DFI image format - Revision history

Philpem: /* New-style images */ Fix another stupid logic bug

2012-04-25T23:45:01Z

‎New-style images: Fix another stupid logic bug

Philpem: /* New-style images */

2012-04-25T21:34:31Z

‎New-style images

Philpem: /* New-style images */ Fix to NEW-style decode algorithm

2012-04-25T21:31:58Z

‎New-style images: Fix to NEW-style decode algorithm

Balrog at 01:45, 16 January 2012

2012-01-16T01:45:40Z

Balrog: Added comments, thanks fjkraan!

2012-01-15T22:54:26Z

Added comments, thanks fjkraan!

Philpem at 18:27, 7 January 2012

2012-01-07T18:27:23Z

Philpem: Created page with "Category:Disc image files '''Format originator:''' DiscFerret software. == Overall format == All integers in this file format are stored in big-endian form. The DFI file ..."

2012-01-07T18:20:54Z

Created page with "Category:Disc image files '''Format originator:''' DiscFerret software. == Overall format == All integers in this file format are stored in big-endian form. The DFI file ..."

New page

[[Category:Disc image files]]

'''Format originator:''' DiscFerret software.

== Overall format ==

All integers in this file format are stored in big-endian form.

The DFI file consists of a 4-byte magic string, followed by a series of disc sample blocks.

The magic string is "DFER" for old-style DiscFerret images, or "DFE2" for new-style DiscFerret images.

Each sample block has a header --
<nowiki>
uint16_be cylinder;
uint16_be head;
uint16_be sector;
uint32_be data_length;
</nowiki>

The cylinder number starts at zero and counts up to the number of cylinders on the disk. The head number follows the same rule (starts at zero, increments for each additional head). The sector number is optional, and only used for hard-sectored discs. For soft-sectored discs, it is set to zero. Data_length indicates the number of bytes of data which follow.

== Decoding data ==

=== Old-style images ===
<nowiki>
carry = 0
For every byte in the stream:
if (byte AND 0x7f) == 0x00:
carry = carry + 127
else:
emit((byte AND 0x7f) + carry)
carry = 0
if carry > 0:
emit(carry)
</nowiki>

=== New-style images ===
<nowiki>
carry = 0
For every byte in the stream:
if (byte AND 0x7f) == 0x7f:
carry = carry + 127
else:
emit((byte AND 0x7f) + carry)
carry = 0
if carry > 0:
emit(carry)
</nowiki>

@@ Line 47: / Line 47: @@
      carry = carry + (byte & 0x7F)  // add lower 7 bit value to carry and absolute-position
      abspos = abspos + (byte & 0x7F)
-     add_index_position(carry)      // this store was caused by an index pulse: save its absolute position
+     add_index_position(abspos)     // this store was caused by an index pulse: save its absolute position
    else:                            // here byte < 0x7f
      emit((byte AND 0x7f) + carry)  // this store was caused by a data transition: store the time-delta since last transition

@@ Line 55: / Line 55: @@
 // Carry may be nonzero at the end of this loop. In this case, there was an incomplete transition, which should be discarded.
-// emit() stores the timing value
+// emit()                 stores the timing delta for a data pulse
-// add_index_position() stores the timing position of an index pulse
+// add_index_position()   stores the absolute timing position of an index pulse
 //
 </nowiki>

@@ Line 38: / Line 38: @@
 === New-style images ===
   <nowiki>
-carry = 0
+carry = 0   // running carry
 For every byte in the stream:
    if ((byte AND 0x7f) == 0x7f):    // if lower 7 bit value is 0x7f
-     carry = carry + 127
+     carry = carry + 127            // ... then there was a carry
    else if (byte AND 0x80) != 0:    // if high bit set in byte
-     carry = carry + (byte & 0x7F)  // add lower 7 bit value to carry
+     carry = carry + (byte & 0x7F)  // add lower 7 bit value to carry and absolute-position
-     add_index_position(carry)      // add high bit to carry
+    abspos = abspos + (byte & 0x7F)
    else:                            // here byte < 0x7f
-     emit((byte AND 0x7f) + carry)  // report carry + byte
+     emit((byte AND 0x7f) + carry)  // this store was caused by a data transition: store the time-delta since last transition
      carry = 0                      // reset carry
-if carry > 0:                      //
-  emit(carry)                      // report carry
+// Carry may be nonzero at the end of this loop. In this case, there was an incomplete transition, which should be discarded.
 // emit() stores the timing value

@@ Line 46: / Line 46: @@
      add_index_position(carry)      // add high bit to carry
    else:                            // here byte < 0x7f
-     emit((byte AND 0x7f) + carry)  // ?? the AND 0x7f is superfluous here. report carry + byte
+     emit((byte AND 0x7f) + carry)  // report carry + byte
      carry = 0                      // reset carry
 if carry > 0:                      //
    emit(carry)                      // report carry
-// emit() looks like a written value
+// emit() stores the timing value
-// add_index_position() is just setting the high bit of the last data value
+// add_index_position() stores the timing position of an index pulse
 //
 </nowiki>

@@ Line 40: / Line 40: @@
 carry = 0
 For every byte in the stream:
-   if ((byte AND 0x7f) == 0x7f):
+   if ((byte AND 0x7f) == 0x7f):    // if lower 7 bit value is 0x7f
      carry = carry + 127
-   else if (byte AND 0x80) != 0:
+   else if (byte AND 0x80) != 0:    // if high bit set in byte
-     carry = carry + (byte & 0x7F)
+     carry = carry + (byte & 0x7F)  // add lower 7 bit value to carry
-     add_index_position(carry)
+     add_index_position(carry)      // add high bit to carry
-   else:
+   else:                            // here byte < 0x7f
-     emit((byte AND 0x7f) + carry)
+     emit((byte AND 0x7f) + carry)  // ?? the AND 0x7f is superfluous here. report carry + byte
-     carry = 0
+     carry = 0                      // reset carry
-if carry > 0:
+if carry > 0:                      //
-   emit(carry)
+   emit(carry)                      // report carry
 </nowiki>