Chapter Four


DUP gets control whenever the system is booted or the RESET key is pressed. DUP will call the FMS initialization routine, DINIT at $7E0.


1) Determine how many (and what type of) disk drives will be used.
2) Set up a drive table and allocate a drive buffer for each drive.
3) Allocate sector buffers and build the sector buffer table.
4) Clear the FCBs to zero.
5) Set MEMLO.
6) Enter the D: device into the Device Handler Table.
7) Exit to caller via RTS.
Drive Determination
The DRVBYT byte at $70A is used to tell FMS how many disk drives will he used and what the drive number of the drives will be. The rightmost bit (bit $01) indicates drive 1. The next left bit ($02) indicated drive 2 and so forth. If the bit is one, then the drive is to be used. If the drive is zero then the drive is not to be used. The code will allocate up the eight drives, even though the 810 hardware only has switches for drives 1,2,3 and 4.
     If DRVBYT indicates that a drive is to be used, then FMS issues a status command to that drive to determine if it is active and what type (810 or 815) of drive it is.

Drive Allocations
The drive determination process sets up two tables (Figure 4-1). The first table is the DRVTBL. This table is indexed into by the drive number (minus one). If the value in the table is zero then the drive is not to be used. If the value is one, then the drive is an active 810 and requires one drive buffer. If the value is two, then the drive is an 815 and requires two 128 byte buffers.
     The second table is the drive buffer table. The drive buffer table contains the address of the drive buffer to be used for each drive. This Drive Buffer will be used to hold the VTOC sector on the diskette in the drive. The table is separated into two sections: DBUFAL contains the least significant address byte and DBUFAH which contains the most significant address byte. The drive buffer table is also accessed by the drive number (minus one).
     When a file is being processed, the Drive number is obtained from the IOCB Device Number field, ICDNO. The obtained value is decremented by one and is then used as an index into the Drive Tables. The Drive Type is copied from the DRVTBL entry to DRVTYP ($12FE) for easy access by FMS. The Drive Buffer address is copied from the DBUFAL and DBUFAH table entries to the zero page drive buffer pointer, ZDRVA ($45).

Sector Butter Allocations
The SABYTE at location $709 is used to inform FMS about the number of 128 areas to he allocated as sector buffers. One 128 buffer is required for each file which is to he processed concurrently on 810 drives. Two 128 byte buffers are required for each file which is to be processed concurrently on 815 drives.
     The Sector Buffer Allocation table, SECTBL at $1319, is used to indicate if a buffer is available for allocation to a file (Figure 4-2). If a buffer is available, the entry is set to zero. If the buffer is not available, the entry is a minus value. The table is 16 bytes in size and therefore can be used to allocate up to sixteen 128 byte buffers. During the initialization process, entries which are to be unused are set to a minus value.
     The Sector Buffer Address Table is a table of addresses which point to the individual sector buffers. The table is divided into two parts: SABUFL contains the least significant address byte, SABUFH contains the most significant address byte.
     When a file is being processed, an available buffer number is found in SECTBL by search for a zero valued entry. The located buffer is allocated to the file by entering a minus value ($80) into the table and placing the corresponding buffer number into the DCB buffer number field, FCBBUF. When the file processing is done, the buffer is deallocated by setting the SECTBL entry to zero.

Setting MEMLO
The Atari MEMLO location ($2E7) is set after the FMS buffers have been allocated. The address of the last sector buffer allocated is incremented by 128. This value is then placed into MEMLO.

Device Handler Table Entry
The Device Handler Table ($31A) is searched for a D entry or the first (from the top) empty entry. When an appropriate entry is found, FMS inserts (or reenters) D as a DEVICE NAME and sets the DEVICE vector entry to point to the FMS Device Vector table at DFMSDH ($7CB).

Drive Tables

Sector Allocation Tables

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