Differences

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

--- doc:cbm:disk:image:g64 [2020/05/31 23:39] – eek
+++ doc:cbm:disk:image:g64 [2020/05/31 23:47] (current) – [Analysing the GCR data stream] eek
@@ Line 215: / Line 215: @@
 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
+one for the sector data. In the above example, there is 2 groups of ''"FF FF FF FF FF"''.
-FF FF FF". The first one is the header SYNC and the second one is the  data
+The first one is the header SYNC and the second one is the  data
 SYNC.
@@ Line 246: / Line 246: @@
 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
+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
+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
@@ Line 270: / Line 270: @@
 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