Family Computer Disk System

The Famicom Disk System was a Japan-exclusive storage device for the Famicom, designed to reduce Nintendo's cost of making copies of games by switching from mask ROM chips to a storage medium based on Mitsumi's Quick Disk. Unfortunately for Nintendo, it also reduced the pirates' cost of making copies of games.

.FDS format

fwNES was an NES emulator developed by Fan Wan Yang. Its most lasting contribution to the NES scene was its disk image file format, an image of the Quick Disk media.

The FDS format (file name suffix .fds) is a way to store Famicom Disk System disk data. It consists of the following sections, in order:

Header (16 bytes)
Disk data (65500 * x bytes)

The format of the header is as follows:

0-3: Constant $46 $44 $53 $1A ("FDS" followed by MS-DOS end-of-file)
4: Number of disk sides in 65500 byte units
5-15: Zero filled

Note: Most games are an even number of sides. Ports from NROM were one side. No commercial FDS game had an odd number of sides greater than 1.

Overview

PRG ROM : 8 KB BIOS at $E000-$FFFF
PRG RAM: 32 KB at $6000-$DFFF
CHR capacity: 8 KB RAM
Nametable mirroring: Controlled by mapper
Subject to bus conflicts: No

Games are stored on one or multiple disk sides. The FDS BIOS is used to load data from disks to PRG RAM or VRAM, and games can execute form there.

FDS Disk Side format

Each disk side must be structured into block as follows :

1, 2, 3, 4, 3, 4, ...., 3, 4

The 3, 4 pattern should be repeated once per file present on the disk.

From the last file, fill the side with all 0 so that the side has exactly 65550 is reached.

Block format

Disk info block (block 1)

SIZE   CONTENTS
1      $01
14     FC Disk String  "*NINTENDO-HVC*" (not including the ")
1      Manufacture Code  
       Same code as used in GameBoy(?)
4      Game Name Code
1      Game Version Number (start with $00, increase each revision)
1      Side Number  
         $00: Side-A  
         $01: Side-B
1      Disk Number (fist disk is actually $00)
1      Err.9  (indicate disk # ?, usually $00)
1      Err.10 (indicate disk # ?, usually $00)
1      Boot Read File Code
5      Unknown
3      Manufacture Permit Date
       Recorded in BCD, in the year of "showa"(+1925)
       For example, 21 march 2010 becomes $85, $03, $21 (because 2010-1925 = 85)
10     Unknown
3      Created Date  
       Recorded in BCD, in the year of "showa"(+1925)
9      Unknown

The *NINTENDO-HVC*, stored in ASCI standard, strings proves this is a FDS disk, and if the string doesn't exactly match the BIOS will refuse to read the disk further. If the FDS is started with a disk whose side number and disk number aren't both 0, it will be prompted to insert the first disk side. Some games do however make this number 0 even for the second disk to make it bootable as well.

All files which IDs is smaller or equal than the boot read file code will be loaded when the game is booting.

The FDS also has a trademark security system similar to what Sega used on some of its consoles. The 224-byte text at PPU $2800-$28DF must match data in the BIOS starting at $ED37. This data says roughly "produced by or under license from Nintendo" in the SMB1/Zelda character encoding. Traditionally, the first file on a disk is a nametable type file loaded into $2800 which is named KYODAKU- (きょだく or 許諾 means approval).

File amount block (block 2)

This block contains the total number of files recorded on disk. The info in this block is checked only at system boot. Since there may exist more files than the number recorded in this block, emulators should ignore the value recorded in this block.

SIZE   CONTENTS
1        $02
1       File Amount

File header block (block 3)

SIZE     CONTENTS
1        $03
1        File Number  
1        File Indicate Code (file identification code)  
         ID specified at disk-read function call
8        File Name
2        File Address (16-bit little endian)
         the destination address when loading
2        File Size (16-bit little endian)
1        Kind of File  
         0:Program (PRAM)  
         1:Character (CRAM)  
         2:Name table (VRAM)

The file Number must go in increasing order, first file is 0. File IDs can be freely assigned, and this is the number which will decide which file is loaded from the disk (instead of the file number). An ID smaller than the boot number means the file is a boot file, and will be loaded on first startup.

File names are uppercase ASCII.

File data block (block 4)

SIZE      CONTENTS
1         $04
--        disk data

Registers

$402x registers are write-only $403x registers are read-only

IRQ timer low ($4020)

7  bit  0
---------
LLLL LLLL
|||| ||||
++++-++++- 8 LSB of IRQ timer

IRQ timer high ($4021)

7  bit  0
---------
LLLL LLLL
|||| ||||
++++-++++- 8 MSB of IRQ timer

IRQ timer enable ($4022)

7  bit  0
---------
xxxx xxEx
       |
       +-- Enable IRQ timer

Each CPU clock cycle the timer is decremented by one if the enable flag is set. When the counter reach 0, an IRQ is generated. Read $4030 to acknowledge the IRQ.

Note : Since the disk transfer routine also uses IRQs, it's very important to disable timer IRQs before doing any access to the disk.

Master I/O enable ($4023)

7  bit  0
---------
xxxx xxSD
       ||
       |+- Enable disk I/O registers
       +-- Enable sound I/O registers

This register sounds obscure. FDS bios just writes $00 then $83 to it.

Write data register ($4024)

The data that this register is programmed with will be the next 8-bit quantity to load into the shift register (next time the byte transfer flag raises), and to be shifted out and appear on pin 5 of the RAM adaptor cable (2C33 pin 52).

FDS Control ($4025)

7  bit  0
---------
IS1B MRTD
|||| ||||
|||| |||+- Drive Motor Control  
|||| |||     0: Stop motor
|||| |||     1: Turn on motor
|||| ||+-- Transfer Reset
|||| ||        Set 1 to reset transfer timing to the initial state.
|||| |+--- Read / Write mode
|||| |     (0: write; 1: read)
|||| +---- Mirroring (0: horizontal; 1: vertical)
|||+------ CRC control (set during CRC calculation of transfer)
||+------- Always set to '1'
|+-------- Read/Write Start  
|            Turn on motor.  Set to 1 when the drive becomes ready for read/write
+--------- Interrupt Transfer  
             0: Transfer without using IRQ
             1: Enable IRQ when the drive becomes ready for

A FDS game that wants to change mirroring probably don't want to touch motor related bits, so it should do a read-modify-write from the pseudo registers (see below).

External connector ($4026)

Output of expansion terminal where there's a shutter on the back of the ram card. The outputs of $4026 (open-collector with 4.7K ohm pull-ups (except on bit 7)), are shared with the inputs on $4033.

Disk Status Register 0 ($4030)

7  bit  0
---------
IExB xxTD
||||   ||
||||   |+- Timer Interrupt (1: an IRQ occurred)
||||   +-- Byte transfer flag. Set every time 8 bits have been transfered between the RAM adaptor & disk drive (service $4024/$4031). 
||||       Reset when $4024, $4031, or $4030 has been serviced.
|||+------ CRC control (0: CRC passed; 1: CRC error)
|+-------- End of Head (1 when disk head is on the most inner track)
+--------- Disk Data Read/Write Enable (1 when disk is readable/writable)

Read data register ($4031)

This register is loaded with the contents of an internal shift register every time the byte transfer flag raises. The shift register recieves it's serial data via pin 9 of the RAM adaptor cable (2C33 pin 51).

Disk drive status register ($4032)

7  bit  0
---------
xxxx xPRS
      |||
      ||+- Disk flag  (0: Disk inserted; 1: Disk not inserted)
      |+-- Ready flag (0: Disk read; 1: Disk not ready)
      +--- Protect flag (0: Not write protected; 1: Write protected or disk ejected)

External connector read ($4033)

7  bit  0
---------
BIII IIII
|||| ||||
|+++-++++- Input from expansion terminal where there's a shutter on the back of the ram card.
+--------- Battery status (0: Good; 1: Voltage is low).

When a bit is clear in $4026 port it will read back as '0' here (including battery bit) because of how open collector input works.

Sound ($4040-$4089)

For details on sound information, see FDS audio.

Pseudo-registers

Those registers are used by the FDS bios and FDS programs. They are used to overcome the problem that NES/FDS registers are write only, so it is effectively possible to modify only one bit of them without affecting other bits.

[$FF]:  value last written to $2000   $80 on reset.
[$FE]:  value last written to $2001   $06 on reset
[$FD]:  value last written to $2005#1 0'd on reset.
[$FC]:  value last written to $2005#2 0'd on reset.
[$FB]:  value last written to $4016   0'd on reset.
[$FA]:  value last written to $4025   $2E on reset.
[$F9]:  value last written to $4026   $FF on reset.
$F5..$F8 : Used by controller read routines
$00..$0F is used as temporary memory for the BIOS. The main program can use it as temporary memory too.

Those values are used by the BIOS to determine action on interrupt.

($0102):	PC action on reset
[$0101]:	PC action on IRQ. set to $80 on reset
[$0100]:	PC action on NMI. set to $C0 on reset
($DFFE):	disk game IRQ vector    (if [$0101] = 11xxxxxxB)
($DFFC):	disk game reset vector  (if ($0102) = $5335, or $AC35)
($DFFA):	disk game NMI vector #3 (if [$0100] = 11xxxxxxB)
($DFF8):	disk game NMI vector #2 (if [$0100] = 10xxxxxxB)
($DFF6):	disk game NMI vector #1 (if [$0100] = 01xxxxxxB)

A few important notes :

After loading the boot files, $102 is set to $35 so that the ($DFFC) vector is used and the BIOS is skipped.
$103 indicate reset type : $AC = first boot of the game, $53 = the game was soft-reseted by the user
To use your own IRQ routine, you must manually write $c0 to $101
There is 3 possible NMI vectors, #3 is used by default.
On first start, the mirroring is set to horizontal, the stack pointer is $ff, and the I flag is clear. System RAM is filled with values used by the BIOS, and PRG RAM is uninitialized, except for parts of it which has files loaded in.

BIOS calls

Disk acess routines

Routines takes one or two pointers as arguments. Those are placed directly after the jsr instruction : The subroutines uses the return address in stack to fetch the pointers and fix the return address.
Memory at $00-$0f will be affected by those routines
Don't expect disk calls to return quick; it may take several seconds to complete.
All non-disk IRQ sources (timer, DMC and APU frame) should be properly disabled before calling any of these routines. The value at [$0101] however, is preserved on entry, and restored on exit.
On return of those routines, A = $00 means no error occurred, other number is error #. Main program should test if an error occurred with the BEQ or BNE instruction, BEQ will branch if no error, and BNE will branch if there is an error.
The structures defined below are used to identify files & disks in the access routines. The argument pointers should point to these structures in the program.

Adress	Name	Input parameters	Output parameters	Description
$e1f8	LoadFiles	Pointer to Disk ID, Pointer to File List	A = error #, Y = # of files loaded	Loads files specified by DiskID into memory from disk. Load addresses are decided by the file's header.
$e237	AppendFile	Pointer to Disk ID, Pointer to File Header	A = error #	Appends the file data given by DiskID to the disk. This means that the file is tacked onto the end of the disk, and the disk file count is incremented. The file is then read back to verify the write. If an error occurs during verification, the disk's file count is decremented (logically hiding the written file).
$e239	WriteFile	Pointer to Disk ID, Pointer to File Header, A = file #	A = error #	Same as "Append File", but instead of writing the file to the end of the disk, A specifies the sequential position on the disk to write the file (0 is the first). This also has the effect of setting the disk's file count to the A value, therefore logically hiding any other files that may reside after the written one.
$e23a	GetDiskInfo	Pointer to Disk Info	A = error #	Fills DiskInfo up with data read off the current disk.
$e2bb	AdjustFileCount	Pointer to Disk ID, A = number to reduce current file count by	A = error #	Reads in disk's file count, decrements it by A, then writes the new value back.
$e2b7	CheckFileCount	Pointer to Disk ID, A = # to set file count to	A = error #	Reads in disk's file count, compares it to A, then sets the disk's file count to A.
$e305	SetFileCount	Pointer to Disk ID, A = file count	A = error #	Set the file count to A
$e301	SetFileCount1	Pointer to Disk ID, A = file count minus one = # of the last file	A = error #	Set the file count to A + 1

Exemple code how to load files

Load
   jsr LoadFiles
   .dw DiskID
   .dw LoadList
   bne _Error        ;Check if there is an error
   rts
_Error
   jsr PrintError      ;If so print the error number and message to screen (include side/disk changing prompts)
_sideError
   lda $4032
   and #$01
   beq _sideError     ;Wait until disk is ejected
_insert
   lda $4032
   and #$01
   bne _instert      ;Wait until disk is inserted
   jmp Load

DiskID
   .db $01        ;Manufacturer code
   .db "NAME"     ;4-letter code of game
   .db $00        ;Version
   .db $01        ;Disk side
   .db $00        ;Disk number
   .db $00, $00   ;Extra disk IDs

LoadList          ;In this example the files with IDs equal to $02, $03 or $04 will be loaded into memory
   .db $02, $03, $04, $ff

Error list

Message in bold is the official BIOS message (if there is one) followed by an explanation

$00: no error
$01: disk set, ($4032.0) disk not set
$02: battery, ($4033.7) power supply failure
$03: ($4032.2) disk is write protected
$04: Wrong maker ID
$05: Wrong game
$06: Wrong game version
$07: a,b side, wrong side number
$08: disk no., wrong disk number
$09: wrong additional disk ID 1
$0a: wrong additional disk ID 2
$20: disk trouble, approval check failed
$21: disk trouble, '*NINTENDO-HVC*' string in block 1 doesn't match
$22: disk trouble, block type 1 expected
$23: disk trouble, block type 2 expected
$24: disk trouble, block type 3 expected
$25: disk trouble, block type 4 expected
$27: disk trouble, ($4030.4) block failed CRC
$28: disk trouble, ($4030.6) file ends prematurely during read
$29: disk trouble, ($4030.6) file ends prematurely during write
$30: disk trouble, ($4032.1) disk is full

Disk ID structure

This is a commonly used string. It consists of 10 bytes which are all compared directly against bytes 15..24 (right after the '*NINTENDO-HVC*' string) of the disk's header block (block type 1; always the first one on the disk). If any of the bytes fail the comparison, an appropriate error # is generated. Comparisons of immaterial data can be skipped by placing an $FF byte in the appropriate place in the DiskID string (for example, when the ROM BIOS boots a disk, it sets all the fields in the DiskID string to -1, except disk side #, and disk #, which are set to 0 (so these fields have to match 0)). The following chart describes the DiskID structure, and the error #'s returned when a comparison fails.

offset	size	error#	description
------	----	------	-----------
0	1	$04	game manufacturer code
1	4	$05	game ASCII name string
5	1	$06	game version
6	1	$07	disk side #
7	1	$08	disk #
8	1	$09	extra disk # data
9	1	$10	extra disk # data
A			-

File list structure

This is a list of 1-byte IDs of files to load. All files that matches any ID in the list are loaded. A list of up to 20 IDs is possible at a time, smaller lists should be terminated by a $ff byte (this implies a file ID can never be $ff).

File header structure

This structure is specified when a file is to be written to the disk. The first 14 bytes of this structure directly specify the data to use for generating a file header block (type 3, bytes [2..15]) to write to disk. The last 2 entries concern the file data to be written to disk (block type 4). The following is a table describing the FileHeader structure.

offset	size	description
------	----	-----------
00	1	file ID code
01	8	file name
09	2	load address
0B	2	file data size
0D	1	file type ($00 : Program; $01 : Character; $02 : Nametable)
0E	2	source address of file data (NOT written to disk)
10	1	source address type ($00 : RAM, $01 : VRAM)
11		-

Disk information structure

This is a data structure returned by a subroutine, of collected information from the disk (list of files on disk, disk size, etc.). The following table is a description of that structure.

offset	size
------  ----
0	1	game manufacturer code
1	4	game ASCII name string
5	1	game version
6	1	disk side #
7	1	disk #
8	1	extra disk # data
9	1	extra disk # data
A	1	# of files on disk

(the following block will appear for as many files as the "# of files on disk" byte indicates)

B	1	file ID code
C	8	file name (ASCII)

(the following is present after the last file info block. Disk size is equal to the sum of each file's size entry, plus an extra 261 per file.)

x	1	disk size high byte
x+1	1	disk size low  byte
x+2		-

Other BIOS calls

Adress	Name	Input parameters	Output parameters	Affected RAM/Registers	Description
$e149	Delay132				132 clock cycle delay
$e153	Delayms			X, Y	Delay routine, Y = delay in ms (approximate)
$e161	DisPFObj			A, $fe	Disable sprites and background
$e16b	EnPFObj			A, $fe	Enable sprites and background
$e170	DisObj			A, $fe	Disable sprites
$e178	EnObj			A, $fe	Enable sprites
$e17e	DisPF			A, $fe	Disable background
$e185	EnPF			A, $fe	Enable background
$e1b2	VINTWait			$ff	Wait until next Vblank NMI fires, and return (for programs that does it the "everything in main" way). NMI vector selection at $100 is preserved, but further VBlanks are disabled.
$e7bb	VRAMStructWrite	Pointer to VRAM Structure to be written		A, X, Y, $00, $01, $ff	Set VRAM increment to 1 (clear $2000/$ff bit 2), and write a VRAM structure data to VRAM. Read below for information on the structure.
$e844	FetchDirectPtr		$00, $01 = pointer fetched	A, X, Y, $05, $06	Fetch a direct pointer from the stack (the pointer should be placed after the return address of the routine that calls this one (see "important notes" above)), save the pointer at ($00) and fix the return address.
$e86a	WriteVRAMBuffer			A, X, Y, $301, $302	Write a VRAMBuffer structure to VRAM. Read below for information on the structure.
$e8b3	ReadVRAMBuffer	X = start adress in buffer, Y = # of bytes to read		A, X, Y	Read individual bytes from VRAM to the VRAMBuffer. Apparently this routine has been made so that, when combined to WriteVRAMBuffer, it can easily do read-modify-write operations to individual VRAM bytes.
$e8d2	PrepareVRAMString	A = High VRAM adress, X = Low VRAM adress, Y = string length, Direct Pointer = data to be written to VRAM	A = $ff : no error, A = $01 : string didn't fit in buffer	A, X, Y, $00, $01, $02, $03, $04, $05, $06	This routine puts pointed data in the VRAM buffer.
$e8e1	PrepareVRAMStrings	A = High VRAM adress, X = Low VRAM adress, Direct pointer = data to be written to VRAM	A = $ff : no error, A = $01 : strings didn't fit in buffer	A, X, Y, $00, $01, $02, $03, $04, $05, $06	This routine put pointed data in VRAM buffer across multiple strings which are starts 32 bytes apart (one nametable row). The first byte tells the dimention of the strings : High nybble = # of rows, low nybble = # of columns.
$e94f	GetVRAMBufferByte	X = starting index, Y = # of byte to read in buffer (starting at 1), $00, $01 = adress to read from	carry clear : a byte was read, carry set : no byte was read	A, X, Y	This routine is somewhat obscure, and was probably meant to be used in pair with ReadVRAMBuffer. First the index is calculated as x+3*(y-1). Then the adress stored at that index is compared with the adress in ($00). If match, the third byte is read and the routine exit with c clear. If not match, the adress is overwritten by the adress in ($00) and the routine exit with c set. Exact details in how this could be any useful is under analysis.
$e97d	Pixel2NamConv	$02 = Pixel X cord, $03 = Pixel Y cord	$00 = High Nametable address, $01 = Low Nametable address	A	This routine convert pixel screen coordinates to corresponding nametable adress (assmue no scrolling, and points to first nametable at $2000-$23ff).
$e997	Nam2PixelConv	$00 = High nametable address, $01 = low nametable address	$02 = Pixel X cord, $03 = Pixel Y cord	A	This routine convert a nametable adress to corresponding pixel coordinates (assume no scrolling).
$e9b1	Random	X = Zero Page adress where the random bytes are placed, Y = # of shift register bytes (normally $02)		A, X, Y, $00	This is a shift-register based random number generator, normally takes 2 bytes (using more won't affect random sequence). On reset you are supposed to write some non-zero values here (BIOS uses writes $d0, $d0), and call this routine several times before the data is actually random. Each call of this routine will shift the bytes right.
$e9c8	SpriteDMA			A	This routine does sprite DMA from RAM $200-$2ff
$e9d3	CounterLogic	A, Y = end zeropage address of counters, X = start zeropage adress of counters		A, X, $00	This decrements several counters in Zeropage. The first counter is a decimal counter 9 -> 8 -> 7 -> ... -> 1 -> 0 -> 9 -> ... Counters 1...A are simply decremented and stays at 0. Counters A+1...Y are decremented when the first counter does a 0 -> 9 transition, and stays at 0.
$e9eb	ReadPads		$f5 = Joypad #1 data, $f6 = Joypad #2 data	A, X, $00, $01,	This read hardwired famicom joypads.
$ea1a	ReadDownPads		$f5 = Joypad #1 up->down transitions, $f6 = Joypad #2 up->down transitions $f7 = Joypad #1 data, $f8 = Joypad #2 data	A, X, $00, $01	This reads hardwired famicom joypads, and detect up->down button transitions
$ea1f	ReadOrDownPads		$f5 = Joypad #1 up->down transitions, $f6 = Joypad #2 up->down transitions $f7 = Joypad #1 data, $f8 = Joypad #2 data	A, X, $00, $01	This read both hardwired famicom and expansion port joypads and detect up->down button transitions.
$ea36	ReadDownVerifyPads		$f5 = Joypad #1 up->down transitions, $f6 = Joypad #2 up->down transitions $f7 = Joypad #1 data, $f8 = Joypad #2 data	A, X, $00, $01	This reads hardwired famicom joypads, and detect up->down button transitions. Data is read until two consecutive read matches to work around the DMC reading glitches.
$ea4c	ReadOrDownVerifyPads		$f5 = Joypad #1 up->down transitions, $f6 = Joypad #2 up->down transitions $f7 = Joypad #1 data, $f8 = Joypad #2 data	A, X, $00, $01	This read both hardwired famicom and expansion port joypads and detect up->down button transitions. Data is read until two consecutive read matches to work around the DMC reading glitches.
$ea68	ReadDownExpPads	$f1-$f4 = up->down transitions, $f5-$f8 = Joypda data in the order : Pad1, Pad2, Expansion1, Expansion2		A, X, $00, $01	This read both hardwired famicom and expansion port joypad, but stores their data separately instead of ORing them together like the other routines does. This routine is NOT DMC fortified.
$ea84	VRAMFill	A = High VRAM Adress (aka tile row #), X = Fill value, Y = # of tile rows OR attribute fill data		A, X, Y, $00, $01, $02	This routine does 2 things : If A < $20, it fills pattern table data with the value in X for 16 * Y tiles. If A >= $20, it fills the corresponding nametable with the value in X and attribute table with the value in Y.
$ead2	MemFill	A = fill value, X = first page #, Y = last page #		A, X, Y, $00, $01	This routines fills RAM pages with specified value.
$eaea	SetScroll			A	This routine set scroll registers according to values in $fc, $fd and $ff. Should typically be called in VBlank after VRAM updates
$eafd	JumpEngine	A = Jump table entry		A, X, Y, $00, $01	The instruction calling this is supposed to be followed by a jump table (16-bit pointers little endian, up to 128 pointers). A is the entry # to jump to, return address on stack is used to get jump table entries.
$eb13	???				Analyzing at present (related to something that plugs into expansion port, light gun maybe ?)
$eb66	LoadTileset	A = Low VRAM Adress & Flags, Y = Hi VRAM Adress, X = # of tiles to transfer to/from VRAM		A, X, Y, $00, $01, $02, $03, $04	This routine can read and write 2BP and 1BP tilesets to/from VRAM. See appendix below about the flags.

VRAM Structures

The VRAM structures is as following

SIZE   CONTENTS
2      VRAM Adress (big endian)
1      bit 0-5 length of data ($0 means 64 byte data)
       bit 6 : 0 = copy, 1 = fill
       bit 7 : 0 = increment by 1, 1 = increment by 32
n      Data to copy to VRAM
.....  repeated as many times as needed
1      $ff

The main structure is terminated by a $ff byte (High adress is always supposed to be in $00..$3f range)
$4c is a "call" command. 2 bytes that follow is adress of a sub-VRAM structure. The sub-structure can call another sub-structure and so on.
$60 is a "return" command. It will terminate a sub-structure.
If Fill mode is used, the routine takes only 1 byte of data which is repeated.

VRAM Buffer considerations

VRAM buffer is located at $300-$3xx. $300 holds the size of the buffer (maximum), and $301 the end index of the buffer. The actual buffer lies at $302-$3xx.

$300 is initialized to value $7d, effectively making the buffer lie at $300-$37f. It's possible to change the value here to make it bigger or smaller, but you the biggest possible value is $fd, making the buffer lie at $300-$3ff.
Format of the buffer is equivalent to VRAM structure above, exept that there is no sub-structures, no increment by 32 flag and no fill flag.
For this reason, the VRAM buffer at $302 can be used as a sub-structure.
Read routines are still obscure and analyzed at the moment

Load Tileset considerations

The flags parameters comes as follow :

7  bit  0
---------
AAAA MMIT
|||| ||||
|||| |||+- Fill bit
|||| ||+-- Transfer direction (0 = Write tiles, 1 = Read tiles)
|||| ++--- Bitplane type (see below)
++++------ Low VRAM Adress (aka tile # within a row)

        1st bitplane	2nd bitplane
        -----------	-----------
    0:  data	        data+8
    1:  data	        fill bit
    2:  fill bit        data
    3:  data^fill bit	data

This makes it possible for 1BP tiles to take all possible color schemes when they are written as 2BP. In read mode all non "data" bitplanes are replaced by dummy reads.

References

FDS Lister by ccovell