TRS-80 DOS - LDOS 5.3.1 for the Model I - Disassembled

The LDOS 5.3.1 bootstrap program written to a diskette by FORMAT and SYSGEN. The Model I boot ROM loads track 0, sector 0 of drive 0 to 4200H and jumps to it; that sector reads the resident system loader's directory record from the directory cylinder, streams the file as an LDOS load module, and transfers to its entry point. Disk access is direct to the memory-mapped WD1771 (Seek 1BH, Read Sector 88H, Force Interrupt 0D0H). The 1792-byte file also stores an alternate drive-geometry-aware boot sector, a Drive Code Table (DCT) initialization-template table, the day/month name tables, the "LDOS531 07/14/91" identification string, and a secondary loader.

A disassembly of BOOT/SYS can be found here.

SYS0/SYS is not an overlay - it is the resident part of the operating system (SYSRES). It remains permanently in memory and must be present on any disk used to boot the system, but may be removed from disks not used for booting.

From the disassembly, SYS0/SYS spans 3C00H through 50FFH and contains the sign-on text and block-graphics "LDOS" logo, the RST/DOS vectors at 400CH, the device control blocks for *KI/*DO/*PR and *JL/*SI/*SO, the clock and interrupt workspace and task-vector table, the SVC vector table (4420H-4454H), the low-level WD1771 disk I/O engine, the record-positioning and file-access routines, the directory read/write and granule-allocation code, the 16-bit multiply/divide primitives, the RST 28H supervisor dispatcher and overlay loader, and the boot-time initialization and date/time prompt code.

It provides the fundamental, non-changing operating-system services:

• Low-level disk I/O (reading and writing raw sectors via the memory-mapped WD1771).
• Basic file-system primitives (directory search, granule allocation, record positioning, open/read/write/close support).
• Interrupt handling for the real-time clock and the task scheduler.
• Fixed entry points for character output, file services, and program exit/error handling through the resident vector table.

A detailed, commented disassembly of SYS0/SYS can be found here.

SYS1/SYS contains the DOS command interpreter, together with the routines that service the @FEXT (default file-extension), @FSPEC (file-specification parsing), and @PARAM (parameter-list parsing) system vectors. It holds the built-in command-name table (APPEND through VERIFY) and routes each typed command either directly within the overlay or to the Library A/B command overlays through RST 28H request codes 88H and 89H. This overlay must be available on all SYSTEM disks.

A disassembly of SYS1/SYS can be found here.

SYS2/SYS opens and initializes disk files and logical devices. It also contains the routines that check the availability of a disk pack (servicing the @CKDRV vector) and the routines that hash file specifications and passwords. This overlay must reside on all SYSTEM disks; along with SYS3/SYS it must remain on any booting disk if a SYSGEN configuration file is to be loaded.

A disassembly of SYS2/SYS can be found here.

SYS3/SYS contains all of the system routines needed to close files and logical devices, plus the routines that service the @FNAME system vector. This overlay must not be eliminated, and together with SYS2/SYS must remain on any booting disk if a SYSGEN configuration file is to be loaded.

When a disk file is closed, SYS3/SYS finalizes its directory entry - flushing any buffered record, updating the end-of-file byte and ending record number, and, on drives whose dating is enabled, stamping the modification date and time - and then releases any pre-allocated but unused granules back to the Granule Allocation Table, unlinking and de-hashing any secondary (FXDE) directory record that becomes empty. Closing a logical device instead rebuilds its control block into a re-openable device specification. The @FNAME service (RST 28H code A5H) builds the printable NAME/EXT:d specification for a directory record into a caller-supplied buffer.

A disassembly of SYS3/SYS can be found here.

SYS4/SYS holds the system error dictionary - the text of messages such as "File not in directory" and "Directory read error." It is the overlay that services the @ERROR system call (RST 28H request code 96H): it turns a numeric error code into readable text, frames it as "*** Error code = NN, message ***", and displays it. If this overlay is purged, all system errors instead produce the terse message "SYS ERROR." Because it occupies only a single granule, it is recommended that it be kept.

To fit its 46 messages into one granule, SYS4/SYS does not store them as literal strings. It keeps a dictionary of 56 English words once, a compact per-error table that lists each message as a short sequence of word numbers, and a one-byte index that locates each message; every dictionary word is marked by setting the high bit on its final character. When an error is displayed, the routine can also append the identity of the offending object - either <Device=*XX> for a logical device or <File=NAME/EXT:D> for a disk file read from the directory - and a "Referenced at X'NNNN'" line giving the address from which the failing service was called. Option bits in the caller's accumulator choose how much of this framing is produced and whether control returns to the caller or aborts to DOS.

A disassembly of SYS4/SYS can be found here.

SYS5/SYS is the LDOS 5.3.1 DEBUG overlay for the Model I. It provides an interactive machine-language monitor: display and edit of the full Z80 register set (captured on entry into the resident DEBUG save area DBGSV$ at 405DH-407CH), hexadecimal and ASCII memory examination and patching, execution breakpoints planted as RST 30H (F7H) opcodes, and single-step or trace execution driven by a built-in instruction-length decoder.

DEBUG is loaded at 4E00H by the resident RST 28H overlay loader (request code 87H) and re-entered by the RST 30H breakpoint hook at 400FH. It performs no disk I/O of its own; it drives the screen and keyboard through the Model I ROM (0033H, 002BH, 0049H) and reuses the resident video DCB cursor field at 4020H. The command set is single-key: g (Go), r (register modify), m (memory modify), d (set memory window), ; and - (scroll), a and h (ASCII/hex display), s and x (screen layout), c (step over), i (step into), and u (live update).

A disassembly of SYS5/SYS can be found here.

SYS6/SYS contains the algorithms and routines that service the primary command library - the commands identified as "Library A" by the LIB command. Very limited use can be made of DOS if this overlay is removed from a working SYSTEM disk. It also contains the DO command; if SYS6/SYS is purged there is no purpose in keeping the JCL processor (SYS11/SYS).

A disassembly of SYS6/SYS can be found here.

SYS7/SYS contains the routines that service the secondary command library - the commands identified as "Library B" by the LIB command. These are more specialized functions; a great deal of use can be made of DOS even without this overlay, so it may be purged with the PURGE command if its commands are not needed on a working SYSTEM diskette.

A disassembly of SYS7/SYS can be found here.

On DOS version 5 (LDOS for the Model I), SYS8/SYS is the routine that dynamically allocates file space as files are written. It must be present on any working SYSTEM diskette, and must be on the boot disk if a configuration file is to be loaded.

SYS9/SYS contains the routines that service the extended debugging commands available after a DEBUG (EXT) is performed. It may be purged if the extended debugging commands are not needed; if SYS5/SYS has also been purged, keeping SYS9/SYS serves no purpose.

SYS10/SYS contains the procedures that service the request to delete a file (the @KILL function). It should remain on working SYSTEM diskettes.

SYS11/SYS contains the procedures that perform the Job Control Language (JCL) execution phase. It may be removed from working disks if no JCL functions will be invoked. Because the DO command that launches JCL resides in SYS6/SYS, keeping SYS11/SYS serves no purpose if SYS6/SYS has been purged.

SYS12/SYS contains the routines that service the @DODIR and @RAMDIR directory vectors. These routines are used by the MiniDOS filter and may also be used by other application programs that provide a directory display. It can be removed if the two "mini" directory routines are not needed.

BACKUP/CMD duplicates data from one disk to another. It supports mirror-image and file-by-file (class) backups, with parameters to select, exclude, and filter the files that are copied.

DATECONV/CMD updates LDOS disks created under version 5.1.4 or earlier to the 5.3 style of dating. LDOS 5.3 expanded the date range to 1980-1999 and added a modification time stamp to each directory entry, bringing the on-disk convention into line with LS-DOS 6.3; DATECONV converts older disks so their date/time information is interpreted correctly.

FDUBL/CMD installs the double-density disk driver used with double-density hardware modifications on the Model I under DOS version 5. It patches the floppy driver so the Radio Shack interface (fitted with a supported double-density board) can read and write double-density diskettes.

FORMAT/CMD writes the track, sector, and directory information onto a diskette, organizing it into cylinders, tracks, and sectors so DOS can access it. This is the utility that creates the BOOT/SYS and DIR/SYS system files on a new disk.

PATCH/CMD makes minor changes to existing disk files. It accepts patches in either a symbolic library-command format (Dnn,rr / X'nnnn) or a direct record-and-byte format, and is the standard mechanism for applying official fixes to the system and utility files.

SOLE/CMD is a Model I utility that installs a double-density (DDEN) boot driver under DOS version 5, allowing the system to boot from a double-density system diskette on suitably equipped Model I hardware.

A disassembly of SOLE/CMD can be found here.

TED/CMD is a full-screen ASCII text editor supplied with LDOS. It can be attached to the system as an Immediate Execution Program so it can be invoked directly from the DOS Ready prompt.