83Plus:Software:usb8x/Asm Interface/MSD

The mass storage driver works through a series of U_CALL entry points, much like the HID driver.

It requires two RAM buffers, one for internal use and one for a sector buffer. File handles used for opened files (explained later) also require around 27 bytes. The size of all three of these buffers can be obtained through the MSDVersion U_CALL. The size of the internal buffer is typically 254 bytes and the sector buffer is 512 bytes.

The entry point which requires these buffers is the MSD_Initialize U_CALL. Its memory can be dynamically allocated to appBackUpScreen (taking up practically all of it) with the code below:

U_CALL MSDVersion
ld de,appBackUpScreen
push de
add hl,de
ex de,hl
pop hl
U_CALL MSD_Initialize

For file system operations, you will then make calls to UFI_Initialize and FAT_Initialize. All entry points will return with the carry flag set if there are problems, with more detailed information available via GetErrorCode.

No knowledge of the intimate details of FAT16 are required. However, the "starting cluster" of a directory is used in several routines. This is a 16-bit number uniquely indentifying a directory. It is beneficial to use this number rather than an extremely long ASCIIZ directory path which wastes memory. It is easily obtained with FAT_getStartingCluster by passing an ASCIIZ directory path to it.

Entry Points

If all three of the initializing U_CALLs succeed, the other following entry points can then be called.

MSDVersion	Gets mass storage driver version and memory requirements
MSD_Initialize	Initializes mass storage driver
UFI_Initialize	Initializes connected mass storage device for sector read/write operations
UFI_getCapacity	Gets capacity of entire drive and sets up bytes-per-sector
UFI_Read	Reads sector at (MSDLBA) address into sector buffer
UFI_Write	Writes memory in sector buffer to (MSDLBA) address
msdDetect	Detects and loads 83P/8XP levels based on detection string and specified directory
FAT_Initialize	Initializes connected mass storage device for FAT16 file system operations
DOS_openFile	Opens a file and populates file handle with important information
DOS_fileSeek	Sets seek pointer for file handle to specified offset
DOS_fileRead	Read bytes from a file
DOS_fileWrite	Write bytes to a file and grows it if necessary
DOS_updateAttributes	Update "last modified" date/time and other attributes
DOS_createFile	Create new file in specified directory with size 0
DOS_createDirectory	Create new subdirectory in specified directory
DOS_renameFileEntry	Rename specified file or folder
DOS_deleteFileEntry	Delete file or folder (including contents)
DOS_countFilesInDir	Get number of files/folders in specified directory
DOS_getNextFile	Get first/next file in specified directory
DOS_getFileSize	Get size of file
DOS_getDirEntry	Get directory entry for specified file/folder
FAT_nameConvertTo11	Convert name to 11-character format (ex. "README TXT")
FAT_nameConvertFrom11	Convert name to ASCIIZ format (ex. "readme.txt")
FAT_lookupPath	Find whether specified file/directory exists
FAT_findFileInCluster	Get directory entry for a file in specified folder
FAT_getStartingCluster	Get starting cluster of specified directory
MSD_ImportVariable	Imports 8x* file in specified directory as calculator variable in RAM/Flash
MSD_ExportVariable	Exports calculator variable in RAM/Flash as 8x* file in specified directory

Other File Systems

You can use the MSD_Initialize, UFI_getCapacity, UFI_Read, and UFI_Write entry points to create your own file system driver. For example, you could create a driver for ext2/3, FAT32, or any other file system imaginable.

The low-level parts of the mass storage driver work by reading and writing sectors to the drive based on the current LBA address. This is located in (MSDLBA) in the mass storage driver's memory. To create your own file system driver, you will need to implement all of the details specific to that file system.

The MSD_Initialize entry point is required for any communication with the drive. When this entry point succeeds, you would then call UFI_getCapacity to set up (MSDBPS) in the driver memory. This is necessary for the UFI_Read and UFI_Write entry points to succeed.

Once both entry points succeed, you are free to read and write as much as you like. The boot sector is located at LBA address 0x00000000. Most file systems require that you read this first to find out what to do next. Check out the mass storage driver in the usb8x source for more information.