416 lines
10 KiB
NASM
416 lines
10 KiB
NASM
; fs
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;
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; Collapse OS filesystem (CFS) is not made to be convenient, but to be simple.
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; This is little more than "named storage blocks". Characteristics:
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;
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; * a filesystem sits upon a blockdev. It needs GetC, PutC, Seek.
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; * No directory. Use filename prefix to group.
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; * First block of each file has metadata. Others are raw data.
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; * No FAT. Files are a chain of blocks of a predefined size. To enumerate
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; files, you go through metadata blocks.
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; * Fixed allocation. File size is determined at allocation time and cannot be
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; grown, only shrunk.
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; * New allocations try to find spots to fit in, but go at the end if no spot is
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; large enough.
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; * Block size is 0x100, max block count per file is 8bit, that means that max
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; file size: 64k - metadata overhead.
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;
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; *** Selecting a "source" blockdev
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;
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; This unit exposes "fson" shell command to "mount" CFS upon the currently
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; selected device, at the point where its seekptr currently sits. This checks
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; if we have a valid first block and spits an error otherwise.
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;
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; "fson" takes an optional argument which is a number. If non-zero, we don't
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; error out if there's no metadata: we create a new CFS fs with an empty block.
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;
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; The can only be one "mounted" fs at once. Selecting another blockdev through
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; "bsel" foesn't affect the currently mounted fs, which can still be interacted
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; with (which is important if we want to move data around).
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;
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; *** Block metadata
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;
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; At the beginning of the first block of each file, there is this data
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; structure:
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;
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; 3b: Magic number "CFS"
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; 1b: Allocated block count, including the first one. Except for the "ending"
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; block, this is never zero.
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; 2b: Size of file in bytes (actually written). Little endian.
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; 26b: file name, null terminated. last byte must be null.
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;
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; That gives us 32 bytes of metadata for first first block, leaving a maximum
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; file size of 0xffe0.
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;
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; *** Last block of the chain
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;
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; The last block of the chain is either a block that has no valid block next to
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; it or a block that reports a 0 allocated block count.
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;
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; *** Deleted files
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;
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; When a file is deleted, its name is set to null. This indicates that the
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; allocated space is up for grabs.
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;
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; *** File "handles"
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;
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; Programs will not typically open files themselves. How it works with CFS is
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; that it exposes an API to plug target files in a blockdev ID. This all
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; depends on how you glue parts together, but ideally, you'll have two
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; fs-related blockdev IDs: one for reading, one for writing.
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;
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; Being plugged into the blockdev system, programs will access the files as they
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; would with any other block device.
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;
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; *** Creating a new FS
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;
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; A valid Collapse OS filesystem is nothing more than the 3 bytes 'C', 'F', 'S'
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; next to each other. Placing them at the right place is all you have to do to
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; create your FS.
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; *** DEFINES ***
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; Number of handles we want to support
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; FS_HANDLE_COUNT
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; *** CONSTS ***
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FS_MAX_NAME_SIZE .equ 0x1a
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FS_BLOCKSIZE .equ 0x100
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; Size in bytes of a FS handle:
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; * 2 bytes for starting offset
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; * 2 bytes for file size (we could fetch it from metadata all the time, but it
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; could be time consuming depending on the underlying device).
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; * 2 bytes for current position.
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; Starting offset is the *metadata* offset. We need, when we write to a handle,
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; to change the size of the file.
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FS_HANDLE_SIZE .equ 6
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FS_ERR_NO_FS .equ 0x5
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; *** VARIABLES ***
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; A copy of BLOCKDEV_SEL when the FS was mounted. 0 if no FS is mounted.
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FS_BLKSEL .equ FS_RAMSTART
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; Offset at which our FS start on mounted device
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FS_START .equ FS_BLKSEL+2
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; Offset at which we are currently pointing to with regards to our routines
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; below, which all assume this offset as a context. This offset is not relative
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; to FS_START. It can be used directly with blkSeek.
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FS_PTR .equ FS_START+2
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; This variable below contain the metadata of the last block FS_PTR was moved
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; to. We read this data in memory to avoid constant seek+read operations.
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FS_META .equ FS_PTR+2
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FS_HANDLES .equ FS_META+0x20
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FS_RAMEND .equ FS_HANDLES+(FS_HANDLE_COUNT*FS_HANDLE_SIZE)
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; *** DATA ***
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P_FS_MAGIC:
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.db "CFS", 0
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; *** CODE ***
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; *** Navigation ***
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; Resets FS_PTR to the beginning. Errors out if no FS is mounted.
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; Sets Z if success, unset if error
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fsBegin:
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push hl
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ld hl, (FS_START)
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ld (FS_PTR), hl
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pop hl
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call fsReadMeta
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call fsIsValid ; sets Z
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ret
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; Change current position to the next block with metadata. If it can't (if this
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; is the last valid block), doesn't move.
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; Sets Z according to whether we moved.
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fsNext:
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push bc
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push de
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push hl
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ld a, (FS_META+3)
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cp 0
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jr z, .error ; if our block allocates 0 blocks, this is the
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; end of the line.
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call fsPlace
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ld b, a ; we will seek A times
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ld a, BLOCKDEV_SEEK_FORWARD
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ld hl, FS_BLOCKSIZE
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.loop:
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call blkSeek
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djnz .loop
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; Good, were here. We're going to read meta from our current position.
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; But before we do, let's keep a copy of FS_PTR around. We might need
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; to go back.
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ld de, (FS_PTR)
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call blkTell ; --> HL
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ld (FS_PTR), hl
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call fsReadMeta
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jr nz, .goback ; error! let's bail out
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call fsIsValid
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jr nz, .goback ; error! let's bail out
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; We're good! We have a valid FS block and FS_PTR is already updated.
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; Meta is already read. Nothing to do!
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cp a ; ensure Z
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jr .end
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.goback:
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ld (FS_PTR), de
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call fsReadMeta
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.error:
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call unsetZ
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.end:
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pop hl
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pop de
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pop bc
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ret
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; Reads metadata at current FS_PTR and place it in FS_META.
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; Returns Z according to whether the blkRead operation succeeded.
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fsReadMeta:
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call fsPlace
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push bc
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push hl
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ld b, 0x20
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ld hl, FS_META
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call blkRead ; Sets Z
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pop hl
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pop bc
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ret
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; Writes metadata in FS_META at current FS_PTR.
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; Returns Z according to whether the blkWrite operation succeeded.
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fsWriteMeta:
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call fsPlace
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push bc
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push hl
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ld b, 0x20
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ld hl, FS_META
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call blkWrite ; Sets Z
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pop hl
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pop bc
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ret
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; Initializes FS_META with "CFS" followed by zeroes
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fsInitMeta:
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push af
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push de
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push hl
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ld hl, P_FS_MAGIC
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ld de, FS_META
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ld bc, 3
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ldir
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xor a
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ld b, 0x20-3
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.loop:
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ld (hl), a
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inc hl
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djnz .loop
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pop hl
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pop de
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pop af
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ret
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; Make sure that our underlying blockdev is correcly placed.
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fsPlace:
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push af
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push hl
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xor a
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ld hl, (FS_PTR)
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call blkSeek
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pop hl
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pop af
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ret
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; Create a new file with A blocks allocated to it and with its new name at
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; (HL).
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; Before doing so, enumerate all blocks in search of a deleted file with
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; allocated space big enough. If it does, it will either take the whole space
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; if the allocated space asked is exactly the same, or of it isn't, split the
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; free space in 2 and create a new deleted metadata block next to the newly
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; created block.
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; Places FS_PTR to the newly allocated block. You have to write the new
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; filename yourself.
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fsAlloc:
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push bc
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push de
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push hl ; keep HL for later
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push af ; keep A for later
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; First step: find last block
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call fsBegin
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ret nz ; not a valid block? hum, something's wrong
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.loop1:
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call fsNext
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jr z, .loop1
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call fsPlace ; Make sure that our block device points to
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; the beginning of our FS block
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; We've reached last block. Two situations are possible at this point:
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; 1 - the block is unallocated (0 alloc size)
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; 2 - the block is allocated, but there are no next block
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; So, what we need to do is check our allocation size
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call fsAllocatedBlocks
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cp 0
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jr z, .proceed ; 0 allocated blocks? this is our block
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; > 0 allocated blocks. We need to allocate further
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ld b, a ; we will seek A times
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ld a, BLOCKDEV_SEEK_FORWARD
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ld hl, FS_BLOCKSIZE
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.loop2:
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call blkSeek
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djnz .loop2
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.proceed:
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; At this point, the blockdev is placed right where we want to allocate
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; But first, let's prepare the FS_META we're going to write
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call fsInitMeta
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pop af ; now we want our A arg
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ld a, 1
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ld (FS_META+3), a
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pop hl ; now we want our HL arg
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ld de, FS_META+6
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ld bc, FS_MAX_NAME_SIZE
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ldir
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; Good, FS_META ready. Now, let's update FS_PTR because it hasn't been
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; changed yet.
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call blkTell
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ld (FS_PTR), hl
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; Ok, now we can write our metadata
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call fsWriteMeta
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pop de
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pop bc
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ret
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; *** Metadata ***
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; Sets Z according to whether the current block in FS_META is valid.
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; Don't call other FS routines without checking block validity first: other
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; routines don't do checks.
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fsIsValid:
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push hl
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push de
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ld a, 3
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ld hl, FS_META
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ld de, P_FS_MAGIC
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call strncmp
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; The result of Z is our result.
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pop de
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pop hl
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ret
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; Return, in A, the number of allocated blocks at current position.
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fsAllocatedBlocks:
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ld a, (FS_META+3)
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ret
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; Return, in HL, the file size at current position.
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fsFileSize:
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ld hl, (FS_META+4)
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ret
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; Return HL, which points to a null-terminated string which contains the
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; filename at current position.
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fsFileName:
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ld hl, FS_META+6
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ret
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; Change name of current file to name in (HL)
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fsRename:
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push af
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push hl ; save filename for later
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call fsPlace
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ld a, BLOCKDEV_SEEK_FORWARD
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ld hl, 6
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call blkSeek
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pop hl ; now we need the filename
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push hl ; ... but let's preserve it for the caller
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.loop:
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ld a, (hl)
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cp 0
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jr z, .end
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call blkPutC
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inc hl
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jr .loop
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.end:
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pop hl
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pop af
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ret
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; *** Handling ***
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; Open file at current position into handle at (HL)
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fsOpen:
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ret
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; Ensures that file size in metadata corresponds to file size in handle as (HL).
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fsCommit:
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ret
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; Read a byte in handle at (HL), put it into A and advance the handle's
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; position.
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; Z is set on success, unset if handle is at the end of the file.
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fsRead:
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ret
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; Write byte A in handle at (HL) and advance the handle's position.
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; Z is set on success, unset if handle is at the end of the allocated space.
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fsWrite:
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ret
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; Sets position of handle (HL) to DE. This position does *not* include metadata.
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; It is an offset that starts at actual data.
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; Sets Z if offset is within bounds, unsets Z if it isn't.
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fsSeek:
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ret
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; *** SHELL COMMANDS ***
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; Mount the fs subsystem upon the currently selected blockdev at current offset.
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; Verify is block is valid and error out if its not, mounting nothing.
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; Upon mounting, copy currently selected device in FS_BLKSEL.
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fsOnCmd:
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.db "fson", 0, 0, 0
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push hl
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call blkTell
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ld (FS_PTR), hl
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call fsReadMeta
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jr nz, .error
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call fsIsValid
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jr nz, .error
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; success
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ld (FS_START), hl
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xor a
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jr .end
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.error:
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ld a, FS_ERR_NO_FS
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.end:
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pop hl
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ret
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; Lists filenames in currently active FS
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flsCmd:
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.db "fls", 0, 0, 0, 0
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call fsBegin
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jr nz, .error
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.loop:
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call fsFileName
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call printstr
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call printcrlf
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call fsNext
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jr z, .loop ; Z set? fsNext was successfull
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xor a
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jr .end
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.error:
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ld a, FS_ERR_NO_FS
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.end:
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ret
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; Takes one byte block number to allocate as well we one string arg filename
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; and allocates a new file in the current fs.
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fnewCmd:
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.db "fnew", 0b001, 0b1001, 0b001
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push hl
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push de
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ld a, (hl)
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ex de, hl
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inc de
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call intoDE
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ex de, hl
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call fsAlloc
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pop de
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pop hl
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xor a
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ret
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