Last night I changed the PushBit() routine. After a Sync marker is found it now builds an array of bytes filled with MFM data, up to a maximum of 1080 bytes. This is 4 bytes of the data above, 16 bytes sector label, 4 bytes header checksum, 4 bytes data checksum and 512 bytes sector data. 540 bytes in total times 2 since we’re storing clock data too.
Once filled I’m just writing the 540 bytes out to disk for inspection with a hex editor.
Look what we have here:
00000000 FF 00 00 0B 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000010 00 00 00 00 00 00 00 04 54 15 11 00 44 4F 53 00 ........T...DOS.
00000020 E3 3D 0E 73 00 00 03 70 43 FA 00 3E 70 25 4E AE .=.s...pC..>p%N.
00000030 FD D8 4A 80 67 0C 22 40 08 E9 00 06 00 22 4E AE ..J.g."@....."N.
00000040 FE 62 43 FA 00 18 4E AE FF A0 4A 80 67 0A 20 40 .bC...N...J.g. @
00000050 20 68 00 16 70 00 4E 75 70 FF 4E 75 64 6F 73 2E h..p.Nup.Nudos.
00000060 6C 69 62 72 61 72 79 00 65 78 70 61 6E 73 69 6F library.expansio
00000070 6E 2E 6C 69 62 72 61 72 79 00 00 00 00 00 00 00 n.library.......
00000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000000F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000100 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000120 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000140 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000160 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000170 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001C0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001D0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
000001F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000200 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000210 00 00 00 00 00 00 00 00 00 00 00 00 ............It’s the first block of a boot block! Excellent – working a treat.
The next thing is – tidy the code to make it more modular. Then calculate the checksum. Once I’ve done all this I’m going to see about changing the PIC code to read the whole disk so I can analyse more data.
I got in contact with a guy called Keith. He’s doing some Amiga floppy stuff on his blog at techtravels.org. Very helpful and has lots of experience doing this stuff. It’s uncanny how close our thought patterns are. Be sure to check out his site.
Another excellent resource is this site here http://lclevy.free.fr/adflib/adf_info.html. The checksum logic does not appear to be explained at all in the RKRM and Laurent’s site was quite useful. The source from ADFRead also confirmed what was going on: http://www.winuae.net/files/stuff/adfread-1.1.zip