TRS-80 DOS - NEWDOS/80 v2.0 for the Model I - DIRCHECK Disassembled

DIRCHECK

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DIRCHECK is a comprehensive disk integrity checking and repair utility for NEWDOS/80 v2.0. This program performs detailed analysis of diskette directory structures, validates the Granule Allocation Table (GAT), checks file extent chains, and detects various types of disk corruption including cross-linked files, lost clusters, orphaned extensions, and GAT inconsistencies. The utility can operate in both check-only and repair modes, providing detailed diagnostic output of any problems found.

The program prompts the user for a drive number to check, loads and analyzes the Directory Entry Table (DET) and Hash Index Table (HIT), validates all file entries and their extent chains, cross-references granule allocations against the GAT, and reports statistics including file count, extent count, sector usage, free granules, and locked-out granules.

5367H	Drive Type Detection	Determines single vs double-density and configures display parameters
53C0H	GAT Cross-Reference Check	The critical routine that compares GAT entries against actual file allocations, checking for errors like: Granules marked free but assigned to files Granules marked allocated but not assigned Locked-out granules incorrectly marked as free Cross-linked files (same granule assigned to multiple files)
543AH	Directory Entry Validation	Main loop that scans all directory entries checking: File extent chains for validity Granule numbers within disk bounds Extent elements for proper formatting Extension linkages between FPDEs and FXDEs
54E1H	FXDE Validation	Validates File eXtension Directory Entries including: Checking inactive FXDEs are truly empty Backward linkage to primary entries Forward linkage verification (bidirectional consistency)
5535H	File Count Validation	Ensures no more than 31 files exist on disk (NEWDOS/80 limit)
554EH	Granule Allocation Error Analysis	The main error detection engine that: Scans all granules checking status bit patterns Identifies 8 different error types based on bit combinations Reports errors like: locked-out but free, allocated but unassigned, free but assigned, cross-linked files For errors involving file assignments, scans directory to find which files use the problem granule 55DCH - File Detail Display - Shows which specific files are using problem granules
5611H	Granule Counter Advance	Manages the scan loop progression
5625H	Directory Listing Generator	Creates the comprehensive file listing showing: Entry number and filename/extension System and Invisible flags (S/I indicators) Protection levels (P=n format) EOF position (record/byte format) Extent count and total sector usage
571BH	Final Statistics Display	Uses alternate register set (EXX) to simultaneously count free and locked-out granules, displaying final disk usage summary
5758H	Extent Element Decoder	Critical routine that decodes NEWDOS/80's packed extent format: Extracts lump number from upper 3 bits Calculates starting granule: (lump × granules_per_track) + offset Extracts granule count from lower 5 bits (1-32 sectors) Validates against disk bounds
5786H	Directory Checksum Calculator	Computes checksum over 11 bytes (filename + extension) using XOR and rotate operations
579EH	Checksum Validator	Retrieves stored checksum and compares against current entry
57AEH, 57B1H	Filename Display	Formats and displays filenames with proper handling of: Trailing space suppression Slash separator between name and extension Clean output like "FILENAME/EXT"
57E0H	Directory Sector Loader	Maps entry index to physical disk location and loads sectors
5801H	Sector Read with Repair Flag	Reads sectors and marks for write-back if repairs made
5815H	"Writing Corrections" Message
5820H	DOS Sector I/O Wrapper	Interfaces with ROM disk routines at 4436H
5829H and 582EH	File error display with entry numbers
583FH	General error message formatter
584DH	Granule number display in lump,offset format (e.g., "5,3")
5867H	Entry number display helper
586AH	Directory entry loader (critical navigation routine that maps entry numbers to physical sectors)
5893H	Extension (FXDE) entry loader (uses separate buffer at 62BAH)
58BDH	Directory counter increment
58C5H	Write-back routine	The complete sector write operation including: Write-protect check Sector-by-sector write loop Clearing of inactive directory entries DOS write command execution
5932H	Program completion handler with user menu (N/Y/I/W/C options)
595FH-5973H	Display utilities (message display, multi-line paging)
5977H-59B6H	String and character output routines (ETX-terminated strings, spaces, input)
59D1H-59EEH	Screen/printer mode switching
59EFH-5A26H	Screen paging and line counting
5A27H-5A67H	Main character output with line tracking and keyboard pause handling
5A69H-5AADH	Output routing with self-modifying code for I/O vectoring
5AB5H-5AC8H	Utilities (uppercase conversion, error handler, termination)
5ACEH-5AEDH	Math routines (BCD conversion)
5AEEH-5B00H	Division routine (16-bit ÷ 8-bit)

Address	Bytes	Purpose
5200H-5201H	2	Program identification bytes: 82H, 60H (possibly header/signature)
5202H-5205H	4	Reserved/padding (NOP bytes)
5206H	1	Repair mode flag (00H=check only, non-zero=repair mode)
5207H	1	Initialized to FFH
5208H-520BH	4	Reserved (initialized to 00H)
520CH-520DH	2	Word variable (initialized to FFFFH)
520EH-521FH	18	Reserved area (filled with FFH)
5260H-5267H	8	Drive specification block (copied from ROM at 430AH)
5268H	1	Drive number being checked (0-3)
526EH	1	Status flag (initialized to 00H)
526FH	1	Computed value (granules per track * 5)
5270H-5271H	2	Sector count calculation storage
5272H	1	Current track number (initialized to 00H)
5273H	1	Current sector number (initialized to 01H)
5274H-5275H	2	Calculated pointer to sector buffer area
5276H-5280H	11	Additional reserved area
60BAH-61B9H	256	Work buffer area for sector data
61BAH	varies	Stack pointer initialization address
62BAH-63B9H	256	Secondary work buffer (directory sector buffer)
63BAH-64B9H	256	Tertiary work buffer (GAT buffer)
65BAH-6BBFH	1536	Large work area (cleared at initialization, 0600H bytes)

5200H - Program Header and Data Area

This section contains the program header bytes and pre-allocated data storage areas. The first two bytes may serve as a program identifier or signature. The remainder consists of variable storage that will be initialized during program execution, including the critical repair mode flag at 5206H which determines whether the program operates in check-only or repair mode.

5260H - Drive Specification Storage

Eight-byte storage area that will receive a copy of the current drive's specification block from ROM address 430AH. This includes drive geometry parameters needed for disk access operations.

5268H - Drive Number Storage

Single-byte storage for the drive number (0-3) that the user specifies to check. This value is validated and stored here after user input.

526EH - Status and Working Variables

Collection of single-byte and multi-byte working variables used throughout the program for various calculations, flags, and temporary storage.

5276H - Additional Reserved Area

Additional padding and reserved space before program entry point.

5281H - Program Entry Point and Initialization

This is the main entry point for DIRCHECK (as indicated by "PROGRAM ENTRY = 5281H" in the listing). The initialization sequence sets up the stack, prompts for drive number and repair mode, clears working buffers, initializes variables, loads the drive specification block, and prepares for disk checking operations. Entry: Called by DOS when DIRCHECK is executed. Exit: Branches to disk checking routines or error handlers.

5367H - Drive Type Detection and Parameter Setup

This section analyzes the drive specification to determine if it's a single or double-density drive, then configures sector size parameters and display formatting accordingly. The code checks byte offset +1 in the drive specification block and adjusts parameters based on density type. Entry: Drive specification block has been loaded at 5260H. Exit: Display parameters configured at 53E5H and 53E1H.

53C0H - GAT Cross-Reference Check

This routine performs the critical GAT (Granule Allocation Table) cross-reference operation. It compares the GAT entries against actual file allocations stored in the working buffer at 65BAH. The routine uses index registers IX (pointing to GAT at 63BAH) and IY (pointing to allocation tracking buffer at 65BAH) to simultaneously scan both tables, checking each granule's status bit-by-bit. For each granule, it verifies that the GAT allocation status matches the actual usage determined from file extent chains. Entry: GAT loaded at 63BAH, file allocation tracking at 65BAH. Exit: Errors flagged in working buffer.

5431H - GAT Error Reporter

Simple error reporting subroutine that displays a GAT allocation error message. The granule number in Register A and error message address in HL are used to format and display the error. Entry: A=granule number, HL preserved. Exit: Error message displayed, registers restored.

543AH - Directory Entry Validation Main Loop

This is the main directory entry validation routine that scans through all directory entries in the Hash Index Table (HIT), checking file primary directory entries (FPDEs) and file extension directory entries (FXDEs) for consistency. The routine validates extent chains, checks directory entry linkages, verifies that active entries have valid data, and ensures extensions properly link back to their primary entries. Entry: Directory sectors loaded, IY points to allocation tracking buffer. Exit: All directory entries validated, errors reported.

54E1H - File Extension (FXDE) Validation

This routine validates File eXtension Directory Entries (FXDEs). It checks that the FXDE is truly inactive (all extent elements are zero) if it appears to be unused, validates the backward linkage to the primary entry, and processes the extent chain if active. FXDEs contain additional extent elements when a file's allocation exceeds the 5 extents available in the primary directory entry. Entry: IY points to FXDE, (IY+00H) has bit 7 set. Exit: FXDE validated or error reported.

5535H - Check for Files Beyond Allowed Count

This routine scans the directory buffer at 64BAH to verify that the number of files on the disk doesn't exceed the limit of 31 files (1FH). NEWDOS/80 has a maximum directory entry limit, and this routine ensures that limit is not violated. Entry: Directory data loaded at 64BAH. Exit: Error reported if more than 31 files found.

554EH - Granule Allocation Error Analysis

This is the main granule allocation error analysis routine. It scans through the allocation tracking buffer at 65BAH checking each granule's status bits to identify and report various allocation errors. The routine uses bit flags to determine error types: bit 7=locked-out, bit 6=allocated in GAT, bit 5=assigned to file, bit 4=cross-linked. By examining combinations of these bits, it can detect conditions like granules allocated but unassigned, granules free but assigned, locked-out granules improperly marked, and cross-linked files. Entry: Allocation tracking buffer populated at 65BAH. Exit: All allocation errors reported, statistics displayed.

55DCH - Display File Using Problem Granule

This subroutine displays the filename and extent information for a file that uses the problem granule. It's called when the granule allocation scanner finds a file referencing a granule that has allocation errors. The routine displays the entry number, filename with extension, and EOF marker. Entry: IY points to directory entry, (5279H) contains entry number. Exit: File information displayed.

5611H - Advance to Next Granule in Scan

This routine advances the granule counter and buffer pointer to the next granule in the allocation tracking scan. It also checks if we've scanned beyond the valid disk space and returns when complete. Entry: HL points to current granule in buffer, DE contains granule number. Exit: Pointers advanced, returns if scan complete.

5625H - Display File Directory Listing

This routine generates a formatted directory listing showing all active files on the disk. For each file, it displays the entry number, filename with extension, protection flags (System/Invisible), EOF position, extent count, and sector count. The display is formatted with proper spacing and uses symbols to indicate special file attributes. Entry: Directory data loaded. Exit: Complete file listing displayed.

571BH - Display Final Disk Statistics Summary

This routine generates the final summary statistics showing total free granules and locked-out granules on the disk. It scans through the entire allocation tracking buffer, counting granules in each state by examining status bits. Uses the alternate register set (EXX) to maintain a separate counter for locked-out granules while the primary registers track free granules. The results are displayed with descriptive labels. Entry: Allocation tracking buffer at 65BAH contains final status. Exit: Statistics displayed, routine returns.

5758H - Decode Extent Element to Granule Range

This critical subroutine decodes a 2-byte extent element into its component parts: starting granule number (returned in DE) and granule count (returned in C). NEWDOS/80 extent elements encode the starting lump number and offset within lump in the high byte, and the granule count in the low 5 bits of the low byte. The routine validates that the starting position is within valid disk bounds and performs the arithmetic to calculate the absolute starting granule. Entry: A=extent high byte, (IY+17H)=extent low byte. Exit: Z flag=error, NZ=success with DE=start granule, C=count.

5786H - Calculate Directory Entry Checksum

This subroutine calculates a checksum for a directory entry by XORing and rotating bytes from the entry. The checksum is computed over 11 bytes starting at offset +5 in the directory entry (the filename and extension area). The result is used to detect directory corruption or verify entry integrity. Entry: IY points to directory entry. Exit: Checksum stored at 527CH.

579EH - Validate Entry Against Stored Checksum

This routine retrieves a previously stored checksum from the directory tracking buffer and compares it against the current directory entry. Used to verify that an entry matches what was expected or to detect if entries have been modified. Entry: (527AH) contains entry index, (527CH) contains calculated checksum. Exit: Z flag set if match, NZ if mismatch.

57AEH - Display Entry Number and Filename

Helper routine that first calls the entry number display, then falls through to display the filename and extension from the directory entry pointed to by IY. Entry: IY points to directory entry. Exit: Entry number and filename displayed.

57B1H - Display Filename and Extension

This routine formats and displays a file's name and extension from a directory entry. It outputs the 8-character filename, suppressing trailing spaces, optionally adds a slash separator if an extension exists, then outputs the 3-character extension. The routine handles padding and formatting to produce clean filename display like "FILENAME/EXT" or just "FILENAME" if no extension. Entry: IY points to directory entry at offset +5 (filename start). Exit: Filename and extension displayed.

57E0H - Load Directory Sector Into Buffer

This routine loads a specific directory sector into the buffer pointed to by HL. It calculates the appropriate track and sector number from the entry index in Register A, checks if the sector is already loaded in the current track buffer, and reads it from disk if necessary. The routine handles the mapping from entry index to physical disk location. Entry: A=entry index (0-31), HL=destination buffer. Exit: Z flag set if success, NZ if error, IY points to entry in buffer.

5801H - Read Sector and Check for Write-Back

This routine reads a sector from disk via the DOS sector I/O routine. After reading, it checks if the read was successful and whether any modifications were made to the data (if in repair mode). If errors need to be written back, it marks the sector for write-back by setting a flag. Entry: HL=buffer address, sector parameters set. Exit: Z flag set if success, error code in A if failure.

5815H - Display "Writing Corrections" Message

Simple routine that displays a message to inform the user that corrected data is being written back to disk. This is called when the program is in repair mode and has found and fixed errors. Entry: Called when write-back is about to occur. Exit: Message displayed.

5820H - Disk Sector Read via DOS

This is a thin wrapper around the DOS disk I/O routine. It sets up the sector read parameters by storing the buffer address and command block address, then jumps to the ROM DOS sector read routine at 4436H. The DOS uses a command block at 5200H containing drive parameters. Entry: HL=buffer address for sector data. Exit: Z flag set if success, error code in A if failure.

5829H - Display File Error with Entry Number

This routine displays an error message associated with a specific file entry. It stores an error code, displays the filename and entry number, then shows the error message. Used when errors are detected during file validation that need to be associated with a specific directory entry. Entry: A=error code, IY points to directory entry, HL=error message. Exit: Error details displayed.

583FH - Display Error Message with Entry Number

General error message display routine that shows an entry number followed by the error message text. This is the common path for displaying most disk checker errors. Entry: A=entry number (or data value), HL=message address. Exit: Message displayed with entry number.

584DH - Display Granule Number with Lump/Offset Format

This routine converts a granule number into lump and offset format for display. NEWDOS/80 uses lumps (groups of granules) for organization, so granule numbers are often displayed as "lump,offset" (e.g., "5,3" for granule 43 when lumps are 8 granules each). The routine performs division to extract the lump number and offset. Entry: HL=granule number, DE=message address. Exit: Granule displayed in lump,offset format followed by message.

5867H - Display Entry Number from Counter

Simple helper routine that loads the current entry number from memory and falls through to display it. Entry: Entry number stored at 5279H. Exit: Entry number displayed.

586AH - Load Next Directory Entry into IY

This critical routine advances through the directory by loading the next entry into the IY index register. It handles the complex mapping of entry numbers to physical disk sectors, managing track and sector buffers. The routine extracts the track number (upper 5 bits) and sector offset (lower 5 bits) from the entry counter, loads the appropriate sector if not already cached, and positions IY to point to the entry within the sector. Entry: (5279H) contains entry number. Exit: IY points to entry, Z flag clear if success, set if no more entries.

5893H - Load Extension Entry (FXDE) into IY

Similar to 586AH but specifically for loading File eXtension Directory Entries (FXDEs). FXDEs are stored in a different sector buffer (62BAH) than primary entries. This routine handles the sector mapping for extension entries, checking if the needed sector is already loaded, and positions IY to the extension entry. Entry: A=extension index. Exit: IY points to FXDE, Z flag clear if success.

58BDH - Advance to Next Directory Entry

Simple increment routine that advances the directory entry counter. It adds 20H (32 decimal) to move to the next "block" of entries, implementing wraparound detection. Used to iterate through all directory entries. Entry: (5279H) contains current entry number. Exit: Entry number incremented, NC flag if wrapped around.

58C5H - Write Corrected Data Back to Disk

This is the disk write-back routine that executes when the user confirms they want to write corrections. It verifies the user has a writable disk, then systematically writes back all sectors that were loaded and potentially modified during checking. The routine uses the allocation tracking buffer to determine which sectors need to be written, marks sectors for write in the DOS command block, and calls the DOS write routine. Entry: A contains user's response code. Exit: All corrected sectors written or error occurred.

5932H - Program Completion and User Response Handler

Main completion routine that displays completion message and prompts user for next action: N=exit, Y=check another disk, I=reinitialize, W=write corrections, C=clean directory.

595FH - Display Message and Add Blank Line

5964H - Multi-Line Display with Paging

5974H - Display Message via Mode Handler

5977H - Display ETX-Terminated String with CR/LF

597AH - Output CR/LF

5984H - Display ETX-Terminated String

Core string output. Displays characters until ETX (03H) found.

598EH - Cursor Home Sequence

5996H - Output Escape

599BH - Display B Characters

59A3H - Intelligent Space Output

Outputs spaces with screen width checking.

59AEH - Output B Spaces

59B6H - Display Prompt and Get Input

Shows prompt, gets user input, converts to uppercase.

59D1H - Mode Switching Handler

Switches between screen (00H) and printer (05H) output modes.

59DBH - Switch to Screen Mode

59EFH - Check Line Counter

59F5H - Screen Paging Handler

Resets line counters, outputs form feeds for printer if needed.

5A1CH - Output C Blank Lines

5A27H - Character Output with Line Tracking

Main character output routine. Tracks cursor position, handles line wrapping, manages CR specially.

5A69H - Low-Level Character Router

5A6EH - Alternate Output Entry

5A73H - Post-Output Processing

5A90H - Output Vector Setup (Self-Modifying)

Complex self-modifying code that creates dynamic jumps for I/O routing.

5AB5H - Lowercase to Uppercase

5ABEH - Fatal Error Handler

5AC5H - Display and Terminate

5AC8H - Program Termination

5ACEH - 8-Bit to BCD Entry

5AD0H - 16-Bit to Decimal Conversion

5AD9H - Binary to BCD (Multiply Method)

5AEEH - Fixed Divisor 5 Entry

5AF0H - 16-Bit Division by 8-Bit

Divides HL by A using shift-and-subtract. Returns quotient in HL, remainder in A.

5B01H - Convert Byte to 2-Digit Hex and Display

Converts byte in A to 2-character hexadecimal ASCII string and displays it. Used for displaying entry numbers, granule numbers, etc.

5B13H - Convert Byte to Hex ASCII String

Converts byte in A to 2 ASCII hex digits stored at HL and HL+1. Processes high nibble then low nibble.

5B1CH - Convert Nibble to Hex ASCII

Converts 4-bit nibble (0-F) to ASCII hex digit ('0'-'9' or 'A'-'F'). Stores result at HL and increments HL.

5B29H - 8-Bit to Decimal Entry Point

Helper entry point for converting 8-bit value (0-255) to decimal ASCII. Extends to 16-bit then processes.

5B2BH - Convert 16-Bit to Decimal String

Converts 16-bit value in DE to decimal ASCII string with leading space suppression. Result stored at 5269H-526CH (5 digits maximum).

5B41H - Binary to Decimal Conversion Core

Converts 16-bit binary in DE to 5-digit decimal string using repeated division by powers of 10 (10000, 1000, 100, 10). Each division yields one decimal digit. Uses division-by-subtraction algorithm.

5B69H - Power of 10 Divisor Table

Data table containing 16-bit divisors used by decimal conversion routine. Values stored in little-endian format (low byte, high byte). These are actually negative values (two's complement) used for division-by-addition algorithm.

Message String Data (5B71H-6039H)

All message strings in DIRCHECK are terminated with ETX (03H). Carriage returns (0DH) are embedded where multi-line messages need line breaks. This comprehensive list documents every user-visible message in the program.