DOSPLUS 3.5 for the Model I - SYS3/SYS Disassembled

SYS3/SYS

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DOSPLUS 3.5 SYS3/SYS Disassembly - CLOSE@ and KILL@ (Model I)

SYS3/SYS is the low overlay that closes and removes files: CLOSE@ (request code 23H) and KILL@ (24H), and two routines other overlays request: 25H, which frees the space and the entries of a file by its logical file number (SYS14/SYS, the library command KILL), and 26H, which puts the name of an open file back into its control block without closing it (SYS11/SYS). SYS0/SYS loads it at 4E00H over the low overlay before it.

The file is a load module of 3 blocks, 728 bytes, from 4E00H to 50D7H, with the transfer address 4E00H. It uses the 256 bytes at 5100H, after its end, as the buffer for the granule allocation table (5100H); the hash index table and the directory sectors go through the system buffer at 4200HH.

The Parts of SYS3/SYS

Closing a File

CLOSE@ looks at the type byte of the control block: only an open file (80H-87H) is closed. An open device (90H) and a control block that is not open come back at once with Register A = 00H. A file first has its buffer written, through SYS0/SYS's 4C72H, when it is blocked and holds updated bytes; when that write fails with error 1BH (Disk space full), the end of file byte is set to 00H, the entry is brought up to date all the same and 1BH is returned at the end (4E3AH).

The directory entry is then read (4E49H). When the file has been written to (bit 6 of FCB+02H) the entry gets the modified flag and the month in its +01H, the day and year in its +02H, the end of file byte in +03H and the number of sectors (the ending record number, plus 1 when the end of file byte is not 00H) in +14H-15H, and is written back. The date comes from DATE$ (5078H); bits 2-0 of the year are stored, which is the year less 1980 up to 1987.

Unless the entry is non-shrinkable (bit 7 of its +01H, kept in bit 7 of FCB+02H), the granules the file holds beyond its end are given back, whether or not the file was written to (4EA5H). The granules are counted over every segment of every entry of the file (FEH links to an extended entry), the granules needed are worked out with DIVD@ from the sectors and the sectors per granule, and the difference is freed one granule at a time from the end of the last segment: each granule's bit in the allocation table (read into 5100H) is cleared, the segment's count is lowered, an emptied segment becomes FFH FFH, and an extended entry left with no segment is freed (its first byte and its hash index byte set to 00H) and the entry before it loses its FEH link (4F3AH). The entry and the table are written at the end.

Last, the control block becomes the file's specification again: name, / and the extension, : and the drive's name from DCTTBL, and 03H (4F82H). The password is not put back. Routine 6 does only this part, for an open file, without closing it (4F75H).

Removing a File

KILL@ needs an open file whose access level (bits 2-0 of FCB+01H) is 0 or 1; otherwise it returns error 25H (4FE4H). Routine 5 then frees the file: for each of its entries the segments are cleared in the allocation table (504AH), the entry is made inactive by clearing bit 4 of its first byte, and its hash index byte is set to 00H; the entry keeps its name and segments, which is what lets a recovery tool bring the file back, as the manual says. An entry whose list ends with FEH is followed to the extended entry it names. The table is written back last, and KILL@ clears the 32 bytes of the control block (4FFEH).

The granule bits are cleared by a RES n,(HL) instruction whose second byte is stored before it runs (506BH); the logical file number of the entry before a freed extended entry is stored into the operand of an LD B,n (4F48H).

Differences from the Manual

The manual (T/25) says that after CLOSE@ a device's control block holds the device's name again. DOSPLUS 3.5 returns at once for a device (4E16H); the code after the RET that would put back a name is not reached, and in the Model I file it also lacks the store of the @ that the Model III file has.

The manual does not say that CLOSE@ gives back the granules beyond the end of a file that was only read, nor that a Disk space full error while writing the last buffer still closes the file.

4E00H - The Routine Table

Data: SYS3/SYS's table of routines 2-6, one word each, used by SYS0/SYS's overlay request handler (its 49C9H-49DCH) for the request codes 22H-26H. CLOSE@ (4428H) requests 23H and KILL@ (442CH) 24H; SYS14/SYS requests 25H (its 553AH) and SYS11/SYS 26H (its 56E3H). No file of the diskette requests 22H.

4E0AH - Routine 2

Nothing to do when SYS0/SYS's record locator brings SYS3/SYS back.

4E0CH - CLOSE@ - Close a File or a Device

Routine 3, requested by CLOSE@ (4428H) with Register Pair DE pointing to a file or device control block. An open file (type byte 80H-87H) has its last buffer written, its directory entry brought up to date, the granules beyond its end given back (unless the entry is non-shrinkable) and its name, extension and drive put back into the control block. Anything else (an open device, type 90H, or a control block that is not open) returns at once with Register A = 00H and nothing changed. Register A returns 00H with the Z FLAG, or an error code with the NZ FLAG.

4E49H - Bringing the Directory Entry Up to Date

Reads the sector of the file's directory entry (logical file number FCB+07H, drive FCB+06H). When the file has been written to (bit 6 of FCB+02H), the entry gets the modified flag, today's date, the end of file byte and the number of sectors, and is written back. Then, unless the entry is non-shrinkable (bit 7 of FCB+02H, from bit 7 of the entry's +01H), the granules beyond the end of the file are given back (4EA5H on). It ends at 4F82H, which puts the file name back into the control block.

4F75H - Routine 6 - Putting the Name Back

Requested with code 26H (SYS11/SYS, its 56E3H) for the open file at Register Pair DE: the directory entry is read and the file's name, extension and drive are put back into the control block as a file specification, as CLOSE@ does, but nothing is written to the diskette.

4F82H - The File Specification Put Back

Makes the file control block at Index Register IX the file's specification again: the name (up to 8 characters), a / and the extension when there is one, a : and the drive's name from DCTTBL, and 03H at the end. The password is not put back (the manual, T/25, says the same). The directory sector of the file's entry is read again first when the buffer holds another sector.

4FE4H - KILL@ - Remove a File

Routine 4, requested by KILL@ (442CH) with Register Pair DE pointing to an open file control block. The file needs an access level of 0 or 1 (FCB+01H bits 2-0), else error 25H. Its granules are given back in the granule allocation table, every entry of the file is made inactive (bit 4 of the first byte cleared, the rest left, so a recovery tool can find it) and has its hash index byte cleared; then the 32 bytes of the control block are set to 00H.

5009H - Routine 5 - Freeing a File's Space and Entries

Requested with code 25H (SYS14/SYS, its 553AH) with Register B the logical file number of a file's first entry and Register C its drive; KILL@ comes in at 500CH. For each entry of the file in turn: its segments are freed in the granule allocation table (read into 5100H), the entry is made inactive and written, and its byte in the hash index table is cleared; an entry whose list ends with FEH goes on to the extended entry it names. The table is written back at the end.

504AH - Freeing a Segment

Clears in the granule allocation table at 5100H the bits of the granules of the segment Register E (cylinder), Register D (bits 7-5 the first granule, bits 4-0 the number less 1). The segment may run on into the next cylinders: the granule count wraps at the granules per cylinder (DCT+10H at Index Register IY).

506BH - Freeing a Granule

Clears bit Register A (0-7) of the byte at Register Pair HL, by storing the second byte of a RES n,(HL) instruction (86H + n times 8) at 5076H and running it.

5078H - The Date for the Directory Entry

Returns Register B = the month (bits 3-0 of 4046H) and Register C = the day (bits 4-0 of 4045H) in bits 7-3 and bits 2-0 of the year (4044H, the year less 1900) in bits 2-0, the form of an entry's +02H; Register A returns the month as well. The manual gives bits 2-0 as the year less 1980: 80 is a multiple of 8, so the two agree for 1980-1987.

5090H - Reading and Writing the Tables

5090H reads and 50A2H writes the granule allocation table (directory sector 0) of drive Register C through 5100H; 50B4H reads and 50C6H writes the hash index table (directory sector 1) through 4200H, the system buffer. Each returns the Z FLAG when done, or the NZ FLAG with error 14H, 15H, 16H or 17H.

5100H - The Table Buffer

Not in the file: the granule allocation table of the drive is read here (5090H) and written from here (50A2H), one byte per cylinder from 5100H, bit n set when granule n of the cylinder is in use.