a half-track), and one needs to be added,  the  same
+procedure applies. The location of the actual track or speed zone  data  is
+not important, meaning they do not have to be in linear  order  since  they
+are all referenced by the offsets at the beginning of the image.
+FIXME
+===== Analysing the GCR data stream =====
+Since the information stored in the track data area is in GCR format,  it
+is not as simple to analyse as a normal 256-byte sector would be. Here is a
+dump of a portion of the GCR data, and what to look for...
+FIXME
+We need to establish a marker by which one can  start  to  interpret  the
+data. Always look for a group of at least 10 1-bits (two 'F's in a row  and
+a bit more), as they establish the SYNC mark. The 1541 actually writes  out
+a SYNC mark of 40 'on' bits (10 'F's in a  row).  Note  that  there  are  2
+groups of SYNC marks quite close together, one for the  sector  header  and
+one for the sector data. In the above example, there is 2 groups of ''"FF FF FF FF FF"''.
+The first one is the header SYNC and the second one is the  data
+SYNC.
+An important point here: some documentation refers to  the  minimum  SYNC
+mark as being at least 12 bits wide, and claims that one of  that  size  is
+still not entirely reliable. Thus Commodore chose to use 40  bits  for  the
+SYNC mark, making it impossible for the drive read electronics to miss.
+If the GCR data is not in the standard sector layout, then anything  goes
+for interpreting the data. If no standard SYNC  mark  can  be  found,  then
+there is no simple way to extract any useful data.
+Here's the layout of a standard low-level pattern on a 1541 disk. Use the
+above example to follow along.
+FIXME
+The 10 header info bytes (#2) are GCR encoded and must be decoded down to
+it's normal 8 bytes to be understood. Once decoded,  its  breakdown  is  as
+follows:
+FIXME
+The header gap (#3) is 8 bytes on an early model 1540/1541, but  9  bytes
+on a later model 1541 and 4040. The 1541 doesn't read the header  gap,  but
+simply waits it out to write out the  sector  data.  When  sector  data  is
+written, the SYNC mark is re-written as well.
+There is some controversy over the header gap (#3). Most people assume it
+to be 9 bytes of ''0x55'' characters, but the early 1540/1541 drives used  only
+. This caused an write incompatability with the existing 4040 disks of the
+day. In 1541 ROM revision 901225-3 this error was fixed, and now all drives
+write out 9 of the ''0x55'' characters for the gap. The book "Inside  Commodore
+DOS"  by  Immers/Neufeld  documents  the  write  incompatibilty  and   what
+corruption happens at a low level when writing to a disk with a header  gap
+of 8 bytes on a disk that normally expects a gap of 9 bytes.
+The tail gap (#6) is the unused space between the end of one  data  block
+and the start of the next. It will vary in size depending on what track you
+are on, how fast the drive that created the disk was rotating at, and  what
+program was used to format the disk. The stock 1541 format code is supposed
+to determine how big a track is and divide up the extra unused  space  into
+each tail gap. However, many disks will show a much larger tail gap between
+the last sector and sector 0. In tests that the author conducted on a  real
+disk, gap sizes of 8 to 19 bytes were seen.
+The 325 byte data block (#5) is GCR encoded and must be  decoded  to  its
+normal 260 bytes to be understood. For comparison, ZipCode Sixpack  uses  a
+byte GCR sector (why?), but the last byte (when properly rearranged) is
+not used. The data block is made up of the following:
+FIXME
+The most reliable way to read G64 track data is to read it as  bits,  not
+bytes as there is no way to be sure that all the data is byte-aligned. This
+simulates the way a 1541 drive reads data as well as the  head  only  reads
+bits as well. The starting location of the track data is know, as  well  as
+the track size so the boundaries of the track limits (start  and  end)  are
+obtainable.
+What follows is a very simply  point-form  list  of  how  to  read  data,
+finding sync marks, header blocks and sector blocks.
+  - Search for SYNC (at least 10 or more 1 bits)
+  - Check for header id after SYNC (GCR ''0x52'')
+  - If header, read the remaining 9 header bytes
+  - Decode header and get sector value
+  - Search for SYNC again
+  - Check for data id after SYNC (GCR ''0x55'').
+  - If data, read and store with previous header.
+  - Have we finished reading the track... stop
+  - Start over