TRS-80 - CONVERT Utility - Model I to Model III File Converter Disassembled

CONVERT/CMD

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CONVERT is a TRS-80 Model III utility that converts files from Model I TRSDOS format to Model III TRSDOS v1.3 format. The utility reads files from a Model I formatted diskette and writes them to a Model III formatted diskette, handling the necessary format translation between the two different disk operating systems.

The program prompts the user for source and destination drive numbers, then processes each file on the source diskette. For each file found, it displays the filename and asks the user whether to convert it (Y), skip it (N), or quit (Q). Files that are already open or protected may be flagged as not convertible.

The utility operates at the sector level, reading the Model I directory structure, parsing filenames, and writing data to Model III format using the appropriate DOS calls for each system.

Address	Bytes	Purpose
42B4H	1	System flag storage (set to 55H during initialization)
4413H	1	Maximum drive number allowed
5AFFH	1	Source drive number (0-3)
5B00H	1	Destination drive number (0-3)
5B01H	1	Skip file flag (00H = convert, FFH = skip)
5B02H-5B03H	2	Current directory pointer
5B04H	1	Directory entry counter (starts at 40H = 64 entries)
5B05H	1	Current granule/sector position
5B06H	1	Password length entered by user
5B07H-5B08H	2	File size/transfer address storage
5B09H-5B30H	40	Filename buffer (source filename with extension)
5B27H-5B30H	10	User input buffer
5B31H-5B62H	50	Source filespec buffer
5B63H-5B94H	50	Destination filespec buffer
5B95H+	256	Source DCB (Device Control Block)
5C95H+	256	Destination DCB
5E00H	256	Directory sector buffer
5F00H	256	GAT (Granule Allocation Table) buffer
5FC0H	1	Tracks per disk from GAT
55ABH	1	Drive select value storage (self-modifying)
55AEH-55B0H	3	Track number table
59EEH	1	Drive number for directory operations
59EFH-59F6H	8	Extension buffer (padded with spaces)
59F7H-5A01H	11	Filename buffer for directory search
5A02H-5A05H	4	File position/hash storage

5200H - Main Entry Point and Initialization

This is the main entry point for the CONVERT utility. It saves the command line pointer, sets a system flag, clears the screen, displays the title message, and then parses the command line for optional source:destination drive specifications. If no drives are specified on the command line, it prompts the user for each.

523CH - Prompt for Source Drive

This routine prompts the user to enter the source drive number (the Model I diskette). It displays "Source Drive?" and waits for input. If the user presses BREAK or enters an invalid drive, it returns to DOS or re-prompts.

524EH - Prompt for Destination Drive

This routine prompts the user to enter the destination drive number (the Model III diskette). It displays "Destination Drive?" and validates the input.

5260H - Validate Drives and Initialize Conversion

This section verifies that source and destination drives are different, then initializes the conversion process by reading the source disk's GAT and setting up counters for directory traversal.

5299H - Main Directory Processing Loop

This is the main loop that processes each directory entry on the source disk. It scans through all 64 possible entries, converting files one at a time until complete.

52B0H - Extract Filename from Directory Entry

This routine extracts the filename and extension from a Model I directory entry pointed to by HL. It copies up to 8 characters of the filename and 3 characters of the extension (separated by a slash) into the buffer at 5B09H, terminated with 03H.

52EAH - Copy Filename to Source and Destination Buffers

This routine copies the extracted filename from the buffer at 5B09H to both the source filespec buffer (5B63H) and destination filespec buffer (5B31H), creating complete filespecs for both drives.

5306H - Append Extension Separator and Build Full Filespec

This routine appends a period separator and processes the filespec, building properly formatted source and destination file specifications with drive numbers appended.

533DH - Append Drive Numbers to Filespecs

This routine appends the appropriate drive number specifiers (":n") to the source and destination filespec buffers, completing the full file specifications.

5364H - Append Drive Specifier to Buffer

This helper routine appends a colon, ASCII drive number, and carriage return to the buffer pointed to by HL. Entry: A = drive number (0-3), HL = buffer position.

536EH - Check File Transfer/Load Address

This routine examines the directory entry to determine if the file is a loadable program with a transfer address. It checks if the file loads to 4296H (DOS READY) indicating it's a program file, or if the load address matches the file start, indicating it's not a program.

53BDH - Display Filename with Message

This routine displays the current filename followed by additional status text. It outputs the filename from 5B09H and then a message from 5AFCH.

53D0H - Display Prompt and Get Single Character Input

This routine displays a prompt message at (HL), then waits for the user to enter a single character. If the user enters nothing and just presses ENTER, it displays "Invalid Reply" and loops. Returns with HL pointing to the input buffer and CARRY set if BREAK was pressed.

53EDH - Prompt for File Conversion (Y/N/Q)

This routine displays a prompt asking if the user wants to convert the current file. It shows "Existing File. Use it (Y/N/Q)?" and waits for the user's response. Returns Z flag set if user pressed Q to quit.

540DH - Prompt for Password

This routine prompts the user for a password when converting a password-protected file. It displays "Enter Password for " followed by the filename, then accepts up to 8 characters of input. If the user presses ENTER without typing anything, the skip flag is set at 5B01H.

5442H - Verify Password

This routine verifies the entered password against the file's stored password hash. It computes a hash of the entered password and compares it against the hash stored in the file's directory entry. Returns CARRY set if password is incorrect.

5462H - Validate Drive Number

This routine validates a drive number character. It converts the ASCII digit to binary and checks it against the maximum allowed drive number stored at 4413H. Returns Z flag set if valid, NZ if invalid.

5478H - Open Source and Destination Files

This routine opens both the source file (for reading from Model I disk) and destination file (for writing to Model III disk). It handles the case where the destination file already exists (error 1CH = file exists).

5495H - Process Current Directory Entry

This routine checks the current directory entry. If the entry is empty (first byte = 00H), it advances to the next entry. If the entry is valid, it checks file attributes and prepares for conversion. Returns CARRY set when all entries processed.

54C5H - Advance to Next Directory Entry

This routine advances the directory pointer by 32 bytes (20H) to point to the next entry, updates the sector position counter, and decrements the entry counter. Model I directory entries are 32 bytes each, 8 entries per sector.

54E9H - Convert Single File

This is the main file conversion routine. It checks file type, extracts the filename, prompts the user if needed, opens source and destination files, and performs the actual data transfer.

5555H - Handle File with Transfer Address

This routine handles program files that have a transfer (entry) address. It updates the destination file's directory entry with the correct load and transfer addresses for Model III format.

5592H - Display "NOT Converted" Message

This routine displays the filename followed by "NOT Converted (Protected File)" for files that cannot be converted due to protection attributes.

55ABH - Drive Select Storage and Track Table

This area contains drive select value storage and a track number lookup table used for disk I/O operations.

55B8H - Output Drive Select

This routine outputs the current drive select value to the FDC drive select port (F4H). It masks off the motor-on bit to get the drive select pattern.

55C1H - Seek Track and Read Sector

This routine seeks to the specified track and reads a sector. Entry: C = drive number, D = track number. It handles the FDC command sequencing for Model I format disk access on a Model III.

5607H - Wait for FDC Not Busy

This routine waits for the FDC to become not busy by polling the status register bit 0.

5614H - FDC Timing Delay

This routine provides a short timing delay required between FDC operations. It uses PUSH/POP pairs which consume CPU cycles without affecting registers.

5621H - Read Sector Data

This routine reads a sector of data from the disk using the FDC. It issues the Read Sector command and uses the INI instruction to transfer data from the FDC data port to memory.

5653H - Read Sector with Retry

This is the main sector read routine with retry logic. It attempts to read a sector up to 9 times before returning an error. Entry: D = track, E = sector, HL = buffer address. Returns Z flag set on success.

56B2H - Select Drive and Prepare DCB

This routine selects the appropriate drive and prepares for file operations. It checks the DCB flags and either reads data byte-by-byte for Model I format or uses DOS routines for Model III format.

56CBH - Read Using DOS Routines

This routine reads data using standard DOS routines for Model III format disks. It calls DOS to get track/sector information and then reads the sector.

56E0H - Open File for Reading (Model I Format)

This routine opens a file for reading from a Model I format disk. It sets the Model I format flag in the DCB and calls the byte-read routine.

5709H - Compute Track/Sector from Granule

This routine converts a granule number to track and sector values for Model I disk format. Model I uses 3 granules per track with 5 sectors per granule.

5764H - Multiply A by 5

This routine multiplies Register A by 5. Used to convert granule numbers to sector counts (5 sectors per granule on Model I).

576BH - Read File Data Sector

This routine reads a data sector from the file. Entry: B = sector position, C = drive number. It computes the track/sector from the directory information and reads the sector.

5780H - Read GAT (Granule Allocation Table)

This routine reads the Granule Allocation Table from track 17, sector 0 (the first directory sector on Model I disks) into the buffer at 5F00H.

5792H - Compute Track/Sector from Position

This routine computes the track and sector numbers from the current position. It uses the directory pointer to determine which sector to read.

57A4H - Get Track from Table

This routine looks up the track number from the track table at 55AEH. Entry: C = drive number. Returns: D = track number.

57AEH - Open File (Model I Directory Search)

This is the main file open routine for reading Model I format files. It searches the directory, parses the filename, and sets up the DCB for reading. Entry: HL = DCB address, DE = filespec, B = open mode.

5810H - Check Directory Entry Match

This routine checks if the current directory entry matches the filename being searched for. It compares the 11-byte filename/extension field.

Note: The DCB setup continues through 58E5H, followed by filename parsing (58E6H-5943H), character validation (5944H-595DH), hash computation (595EH-596DH), password hash (596EH-59A9H), disk format detection (59AAH-59D8H), directory reading (59D9H-59EDH), and message strings (5A06H-5AFFH).

595EH - Compute Filename Hash

This routine computes a simple hash value from the 11-byte filename. The hash is used for quick comparison during directory searches.

596EH - Compute Password Hash

This routine computes a 16-bit hash from a password string. The hash uses a complex mixing algorithm with rotates and XORs. Entry: DE = password buffer, B = password length. Returns: HL = hash value.

59AAH - Detect Disk Format (Model I Single Density)

This routine detects if the disk is in Model I single-density format by checking for the index pulse pattern. It waits for specific timing patterns that indicate the disk rotation speed and format. Returns Z flag set if Model I format detected.

59CAH - Check Index Pulse with Timeout

This routine checks the FDC status register for the index pulse (DRQ bit). It decrements the timeout counter and returns with the pulse status. Returns Z flag reflecting bit 1 of status.

59D9H - Read Directory Sector

This routine reads a directory sector from the Model I disk into the buffer at 5E00H. It uses the track table to determine which track to read from. Returns Z flag set on success, with HL pointing to the buffer.

59EEH - Drive Number Storage

Single byte storage for the current drive number being accessed.

5A06H - Message Strings Data Area

This area contains all the text messages displayed by the CONVERT utility. Each string is terminated with 0DH (CR) or 03H (ETX - End of Text). The disassembler incorrectly shows these as instructions, but they are ASCII text data.