The FLOP filesystem is designed for the use with the Mackapar 3.5" Floppy Drive (M35FD). The M35FD consists of 1440 sectors of 512 words each (737,280 words total).
The first seven sectors are the main header, followed by the file list. One block is a sector. It just stores pure data. Blocks are allocated back-to-front, to reduce collision between the file list and the data blocks.
+----------+------------------------------------+
| size | description |
+----------+------------------------------------+
| 1 w | FLOP magic number (0x83df) | ---\
| | | |
| 1 w | version (0x0001) | |
| | | |
| 1 w | header size | |- block 0
| | | |
| 16 w | drive name (0-padd, packed string) | |
| | | |
| 1 w | number of file entries | |
| | | |
| 492 w | reserved for more header data | ---/
|-----------------------------------------------|
| 512 w | block list, 512 * 6 = 3072 | ---\
| 512 w | we have 2 words | | - blocks 1 - 6
| 512 w | per block | ---/
|-----------------------------------------------|
| 512 w | file list (alphabetic order) | ---\
| 512 w . file list (alphabetic order) | |
| 512 w . file list (alphabetic order) | | - blocks 7 .. n
| 512 w | file list (alphabetic order) | ---/
|__________|____________________________________|
+------+----------------------------------+
| size | description |
+------+----------------------------------+
| 1w | H L |
| | lllllllllfffffff |
| | \___ _/___ ____/ |
| | v v |
| | | +----- last set word |
| | +----------- unused flags |
| | |
| 1w | H L |
| | ttttFFFFFFFFFFff |
| | \__/____ ____/_/ |
| | v v | |
| | | | +- unused flags |
| | | +------- file id |
| | +-- type: |
| | 0: unused; 1: header; |
| | 2: blocklist, 3: filelist |
| | 4: data |
|______|__________________________________|
+----------+----------------------------------+
| size | description |
+----------+----------------------------------+
| max 16 w | filename (0-term, packed string) |
| | |
| 1 w | file size (words) |
| | |
| 1 w | # of allocated blocks |
| | |
| n w | block list: |
| | H L |
| | 00000fBBBBBBBBBB |
| | \_ _//____ ____/ |
| | v | v |
| | | | +------- block id |
| | | +-- [is block filled?] | Better not implement that, really sucks for usability
| | +-- unused |
|__________|__________________________________|
getting info about a block X (ex. 258):
SET B, X ; B, X = 258
MOD B, 256 ; B = 2
SUB X, B ; X = 256
DIV X, 256 ; X = 1
SUB B, 1 ; B = 1
MUL B, 2 ; B = 2
SET A, 2 ; load sector X to [Y, Y+512]
SET Y, [temp]
HWI [m35fd_id]
ADD Y, B ; Y = temp + 2
; Y now points to the first word of the block info
checking if a block is free (Y points to the corresponding block info): SET B, [Y+1] ; read 2nd word SHR B, 12 ; shift to type only IFE B, 0 JSR sth ; block isn't used