LDOS 5.3.1 for the Model III - SYS3/SYS Disassembled

SYS3/SYS

Other Navigation

LDOS 5.3.1 SYS3/SYS Disassembly - Close a File or a Device, @FNAME (Model III)

SYS3/SYS closes files and devices for LDOS and serves @FNAME. It is a system file, loaded into the overlay area at 4E00H by the supervisor dispatcher of SYS0/SYS (4BCCH) whenever a request code names it: bits 3-0 of the code are 5. The manual says this overlay must not be left off a system diskette.

The file is a load module of 751 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, three load blocks for 4E00H-50A5H, a load block for the two bytes 4BC8H-4BC9H (02A6H, the 678 bytes that SYS3/SYS takes up in the overlay area) and the transfer address record for 4E00H. SYS3/SYS uses the page above it, 5100H-51FFH, for the granule allocation table, and the system buffer 4300H-43FFH for the directory sectors and the Hash Index Table.

Functions

How a File Is Closed

The Date Stamp in the Directory Record

Errors

About This Page

4BC8H - The Overlay Length (Data)

Every system file from SYS1/SYS to SYS12/SYS carries a load block for the two bytes 4BC8H-4BC9H in the resident area of SYS0/SYS. They hold the length of the file's part in the overlay area: for SYS3/SYS 02A6H, the 678 bytes 4E00H-50A5H. In the file this block is the last load block, after the three blocks of 4E00H-50A5H and before the transfer address record (4E00H). SYS7/SYS reads and writes the word (its 55E4H, 5692H). The two bytes are data, not instructions.

4E00H - Entry: Pick the Function of the Request

The supervisor dispatcher (4BCCH) loads SYS3/SYS into 4E00H for a request code whose bits 3-0 are 5 and calls 4E00H with Register A = the request code (from OVRLY$, 4414H, which always gets the code of the latest request) and the caller's Register Pairs BC, DE and HL. Bits 6-4 of the code pick the function: 10H (95H, @CLOSE 4428H) close a file or a device; 20H (A5H, @FNAME 4293H) build the file specification of a directory record. Any other function returns at once. The dispatcher has dropped the return address of the RST 28H, so the final RET of a function goes straight back to the program that called the vector.

4E0CH - Function 10H (Request 95H): @CLOSE

Register Pair DE = the control block of an open file or device. @CLOSE (4428H) is called by BACKUP/CMD, BASIC/CMD, BASIC/OV2, CMDFILE/CMD, CONV/CMD, HELP/CMD, LCOMM/CMD, PATCH/CMD, SYS6/SYS, SYS7/SYS and TED/CMD. A control block whose byte +00H has bits 7 and 6 clear (10H, an open device) is closed at 5029H. For a file the prologue of SYS0/SYS is run, the file's directory record is read, the file's buffer is written when it was changed, and the end of the file is brought into the directory record (4E2AH).

4E2AH - Put the End of the File into the Directory Record

Register Pair HL = the file's directory record in the sector at 4300H. The end of file byte (+08H) and the ending record number (+0CH-+0DH) of the control block are compared with the record's +03H and +14H-+15H and copied into the record when they differ. Bit 2 of +00H of the control block, set by the sector write of SYS0/SYS (49C6H) whenever a sector of the file is written, then decides: a written file gets the date stamp (4E69H) and its record is written; a changed record of a file that was not written is written without a new date (4ECFH); otherwise the record is not written (4ED5H). Register Pair HL ends at the record's +15H on every path.

4E69H - Stamp the Date and the Time into the Directory Record

Register Pair HL = the record's +15H, Register B = the record number, Register C = the drive. Bit 6 of the record's +01H, the MOD flag (the file was changed since it was last backed up; DIR shows it as +), is set, and the date of DATE$ goes into the record: the month (421CH) into bits 3-0 of +01H, the day (421BH) into bits 7-3 of +02H and bits 2-0 of the year (421AH) into bits 2-0 of +02H. When the bit of the drive in 475DH is set (@CKDRV of SYS2/SYS sets it when the diskette carries the directory date format of this release) the time goes into +12H-+13H too: the hours (4219H) in bits 7-3 of +12H, the minutes (4218H) in bits 2-0 of +12H and bits 7-5 of +13H, and the year as an offset from 1980 (0-31) in bits 4-0 of +13H. The seconds are not kept.

4ECFH - Write the Directory Record and Count the File's Granules

4ECFH writes the sector with the record (Register B, drive Register C) back to the diskette. 4ED5H looks at bit 7 of the record's +01H, the create flag: a file whose space was set by CREATE keeps all of it (the manual: such a file never becomes smaller than its created size). For any other file the granules of its extents are counted into Register Pair DE, following the link (FEH and a record number in place of an extent) to the extension records that hold its further extents.

4F03H - Work Out the Granules the File Does Not Need

Register Pair DE = the granules in the file's extents, Register Pair HL = one byte past the FFH that ends them, in the record whose sector is at 4300H (Register B). The granules the file needs are its ending record number (the sectors it uses) divided by the sectors in a granule, rounded up. When the extents hold more, the difference is freed: the allocation table is read into 5100H and the work starts at the last extent (4F6FH).

4F2EH - Free the Last Granule of an Extent

Register Pair HL = the second byte of the extent, Register Pair DE = the granules still to free, Register B = the record whose sector is at 4300H, Register C = the drive, and the allocation table at 5100H (one byte for each cylinder, a bit set for each granule in use). The extent's last granule is found (its cylinder and its place on the cylinder), its bit in the allocation table is cleared and the extent's count of granules is lowered by one. An extent left with no granule goes on at 4F6FH.

4F6FH - An Extent Has No Granule Left

Register Pair HL = the second byte of an extent with no granule left (or, from 4F2CH, the byte after the FFH that ended the extents). The two bytes become FFH FFH, the mark of an unused entry and of the end of the extents, and Register Pair HL steps back to the extent before. When there is none in this record and the record is an extension record (attribute bit 7), the extension record is freed: its attribute becomes 00H, its byte in the HIT 00H, and the link at +1EH-+1FH of the file's own record (whose number the extension record holds at +01H) becomes FFH FFH. The work then goes on at the last extent of the file's own record.

4FB2H - Write the Directory Record and the Allocation Table

While granules remain to be freed (Register Pair DE) the work goes back to 4F2EH. Then the directory sector at 4300H is written; a write protected diskette (error 0FH) is not an error here: nothing is written and the close goes on. Otherwise the allocation table at 5100H is written too. 4FC5H, where every path of the close meets, reads the sector of the file's own record again when another sector is at 4300H, and points Register Pair DE at the control block for 4FD8H.

4FD8H - Build the File Specification of a Directory Record

Register B = a directory record whose sector is at 4300H, Register C = its drive, Register Pair DE = where the text goes. The text is the name without its trailing spaces, then a / and the extension when the extension is not blank, then a colon, the drive's digit and 03H, the end of text mark of a file specification: NAME/EXT:d. No password is written. After @CLOSE the control block holds this text, so a program can open the file again with the same block. Returns Register A = 00H with the Z FLAG and Register Pair DE at the 03H; Register B is changed.

501CH - Function 20H (Request A5H): @FNAME

Register B = a directory record number, Register C = a drive, Register Pair DE = a buffer of at least 15 bytes. @FNAME (4293H) reads the record and writes its file specification NAME/EXT:d with the 03H mark into the buffer (4FD8H). Returns Register A = 00H with the Z FLAG, or 11H (directory read error) with the NZ FLAG; Register Pairs DE and HL and Register C are kept, Register B is changed. SYS6/SYS (its 53F2H), SYS7/SYS (its 527BH, 52A0H and 531FH) and LCOMM/CMD (its 62D6H, 62F9H) call 4293H. The record is not tested for being in use.

5029H - Close a Device

Register Pair DE = the control block of a device opened by @OPEN of SYS2/SYS (+00H 10H, +01H-+02H the address of its own byte +08H, where SYS2/SYS put the six byte driver LD HL with the device control block and JP 4D62H, +06H-+07H the two letters of the device name). The block is made into the device specification again: an asterisk, the two letters and 03H.

5042H - Make the RES Instruction for a Granule

Register A = a granule's place on its cylinder (0-7), Register B = the cylinder's allocation table byte. The bit number is put into the RES instruction at 504CH, which then clears that bit of Register B.

504FH - Read the Allocation Table

Reads sector 0 of the directory cylinder of drive Register C, the granule allocation table (GAT), into 5100H, the page above SYS3/SYS in the overlay area. Returns the Z FLAG with Register A = 00H, or the NZ FLAG with Register A = 14H (GAT read error). Register Pairs DE and HL are kept.

5062H - Write the Allocation Table

Writes the allocation table at 5100H to sector 0 of the directory cylinder of drive Register C with the deleted data mark, as every directory sector is written, and verifies it. Returns the Z FLAG when written and verified, else the NZ FLAG; Register A = 15H (GAT write error) either way. Register Pairs DE and HL are kept.

507BH - Read the Hash Index Table

Reads sector 1 of the directory cylinder of drive Register C, the Hash Index Table (HIT, one byte for each directory record: the hash code of its file name, 00H for a free record), into 4300H. Returns the Z FLAG when read, else the NZ FLAG; Register A = 16H (HIT read error) either way. Register Pair HL = 4300H; Register Pairs BC and DE are kept.

508DH - Write the Hash Index Table

Writes the HIT at 4300H to sector 1 of the directory cylinder of drive Register C with the deleted data mark and verifies it. Returns the Z FLAG when written and verified, else the NZ FLAG; Register A = 17H (HIT write error) either way. Register Pairs BC and DE are kept.