Olivetti M20 disk format analysis
160K
We don't have any but we can guess.
320K
35 track total, from 0 to 34.
- Track 0: FM encoding, 16 sectors, 128 bytes/sector.
- Track 1-34: MFM encoding, 16 sectors, 256 bytes/sector.
Total disk size 286720 - readable by "m20" utility. That is about 1120 blocks * 256 bytes (considering first sector as a 256 bytes/sector size as many emulators do. IDK)
MAIN DISK BLOCK is at 0x20000 (track 16).
- 0x20000 Disk name if present
- 0x20038 FF
- 0x20039 FF ..... to
- 0x200C0 FF
- 0x200C1 FF and after that some 00
- 0x20200 Start of directory: first filename.
- 0x20FFF end of FF and stuff.
Directory entries
16 bytes for the filename + 2 bytes for the address of the file minus 1 (256 bytes; 1 sector lower, 0x100). Total 18 bytes.
Example
Dir entry: 00020200 4d 45 4d 4f 52 2e 41 53 43 00 00 00 00 00 00 00 |MEMOR.ASC.......| 00020210 00 b2 4d 45 4d 4f 52 2e 4f 00 00 00 00 00 00 00 |..MEMOR.O.......| 00020220 00 00 00 e4 50 43 4f 53 2e 53 41 56 00 00 00 00 |....PCOS.SAV....| As you can see, MEMOR.ASC begin at B300 (00B2 there). MEMOR.O begin at E500 (00E4 there). Here they are: MEMOR.ASC 0000b300 31 30 20 52 45 4d 20 2a 2a 2a 2a 2a 2a 2a 2a 2a |10 REM *********| 0000b310 2a 2a 2a 2a 2a 20 70 65 72 20 72 69 63 65 72 63 |***** per ricerc| MEMOR.O 0000e500 ff 66 a6 00 0a 8f 20 2a 2a 2a 2a 2a 2a 2a 2a 2a |.f.... *********| 0000e510 2a 2a 2a 2a 2a 20 70 65 72 20 72 69 63 65 72 63 |***** per ricerc|
struct m20dir_entry {
cpg_u8_t name[16];
cpg_u32_t size;
cpg_u32_t alloc_size;
cpg_u16_t extents;
cpg_u16_t flags;
cpg_u16_t startsec;
};
)))
(((
#define PCOS_DIR_ENTRIES_BLOCK 14 /* # of dir entries in one diskette block */
#define PCOS_NUM_DIR_BLOCKS 14 /* # of blocks containing the directory */
#define PCOS_MAX_DIR_ENTRIES (PCOS_DIR_ENTRIES_BLOCK * PCOS_NUM_DIR_BLOCKS)
/* size of memory needed to hold full directory */
#define M20DIR_SIZE (PCOS_MAX_DIR_ENTRIES * \
sizeof(struct m20dir_entry) + m20dir_hdr_size)
)))
14 directory entries (files) in a single 0x100 bytes block dir structure;
So 14 blocks for the entire dir structure, each with 14 filenames and pointers inside it.
Total dir is 196 files for a single floppy directory. M20DIR_SIZE IDK what that is.
{{{
/* directory structures used by PCOS on disk */
/* all entries are big-endian! */
cpg_u8_t name[16]; /* in fact 14 only, don't know what the other 2 bytes are... */
/* in my tests they are always 0 */
cpg_u16_t startsec;
} ALIGN_2;
WPACK struct ALIGN_ dir_sector {
struct direntry files[PCOS_DIR_ENTRIES_BLOCK];
cpg_u32_t link;
} ALIGN_2;
/* block free bitmap + etc (sector 0x200) */
#define NUM_FILE_BLOCKS 0x8a /* available blocks for files (-16, directory blocks not considered) */
WPACK struct ALIGN_ disk_header {
cpg_u8_t volname[15]; /* volume name (max. 14 chars w/terminating zero) */
cpg_u8_t unknown1[13]; /* purpose unknown at this time */
cpg_u16_t sernum_or_unknown; /* purpose unknown at this time */
cpg_u8_t unknown2[0x1a]; /* purpose unknown at this time */
cpg_u8_t bfl[NUM_FILE_BLOCKS]; /* block free list */
} ALIGN_2;