LDOS 5.3.1 for the Model III - SYS2/SYS Disassembled

SYS2/SYS

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LDOS 5.3.1 SYS2/SYS Disassembly - Open, Create and Check a Drive (Model III)

SYS2/SYS opens files and devices for LDOS. 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 4. It serves @OPEN (open an existing file or a device), @INIT (open a file, creating it when it is on no diskette) and @CKDRV (check that a drive is ready), and it holds the two hash routines of the directory: the one byte hash code of a file name, kept in the Hash Index Table (HIT, sector 1 of the directory cylinder), and the 16 bit hash code of a password, kept in the directory record.

The file is a load module of 1079 bytes: a record of type 1FH with the text Copyright 1991 MISOSYS, Inc., All rights reserved, six load blocks for 4E00H-51FEH (the bytes 51B8H-51BCH and 51E1H-51F8H are not loaded), a load block for the two bytes 4BC8H-4BC9H (0400H, the 1024 bytes 4E00H-51FFH that SYS2/SYS takes up, its work cell 51FFH included) and the transfer address record for 4E00H. A library command of SYS6/SYS or SYS7/SYS runs at 5200H, above SYS2/SYS, so after it has opened a file it can use the work area at 51E1H-51FFH and call the HIT routine at 51BDH (RENAME, ATTRIB).

Functions

How a File Is Opened

Passwords and Access

The Control Block of an Open File

@CKDRV and the Drive Code Table

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 SYS2/SYS 0400H, the 1024 bytes 4E00H-51FFH, one more than the last loaded byte (51FEH), because 51FFH is a cell of SYS2/SYS that the file does not load. In the file this block is the last load block, after the six blocks of 4E00H-51FEH 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 SYS2/SYS into 4E00H for a request code whose bits 3-0 are 4 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 (94H, @OPEN 4424H) open an existing file or a device; 20H (A4H, @INIT 4420H) open a file or create it; 40H (C4H, @CKDRV 4209H) check a drive; 50H (D4H) the file name hash; 60H (E4H) the password hash. Any other function (00H for request 84H, which SYS6/SYS, BACKUP/CMD and FORMAT/CMD issue to have SYS2/SYS in memory before they need it) returns at once.

4E17H - Function 50H (Request D4H): The File Name Hash

Register Pair HL = 11 bytes, the file name (8 bytes) and the extension (3 bytes), each padded with spaces, as a directory record holds them at +05H-+0FH. The hash code is the Exclusive OR of the 11 bytes, rotated left one bit after each byte; 00H is turned into 01H, because 00H in the Hash Index Table (the HIT, sector 1 of the directory cylinder) marks a free directory record. The code is stored at 51E1H and returned in Register A with the NZ FLAG. @OPEN and @INIT call it at 4F63H; SYS6/SYS's RENAME issues request D4H (its 52D0H).

4E27H - Function 60H (Request E4H): The Password Hash

Register Pair DE = an 8 byte password field, padded with spaces. The bytes are taken from the last to the first and mixed into a 16 bit value that starts at FFFFH; the result is returned in Register Pair HL with the Z FLAG. Eight spaces (no password) give 4296H, the password PASSWORD gives 42E0H, and the master password RS0LT0FF (the manual: it allows access to any file) gives 113DH, the value @OPEN compares with at 4FECH. @OPEN and @INIT call it at 4F69H; SYS7/SYS (its 532CH and 54E3H), BACKUP/CMD (its 6C6BH) and FORMAT/CMD (its 7178H) issue request E4H.

4E52H - Function 40H (Request C4H, @CKDRV): Check a Drive

Register C = the drive (0-7). The drive must be present in its drive code table, its diskette must turn (the index hole must pass the sensor within half a second, 15 ticks of the 30 a second of the Model III, unless bit 4 of byte +04H of the table is set) and the allocation table (sector 0 of the directory cylinder) must read. Then the drive code table is brought into line with the diskette from the allocation table's bytes CCH and CDH: the highest cylinder (+06H), and bit 5 of +04H (two sides; unlike the Model I SYS2/SYS it does not set bit 7 of +04H for a diskette whose cylinder 0 is single density); and bit Register C of 475DH is set when the diskette carries the directory date format of this release (bit 3 of byte CDH), cleared when not. Returns the Z FLAG and Register A = 00H when the drive is ready, the CARRY FLAG then set when the diskette is write protected (the controller's status bit 6 or bit 7 of +03H); otherwise the NZ FLAG. Register Pairs BC, DE and HL and Index Register IY are kept. @OPEN and @INIT call it at 4F76H and 504EH; BACKUP/CMD, CMDFILE/CMD, DATECONV/CMD, HELP/CMD, LOG/CMD, SYS6/SYS and SYS7/SYS call @CKDRV (4209H).

4ECBH - Read the Index Hole, or Give Up at the Time Limit

Called from the three loops at 4E78H-4E85H with Register D = the time limit. When TIMER$ (4288H) has reached the limit it does not return to its caller but jumps to 4ED7H; otherwise it returns the NZ FLAG when the controller's status shows the index hole at the sensor (bit 1), the Z FLAG when not.

4ED7H - The Drive Is Not Ready

Reached from 4ECFH (the time limit, with the return address of the CALL 4ECBH on the stack) and from 4E92H (the allocation table could not be read, with the status saved at 4E87H on the stack). Either word is taken off, Register A gets bit 0 set and the routine returns with the NZ FLAG.

4EDCH - Open a Device

@OPEN and @INIT come here when the control block at Index Register IX starts with 2AH, a device specification that @FSPEC (SYS1/SYS) copied there: 2AH (an asterisk) and the two letters of the device name in +01H-+02H. The name is looked up in the table of device names of SYS0/SYS at 42ECH-42FFH: ten slots of two letters, KI, DO, PR, JL, SI, SO and four slots of 00H 00H for the devices that SET and ROUTE make. Slot n gives the device control block: slots 0-2 the blocks of 8 bytes at 4015H + 8 x n (KIDCB$ 4015H, DODCB$ 401DH, PRDCB$ 4025H), slots 3-9 the blocks of 6 bytes of SYS0/SYS at 42B0H + 6 x n (JLDCB$ 42C2H, SIDCB$ 42C8H, SODCB$ 42CEH and the spare blocks 42D4H, 42DAH, 42E0H, 42E6H). The control block is then made a device control block of its own: +00H 10H, +01H-+02H the address of its own byte +08H, +03H-+05H 00H, +06H-+07H the name, and +08H-+0DH a copy of the six bytes at 4F37H, LD HL with the address of the device's block and JP 4D62H. A request on the open device goes through @DVRHK (4D65H) to that copy, which enters the driver of the device's block. The Model I SYS2/SYS instead searches the names at +06H-+07H of the blocks themselves and stores the block's address at +01H-+02H. A name that is not found returns error 08H (device not available).

4F37H - The Driver Copied into the Control Block of a Device

The six bytes 4F37H-4F3CH are not run here: 4F2EH copies them into bytes +08H-+0DH of the control block of an opened device, after 4F0FH has stored the address of the device control block at 4F38H. A request on the open device goes through @DVRHK (4D65H) to the address at +01H-+02H of the control block, which is this copy: Register Pair HL is loaded with the device control block, and 4D62H in SYS0/SYS makes it Index Register IX and enters the driver of that block.

4F3DH - Function 10H (Request 94H, @OPEN): Open a File

Register Pair DE = a control block holding a file specification NAME/EXT.PASSWORD:D (as @FSPEC leaves it, ended by 03H or 0DH) or a device specification (2AH and the two letters of a device name), Register Pair HL = the address of the file's 256 byte buffer, Register B = the record length (00H for 256). The prologue 49F1H points Index Register IX at the block and makes the final RET restore the caller's registers. The specification is taken apart (5106H) and hashed; then the Hash Index Table of the drive named, or of drives 0-7 in turn when no drive is named, is searched for the name's hash code, each match checked against its directory record, and when the file is found its access is decided from the password and the control block is filled (5096H). Returns the Z FLAG and Register A = 00H, or the NZ FLAG and Register A = the error number: 13H illegal file name, 16H HIT read error, 11H directory read error, 18H file not in directory, 19H file access denied, 20H illegal drive number, 08H device not available. @INIT calls 4F40H.

4F97H - Go On to the Next Drive

Register C = the drive just searched. When the specification named a drive (4F70H not FFH) there is no other drive to search: the NZ FLAG is returned with the CARRY FLAG clear. Otherwise Register C is advanced and the CARRY FLAG is set while it is below 8.

4FA2H - Not This Record: Read the HIT Again

The directory record whose hash code matched is not the file (an extension record, not in use, or another name). Its sector has replaced the HIT in 4300H, so the HIT is read again and the search goes on with the next byte.

4FACH - A Hash Match: Check the Directory Record

Register L = the position in the HIT, which is the directory record number, Register C = the drive. The record is read (DIRRD reads the directory sector that holds it into 4300H) and must be the first record of a file (bit 7 of its attribute clear: SYS8/SYS gives the extension records it makes attribute 90H), in use (bit 4) and have the same name and extension (+05H-+0FH) as 51EAH-51F4H.

4FD2H - The File Is Found: Decide the Access

The name matches. The drive where the file is becomes the drive of the specification (4F70H), the attribute goes to 51FBH, and the access the open gives (bits 2-0 of +01H of the control block, 0 the most access) is decided: 00H when the password's hash code is 113DH (the master password RS0LT0FF) or equals the record's +10H-+11H; otherwise the protection level in bits 2-0 of the attribute, except that level 7 is refused (error 19H, file access denied) and level 6 (execute only) gives 06H, or 05H (read) with bit 1 of SFLAG$ set (run the program without DEBUG) when the open is the program load of @RUN; for level 6 the BREAK trap is also left off when the request returns.

5028H - Function 20H (Request A4H, @INIT): Open or Create a File

The same registers as @OPEN. The file is opened when it is on a diskette (the Z FLAG, the CARRY FLAG clear). When @OPEN returns error 18H (file not in directory), the file is created: on the drive named, or on the first of drives 0-7 that is ready, not write protected and has a free directory record. A new file gets the access 00H, the attribute 10H (in use, protection level 0), or 17H (protection level 7) when the specification holds a password, its record length, its name and extension and the password's hash code; it has no extents. Returns the Z FLAG and the CARRY FLAG for a new file; the NZ FLAG with Register A = 1AH (full or write protected disk) when no drive could take it, or with the error of the open or of a disk access.

5067H - Create the Directory Record

Register Pair HL = the byte of the free record in the HIT at 4300H, so Register L = the record number; Register C = the drive. The name's hash code goes into the HIT and the HIT is written back; then the record's sector is read and the record is made: +00H-+04H from 51FBH-51FFH (the attribute, 00H, 00H, the end of file byte 00H, the record length), +05H-+15H from 51EAH-51FAH (the name, the extension, the password hash code at +10H-+11H, the two bytes 51F7H-51F8H at +12H-+13H, and the ending record number 0000H), +16H-+1FH FFH (no extents). The sector is written back and the control block filled.

5096H - Fill the File Control Block

Register Pair HL = the file's directory record in 4300H, Register B = its record number, Register C = the drive, Index Register IX = the control block. The block gets: +00H 80H (an open file); +01H the access (bits 2-0, from 50A2H), bit 5 (the buffer must be read first), bit 3 when the specification ended with ! (50AAH) and bit 7 when the record length is not 256 (byte mode); +02H 00H; +03H-+04H the buffer (50B2H); +05H 00H, the position in the buffer; +06H the drive; +07H the record number; +08H the end of file byte (record +03H); +09H the record length the caller asked for (51FFH); +0AH-+0BH 0000H, the next record; +0CH-+0DH the ending record number (record +14H-+15H); from +0EH the record's extents (cylinder, granule byte), each one after the first preceded by the number of granules in the extents before it, as far as the record has them; the rest of the 32 bytes FFH. Returns the Z FLAG and Register A = 00H.

50FFH - Fill with FFH

Stores FFH in Register B bytes from Register Pair HL on, then returns Register A = 00H with the Z FLAG. 50FBH ends the control block with it, 5088H the extents of a new directory record.

5106H - Take the File Specification Apart

Register Pair HL = the specification NAME/EXT.PASSWORD:D (the name a letter and up to 7 letters or digits, the extension a letter and up to 2 more, the password the same as the name; capitals only, which @FSPEC makes of small letters). 51E2H-51F4H are filled with spaces, then the name goes to 51EAH-51F1H, the extension to 51F2H-51F4H, the password to 51E2H-51E9H, the drive number to 4F70H (FFH when none) and the ! flag to 50AAH. Returns the Z FLAG, or the NZ FLAG with Register A = 13H (illegal file name: the name does not start with a letter) or 20H (illegal drive number: not 0-7).

5151H - The Password Field

Called when the specification holds a . and so a password: a file created with a password gets the attribute 17H (in use, protection level 7, no access without the password) instead of 10H. Then the password is copied like the name, up to 8 characters.

5156H - Copy a Field

5156H copies a field of up to 8 characters (the name or the password), 5158H a field of up to Register B characters (the extension), from the text at Register Pair HL to Register Pair DE. The first character must be a capital letter A-Z, the others capital letters or digits; any other character ends the field. Returns Register A = the character after the field and Register Pair HL past it, and Register B = the characters not used.

5172H - Find a Free Directory Record

The Hash Index Table of drive Register C is at 4300H (Register H = 43H). Byte n of the HIT stands for directory record n, which is in sector (n AND 1FH) + 2 of the directory cylinder, so only the bytes whose bits 4-0 are below the number of directory record sectors (the sectors of the cylinder less the allocation table and the HIT) stand for records on the diskette. The search starts at the byte that TIMER$ (4288H) gives and goes up to FFH; when it finds no free byte it searches again from 08H. Returns the Z FLAG with Register Pair HL at a free byte (00H), or the NZ FLAG when every record is taken.

51B8H - Not in the File

The file loads nothing at 51B8H-51BCH; no code of SYS2/SYS addresses these bytes, so they keep whatever was there before SYS2/SYS was loaded.

51BDH - Read or Write the Hash Index Table

51BDH reads, 51BEH writes, sector 1 of the directory cylinder (the HIT) of drive Register C, with the buffer 4300H. The bytes F6H AFH at 51BDH are OR AFH, which leaves the NZ FLAG (read); entered at 51BEH, the AFH alone is XOR A, which leaves the Z FLAG (write). A read accepts only a sector with the deleted data mark of a directory sector (error 16H, HIT read error, when it fails); a write puts the deleted data mark on it and verifies it (error 17H, HIT write error). Returns the Z FLAG on success, else the NZ FLAG with Register A = the error; Register Pair HL = 4300H, Register Pairs BC and DE kept. SYS6/SYS's RENAME calls both entries (its 52A7H, 52B8H).

51E1H - The Work Area

The cells where @OPEN and @INIT keep the parts of the specification and the five bytes that start a new directory record. 51E1H-51F8H are not in the file; 51F9H-51FEH are loaded with 00H; 51FFH is the last byte of the 0400H that SYS2/SYS takes up (4BC8H) but is not in the file. SYS6/SYS (RENAME, its 5272H-52B8H) and SYS7/SYS (its 5284H-53AFH) use these cells after an @OPEN.