Difference between revisions of "83Plus:Software:usb8x/Asm Interface/MSD"

Latest revision as of 19:02, 17 February 2009

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.

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.