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|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_Read	Reads sector at (MSDLBA) address into sector buffer
UFI_Write	Writes memory in sector buffer to (MSDLBA) address
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_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