505 lines
13 KiB
NASM
505 lines
13 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" doesn'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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; However, to simplify processing, whenever fsNext encounter a chain end of the
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; first type (a valid block with > 0 allocated blocks), it places an empty block
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; at the end of the chain. This makes the whole "end of chain" processing much
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; easier: we assume that we always have a 0 block at the end.
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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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.equ FS_MAX_NAME_SIZE 0x1a
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.equ FS_BLOCKSIZE 0x100
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.equ FS_METASIZE 0x20
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.equ FS_META_ALLOC_OFFSET 3
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.equ FS_META_FSIZE_OFFSET 4
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.equ FS_META_FNAME_OFFSET 6
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; Size in bytes of a FS handle:
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; * 2 bytes for current position. (absolute)
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; * 2 bytes for starting offset, after metadata
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; * 2 bytes for maximum 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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.equ FS_HANDLE_SIZE 8
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.equ FS_ERR_NO_FS 0x5
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.equ FS_ERR_NOT_FOUND 0x6
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; *** VARIABLES ***
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; A copy of BLOCKDEV routines when the FS was mounted. 0 if no FS is mounted.
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.equ FS_GETC FS_RAMSTART
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.equ FS_PUTC FS_GETC+2
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.equ FS_SEEK FS_PUTC+2
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.equ FS_TELL FS_SEEK+2
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; Offset at which our FS start on mounted device
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.equ FS_START FS_TELL+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 fsblkSeek.
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.equ FS_PTR 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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.equ FS_META FS_PTR+2
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.equ FS_HANDLES FS_META+FS_METASIZE
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.equ FS_RAMEND 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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fsInit:
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xor a
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ld hl, FS_GETC
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ld b, FS_RAMEND-FS_GETC
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call fill
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ret
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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 hl
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ld a, (FS_META+FS_META_ALLOC_OFFSET)
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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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.loop:
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ld a, BLOCKDEV_SEEK_FORWARD
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ld hl, FS_BLOCKSIZE
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call fsblkSeek
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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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call fsblkTell ; --> HL
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ld (FS_PTR), hl
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call fsReadMeta
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jr nz, .createChainEnd
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call fsIsValid
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jr nz, .createChainEnd
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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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.createChainEnd:
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; We are on an invalid block where a valid block should be. This is
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; the end of the line, but we should mark it a bit more explicitly.
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; Let's initialize an empty block
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call fsInitMeta
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call fsWriteMeta
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; continue out to error condition: we're still at the end of the line.
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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 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 fsblkRead 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, FS_METASIZE
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ld hl, FS_META
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call fsblkRead ; 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 fsblkWrite 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, FS_METASIZE
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ld hl, FS_META
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call fsblkWrite ; 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 bc
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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 hl, FS_META+3
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ld b, FS_METASIZE-3
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call fill
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pop hl
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pop de
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pop bc
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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 fsblkSeek
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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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ld c, a ; Let's store our A arg somewhere...
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call fsBegin
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jr nz, .end ; not a valid block? hum, something's wrong
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; First step: find last block
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push hl ; keep HL for later
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.loop1:
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call fsNext
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jr nz, .found ; end of the line
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call fsIsDeleted
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jr nz, .loop1 ; not deleted? loop
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; This is a deleted block. Maybe it fits...
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ld a, (FS_META+FS_META_ALLOC_OFFSET)
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cp c ; Same as asked size?
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jr z, .found ; yes? great!
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; TODO: handle case where C < A (block splitting)
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jr .loop1
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.found:
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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 the "end of line" block
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; 2 - the block is a deleted block that we we're re-using.
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; In both case, the processing is the same: write new metadata.
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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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ld a, c ; C == the number of blocks user asked for
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ld (FS_META+FS_META_ALLOC_OFFSET), a
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pop hl ; now we want our HL arg
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ld de, FS_META+FS_META_FNAME_OFFSET
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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 fsblkTell
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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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.end:
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pop de
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pop bc
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ret
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; Place FS_PTR to the filename with the name in (HL).
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; Sets Z on success, unset when not found.
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fsFindFN:
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push de
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call fsBegin
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jr nz, .end ; nothing to find, Z is unset
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ld a, FS_MAX_NAME_SIZE
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.loop:
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ld de, FS_META+FS_META_FNAME_OFFSET
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call strncmp
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jr z, .end ; Z is set
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call fsNext
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jr z, .loop
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; End of the chain, not found
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call unsetZ
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.end:
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pop de
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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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; Returns wheter current block is deleted in Z flag.
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fsIsDeleted:
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ld a, (FS_META+FS_META_FNAME_OFFSET)
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cp 0 ; Z flag is our answer
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ret
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; *** blkdev methods ***
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; When "mounting" a FS, we copy the current blkdev's routine privately so that
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; we can still access the FS even if blkdev selection changes. These routines
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; below mimic blkdev's methods, but for our private mount.
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fsblkGetC:
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ld ix, (FS_GETC)
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jp _blkCall
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fsblkRead:
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ld ix, (FS_GETC)
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jp _blkRead
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fsblkPutC:
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ld ix, (FS_PUTC)
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jp _blkCall
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fsblkWrite:
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ld ix, (FS_PUTC)
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jp _blkWrite
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fsblkSeek:
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ld ix, (FS_SEEK)
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ld iy, (FS_TELL)
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jp _blkSeek
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fsblkTell:
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ld ix, (FS_TELL)
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jp _blkCall
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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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push hl
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push de
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push af
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ex de, hl
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ld hl, (FS_PTR)
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ld a, FS_METASIZE
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call addHL
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call writeHLinDE
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inc de
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inc de
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call writeHLinDE
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inc de
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inc de
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; Maximum offset is starting offset + (numblocks * 0x100) - 1
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ld a, (FS_META+FS_META_ALLOC_OFFSET)
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; Because our blocks are exactly 0x100 in size, we simple have to
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; increase the H in HL to have our result.
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add a, h
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ld h, a
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call writeHLinDE
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inc de
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inc de
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ld hl, (FS_META+FS_META_FSIZE_OFFSET)
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call writeHLinDE
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pop af
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pop de
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pop hl
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ret
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; Place FS blockdev at proper position for file handle in (DE).
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fsPlaceH:
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push af
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push hl
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push de
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pop ix
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push ix
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ld l, (ix)
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ld h, (ix+1)
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ld a, BLOCKDEV_SEEK_ABSOLUTE
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call fsblkSeek
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pop ix
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pop hl
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pop af
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ret
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; Advance file handle in (IX) by one byte
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fsAdvanceH:
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push af
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inc (ix)
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jr nz, .end
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inc (ix+1)
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.end:
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pop af
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ret
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; Read a byte in handle at (DE), 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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fsGetC:
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push ix
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call fsPlaceH
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push ix ; Save handle in IX for fsAdvanceH
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call fsblkGetC
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; increase current pos
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pop ix ; recall handle in IX
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jr nz, .end ; error, don't advance
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call fsAdvanceH
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.end:
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pop ix
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ret
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; Write byte A in handle (DE) and advance the handle's position.
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; Z is set on success, unset if handle is at the end of the file.
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fsPutC:
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call fsPlaceH
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push ix
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call fsblkPutC
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; increase current pos
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pop ix ; recall
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call fsAdvanceH
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ret
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; Sets position of handle (DE) to HL. 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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push de \ pop ix
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ld (ix), l
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ld (ix+1), h
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ret
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fsTell:
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push de \ pop ix
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ld l, (ix)
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ld h, (ix+1)
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ret
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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_GETC/PUTC/SEEK/TELL.
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fsOn:
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push hl
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push de
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push bc
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; We have to set blkdev routines early before knowing whether the
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; mounting succeeds because methods like fsReadMeta uses fsblk* methods.
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ld hl, BLOCKDEV_GETC
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ld de, FS_GETC
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ld bc, 8 ; we have 8 bytes to copy
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ldir ; copy!
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call fsblkTell
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ld (FS_START), hl
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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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xor a
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jr .end
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.error:
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; couldn't mount. Let's reset our variables.
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xor a
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ld b, 10 ; blkdev routines + FS_START which is just
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; after.
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ld hl, FS_GETC
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call fill
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ld a, FS_ERR_NO_FS
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.end:
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pop bc
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pop de
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pop hl
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ret
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