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### 17.1 The raw tape image format

(This section is based on v1.1 TAP specification by Per Hakan Sundell and Markus Brenner, additional format v2 expansion by Markus Brenner, plus some editing by groepaz)

Designed by Per Hakan Sundell (author of the CCS64 C64 emulator) in 1997, this format attempts to duplicate the data stored on a C64 cassette tape, bit for bit. Since it is simply a representation of the raw serial data from a tape, it should handle *any* custom tape loaders that exist.

The TAP images are generally very large, being a minimum of eight times, and up to sixteen times as large as what a raw PRG file would be. This is due to the way the data is stored, with each bit of the original file now being one byte large in the TAP file. The layout is fairly simple, with a small 14-byte header followed by file data.

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F        ASCII
          -----------------------------------------------   ----------------
    0000: 43 36 34 2D 54 41 50 45 2D 52 41 57 00 00 00 00   C64-TAPE-RAW????
    0010: 51 21 08 00 2F 0F 0D 31 64 1D 26 0D 07 21 0A 12   Q!??/??1d?&??!??
    0020: 4A 2F 2C 34 07 18 0D 31 07 04 23 04 0D 42 0D 1E   J/,4???1??#??B??
    0030: 34 04 42 0D 20 15 5E 04 0D 18 61 0D 26 29 34 0D   4?B???^???a?&)4?
    0040: 23 0D 07 0A 3F 55 04 0A 13 3F 07 0D 12 2B 18 0A   #????U???????+??

Offset | Description
$0000-000B | File signature (`C64-TAPE-RAW` or `C16-TAPE-RAW`)
$000C | `G64` TAP version (see below for description), $00 - Original layout, $01 - Updated, $02 - Halfwave Extension
$000D | Computer Platform (0 = C64, 1 = VIC-20, 2 = C16, Plus/4)
$000E | Video Standard (0 = PAL, 1 = NTSC1, 2 = NTSC2)
000F | reserved for future expansion
$0010-0013 | File data size (not including this header, in LOW/HIGH format) i.e. This image is $00082151 bytes long.
$0014-xxxx | File data

In TAP version $00 files, each data byte in the file data area represents the length of a pulse, when the C64’s hardware will trigger again. This pulse length is determined by the following formula:

        pulse length (in seconds) = (8 * data byte) / (clock cycles)

Therefore, a data value of $2F (47 in decimal) would be:

        (47 * 8) / 985248 = .00038975 seconds.

A data value of $00 represents an "overflow" condition, any pulse length which is more that 255 * 8 in length.

The value of "clock cyvles" from above (985248) is based on the PAL value. Since this file format was developed in Europe, which is predominantly PAL video, this is only logical. The NTSC value would be 1022730, which is very close to the PAL, and therefore won’t cause a compatability problem converting European and NTSC tapes. I would stick to using the PAL value just in case.

In TAP version $01 files, the data value of $00 has been re-coded to represent values greater than 255 * 8. When a $00 is encountered, three bytes will follow which are the actual time (in cycles) of a pulse, and the above formula does not apply. The three bytes are stored in LOW/HIGH format.

Version 2 is an extension made by Markus Brenner for C16 tapes. It is version 1, but each value represents a halfwave, starting with a ’0’->’1’ transition. The time encoding doesn’t change.

The actual interpretation of the serial data takes a little more work to explain. The typical ROM tape loader (and the turbo loaders) will initialize a timer with a specified value and start it counting down. If either the tape data changes or the timer runs out, an IRQ will occur. The loader will determine which condition caused the IRQ. If the tape data changed before the timer ran out, we have a short pulse, or a "0" bit. If the timer ran out first, we have a long pulse, or a "1" bit. Doing this continuously and we decode the entire file.

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