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MEMDISK - the filesystem for memory

MEMDISK is designed to store files in memory areas. The memory can be fragmented, so some I2C EEPROMs can be used as one disk. It is also useful for ramdisks in banked RAM or files stored in EPROM.

The MEM/0 structure was the first idea. In the MEM/1 revision, pointers to parent was added. So now there's only one pointer needed to operate a file or directory and there's no need to remember pointers of the path elements. This simplifies e.g. deleting files or moving to another directory.

Disk structure
shift contents length in bytes description
+0 "M", "E", "M", #01 4 filesystem name
+4 three chars followed by #00 4 phisical disk name
+8 structure F at least 26 file descriptor
+... structure D at least 26 directory descriptor
+... structure A at least 7 data area descriptor

First file desriptor (structure F) is always present. Points to first file or directory on disk, contains amount of useful free space and points to the free area chain (structures A). Also can point to parent directory - value 0 means the mount point is root. So empty disk contains the 8 bytes header, one structure F and at least one structure A.

shift length in bytes contents notes for
structure F
(file descriptor)
structure D
(directory descriptor)
structure A
(data area)
+0 4 pointer to next structure next file or directory in this directory only A
+4 2 length of this structure from 20 to 64 from 1 to 65535
+6 1 version number #00 #80 data area
+7 1 flags
+8 4 pointer to parent structure the directory that contains this file or directory
+12 4 parameter file length
+16 4 pointer to child structure only A first entry in child directory
+20 4 date and time packed
+24 at least 2 name followed by #00 file name directory name

All pointer and length fields are in little Endian (the low meanig byte is first).

Example - empty 8kB disk

Example device is M24C64 - an I2C 8kB EEPROM, connected to AY-3-8912.

The pointer base is #E0A0, what means memory connected to AY (#E0 - bit 0 of port A is the SDA, bit 1 is SCL.) and of internal number #A0. The connection number #E2 is used in TC2068 computerers and means bit 4 of port A is the SDA, bit 5 is SCL. Internal number for M24C64 chips can be range #A0..#AE (only even values) and is set with E0, E1 and E2 inputs of the chip.

addr.   contents              description
#0000   4D 45 4D 01           filesystem name "MEM/1"
#0004   45 45 30 00           disk phisical name "EE0" (zero ended)
#0008 F 00 00 00 00           no next directory entry (structure F starts here)
#000C   19 00                 entry length (#0027-#000E)
#000E / 00 00                 file entry with no flags set
#0010   00 00 00 00           no parent structure present
#0014   D3 1F 00 00           total free area length
#0018   27 00 A0 E0           pointer to A structure, that starts from #0027
#001C   B5 44 22 2C           date and time of formatting: 2002-01-02 08:37:42
#0020 \ 45 45 44 48 53 4B 00  disk name "EEDISK" (zero ended)
#0027 A 00 00 00 00           no next structures (structure A starts here)
#002A   D3 1F                 area length (#2000-#002D)
#002D / FF FF FF FF FF ...    empty area - #1FD3 bytes
#1FFF \ FF                    last byte of empty area

© 2002-08-28 Jarek Adamski zxvgs@yarek.com, http://zxvgs.yarek.com