11 The emulator file formats

This chapter gives a technical description of the various files supported by the emulators.

11.1 The T64 tape image format

(This section was taken from the C64S distribution.)

The T64 File Structure was developed by Miha Peternel for use in the C64S emulator. It is easy to use and allows future extensions.

11.1.1 T64 File structure

11.1.2 Tape Record

11.1.3 File record

Valid entry types are:

Notes:

• VICE only supports file type 1.
• Types 3, 4 and 5 are subject to change (and are rarely used).

11.2 The G64 GCR-encoded disk image format

(This section was contributed by Peter Schepers (schepers@ist.uwaterloo.ca) and slightly edited by Ettore Perazzoli (ettore@comm2000.it).)

This format was defined in 1998 as a cooperative effort between several emulator people, mainly Per Håkan Sundell, author of the CCS64 C64 emulator, Andreas Boose of the VICE CBM emulator team and Joe Forster/STA, the author of Star Commander. It was the first real public attempt to create a format for the emulator community which removed almost all of the drawbacks of the other existing image formats, namely D64.

The intention behind G64 is not to replace the widely used D64 format, as 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 format. The best example is with speeder software such as Action Cartridge in Warp Save mode or Vorpal which write track/sector data in 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.

G64 has a deceptively simply 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 track data areas and speed zones which is really at the heart of this format.

Each track entry in simply the raw stream of GCR data, just what a read head would see when a diskette is rotating past it. How the data gets interpreted is up to the program trying to access the disk. Because the data is stored in such a low-level manner, just about anything can be done. Most of the time I would suspect the data in the track would be standard sectors, with SYNC, GAP, header, data and checksums. The arrangement of the data when it is in a standard GCR sector layout is beyond the scope of this document.

Since it is a flexible format in both track count and track byte size, there is no "standard" file size. However, given a few constants like 42 tracks and halftracks, a track size of 7928 bytes and no speed offset entries, the typical file size will a minimum of 333744 bytes.

Below is a dump of the header, broken down into its various parts. After that will be an explanation of the track offset and speed zone offset areas, as they demand much more explanation.

Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
----  -----------------------------------------------
0000: 47 43 52 2D 31 35 34 31 00 54 F8 1E .. .. .. ..

An obvious question here is "why are there 84 tracks defined when a normal D64 disk only has 35 tracks?" Well, by definition, this image includes all half-tracks, so there are actually 42 tracks and 42 half tracks. The 1541 stepper motor can access up to 42 tracks and the in-between 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.

At first, the defined track size value of 7928 bytes may seem to be arbitrary, but it is not. It is determined by the fastest write speed possible (speed zone 0), coupled with the average rotation speed of the disk (300 rpm). After some math, the answer that actually comes up is 7692 bytes. Why the discrepency between the actual size of 7692 and the defined size of 7928? Simply put, not all drives rotate at 300 rpm. Some can be faster or slower, so a upper safety margin of +3% was built added, in case some disks rotate slower and can write more data. After applying this safety factor, and some rounding-up, 7928 bytes per track was arrived at.

Also note that this upper limit of 7928 bytes per track really only applies to 1541 and compatible disks. If this format were applied to another disk type like the SFD1001, this value would be higher.

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. Following that is a dump of the speed zone offsets.

Addr  00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
----  -----------------------------------------------
0000: .. .. .. .. .. .. .. .. .. .. .. .. AC 02 00 00
0010: 00 00 00 00 A6 21 00 00 00 00 00 00 A0 40 00 00
0020: 00 00 00 00 9A 5F 00 00 00 00 00 00 94 7E 00 00
0030: 00 00 00 00 8E 9D 00 00 00 00 00 00 88 BC 00 00
0040: 00 00 00 00 82 DB 00 00 00 00 00 00 7C FA 00 00
0050: 00 00 00 00 76 19 01 00 00 00 00 00 70 38 01 00
0060: 00 00 00 00 6A 57 01 00 00 00 00 00 64 76 01 00
0070: 00 00 00 00 5E 95 01 00 00 00 00 00 58 B4 01 00
0080: 00 00 00 00 52 D3 01 00 00 00 00 00 4C F2 01 00
0090: 00 00 00 00 46 11 02 00 00 00 00 00 40 30 02 00
00A0: 00 00 00 00 3A 4F 02 00 00 00 00 00 34 6E 02 00
00B0: 00 00 00 00 2E 8D 02 00 00 00 00 00 28 AC 02 00
00C0: 00 00 00 00 22 CB 02 00 00 00 00 00 1C EA 02 00
00D0: 00 00 00 00 16 09 03 00 00 00 00 00 10 28 03 00
00E0: 00 00 00 00 0A 47 03 00 00 00 00 00 04 66 03 00
00F0: 00 00 00 00 FE 84 03 00 00 00 00 00 F8 A3 03 00
0100: 00 00 00 00 F2 C2 03 00 00 00 00 00 EC E1 03 00
0110: 00 00 00 00 E6 00 04 00 00 00 00 00 E0 1F 04 00
0120: 00 00 00 00 DA 3E 04 00 00 00 00 00 D4 5D 04 00
0130: 00 00 00 00 CE 7C 04 00 00 00 00 00 C8 9B 04 00
0140: 00 00 00 00 C2 BA 04 00 00 00 00 00 BC D9 04 00
0150: 00 00 00 00 B6 F8 04 00 00 00 00 00 .. .. .. ..

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
      -----------------------------------------------
0150: .. .. .. .. .. .. .. .. .. .. .. .. 03 00 00 00
0160: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
0170: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
0180: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
0190: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
01A0: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
01B0: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
01C0: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
01D0: 00 00 00 00 03 00 00 00 00 00 00 00 03 00 00 00
01E0: 00 00 00 00 02 00 00 00 00 00 00 00 02 00 00 00
01F0: 00 00 00 00 02 00 00 00 00 00 00 00 02 00 00 00
0200: 00 00 00 00 02 00 00 00 00 00 00 00 02 00 00 00
0210: 00 00 00 00 02 00 00 00 00 00 00 00 01 00 00 00
0220: 00 00 00 00 01 00 00 00 00 00 00 00 01 00 00 00
0230: 00 00 00 00 01 00 00 00 00 00 00 00 01 00 00 00
0240: 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00
0250: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0260: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0270: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0290: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
02A0: 00 00 00 00 00 00 00 00 00 00 00 00 .. .. .. ..

Starting here at $02AC is the first track entry (from above, it is the first entry for track 1.0)

The track offsets (from above) 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). From the track 1.0 entry we see it is set for $000002AC. Going to that file offset, here is what we see...

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
      -----------------------------------------------
02A0: .. .. .. .. .. .. .. .. .. .. .. .. 0C 1E FF FF
02B0: FF FF FF 52 54 B5 29 4B 7A 5E 95 55 55 55 55 55
02C0: 55 55 55 55 55 55 FF FF FF FF FF 55 D4 A5 29 4A
02D0: 52 94 A5 29 4A 52 94 A5 29 4A 52 94 A5 29 4A 52

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.

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
      -----------------------------------------------
1FE0: 52 94 A5 29 4A 52 94 A5 29 4A 52 94 A5 29 4A 52
1FF0: 94 A5 29 4A 52 94 A5 29 4A 52 94 A5 29 4A 52 94
2000: A5 29 4A 52 94 A5 29 4A 52 94 A5 29 4A 52 94 A5
2010: 29 4A 52 94 A5 29 4A 52 94 A5 29 4A 52 94 A5 29
2020: 4A 52 94 A5 29 4A 52 94 A5 29 4A 52 94 A5 29 4A
2030: 55 55 55 55 55 55 FF FF FF FF FF FF FF FF FF FF
2040: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2050: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2060: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2070: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2080: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2090: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20A0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20B0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20C0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20D0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20E0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
20F0: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2100: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2110: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2120: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2130: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2140: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2150: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2160: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2170: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2180: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
2190: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
21A0: FF FF FF FF FF FF .. .. .. .. .. .. .. .. .. ..

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:

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 dump of the speed offset entries above, we see that all the entries are in the range of 0-3. If any entry is less than 4, this is not considered a speed offset but defines the whole track to be recorded at that one speed.

In the example I had, there were no offsets defined, so no speed zone 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:

Speed entry $FF:  in binary %11111111
                             |'|'|'|'
                             | | | |
                             | | | +- 4'th byte speed (binary 11, 3 dec)
                             | | +--- 3'rd byte speed (binary 11, 3 dec)
                             | +----- 2'nd byte speed (binary 11, 3 dec)
                             +------- 1'st byte speed (binary 11, 3 dec)

It was very smart thinking 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 any particular order since they are all referenced by the offsets at the beginning of the image.

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