Model I TRSDOS v2.3 TAPEDISK/CMD Explained

This is the main entry point of the cassette loader/monitor. It initializes the stack and prompts the user with a '?' for command input. The three valid commands are: C (load from Cassette), E (Exit), and F (save File to cassette).

At this point, HL points to the input buffer at 4318H containing the user's command. We now check the first character to determine which command to execute.

The 'F' command saves memory to cassette tape. The command format is: F filename:SSSS EEEE XXXX where SSSS=start address (hex), EEEE=end address (hex), XXXX=execution address (hex). All addresses are entered as 4 hex digits.

Now scan forward through the filename until we find the ':' delimiter.

Found the ':' - now parse the START address (4 hex digits = 2 bytes, high byte first).

Now parse the END address (4 hex digits = 2 bytes, high byte first).

Now parse the EXECUTION address (4 hex digits = 2 bytes, high byte first).

All three addresses have been parsed. Now we initialize the cassette output system and calculate how many bytes need to be saved. KEY INSIGHT: This routine saves the loader program ITSELF (5200H-53F1H) to tape!

Now calculate the length of data to save. The source is THIS PROGRAM from 5200H to 53F1H.

This loop writes the program data to cassette in 256-byte blocks. H contains the number of full pages (256-byte blocks) to write, L contains the remaining bytes for the final partial block.

Done with full 256-byte pages. Now check if there's a partial block remaining.

All program data has been written. Now write the file trailer which contains the EOF marker (02H 02H) followed by the execution address (low byte, high byte).

Trailer written. Now flush any remaining data in the buffer and close the cassette file.

This routine handles errors from cassette operations. It sets the error bit and calls the error reporting routine.

This routine displays "SN ERROR" when an unrecognized command is entered.

This is the cassette load routine, invoked by the 'C' command. It reads a program from cassette tape into memory. The routine first synchronizes with the tape by looking for the sync pattern (A5H followed by 55H), then reads data blocks until an EOF marker is found.

Now search for the first sync byte (A5H) in the cassette data stream.

Found A5H. Now look for the second sync byte (55H).

Both sync bytes found! Now skip past 6 leader bytes before the actual data begins.

This loop reads bytes from the cassette looking for block type markers. Type 3CH indicates a data block, type 78H indicates end of file. The bottom-right character of the screen is toggled to provide a visual "loading" indicator.

Found a data block marker (3CH). Now read the block: first the byte count, then the 2-byte load address, then the actual data bytes, and finally verify the checksum.

Now read B data bytes and store them starting at the address in HL.

All data bytes loaded. Now verify the checksum.

Checksum mismatch! Display an error indicator and try to resync.

EOF marker (78H) was found - loading is complete. Read the execution address and jump to it to run the loaded program.

This subroutine writes one data block to the cassette output buffer. It writes: block type (01H), block length (B+2), load address (DE), and B bytes of data from memory at (DE). The buffer is automatically flushed to tape when full.

Header written. Now write B bytes of actual data from memory starting at DE.

This subroutine writes one byte to the cassette output buffer at 53F4H. Register C is used as the buffer index. When C wraps from FFH to 00H (buffer full), the buffer is automatically flushed to tape by calling 4439H.

Buffer is full (C wrapped to 00H). Flush the 256-byte buffer to cassette tape.

This utility subroutine advances HL past any space characters (20H) in the input buffer, leaving HL pointing to the first non-space character.

This subroutine reads two bytes from cassette and assembles them into a 16-bit address in HL. The bytes are read low-byte first, then high-byte (little-endian order).

This subroutine prints B characters to the screen starting from the address in HL, using the ROM character output routine at 0033H.

This subroutine parses two ASCII hex digit characters from (HL) and (HL+1) and converts them to a single binary byte in Register A. For example, "4A" becomes 4AH (74 decimal). If an invalid hex digit is found, it aborts to the syntax error handler.

Now combine the two nibbles: shift the first digit left 4 bits, then OR with the second digit.

This subroutine converts a single ASCII hex digit character ('0'-'9' or 'A'-'F') in Register A to its binary value (0-15). If the character is not a valid hex digit, it manipulates the stack to abort to the syntax error handler at 52C4H.

Character was less than 'A' - check if it's a decimal digit 0-9.

This is the error abort routine called when an invalid hex digit is encountered. It uses a clever stack manipulation technique: it pops the immediate return address (discarding it), then replaces the NEXT return address on the stack with the error handler address. When the calling routine eventually returns, it goes to the error handler instead.

This subroutine reads 8 bits from the cassette tape and assembles them into a byte in Register A. It calls 53B0H eight times, each call reading one bit and shifting it into position.

This is the low-level bit-reading routine. It waits for a rising edge on the cassette input (bit 7 of port FFH), then uses precise timing delays to sample the bit value. The alternate register set (EXX, EX AF,AF') is used to preserve the caller's registers during the timing-critical operations.

Wait for a rising edge on the cassette input signal (transition from 0 to 1).

Rising edge detected! Now use precise timing delays to sample the bit value.

This subroutine turns the cassette motor OFF by clearing bit 2 of port FFH. It uses the common port control routine at 53DDH with HL=FB00H to clear the motor bit.

This subroutine turns the cassette motor ON by setting bit 2 of port FFH. It uses the common port control routine at 53DDH with HL=FF04H to set the motor bit.

This subroutine is called during bit reading for hardware timing adjustment. It preserves all bits (AND FFH) and ORs with 00H (no change), but the act of writing to the port may reset edge detection hardware.

This is the common cassette port control routine. It reads the current port state from the shadow register at 403DH, applies an AND mask (H) and OR mask (L), writes to port FFH, and updates the shadow register. Returns with the Z flag set based on bit 2 (motor state).

This is the "SN ERROR" message displayed when an unrecognized command is entered or invalid hex digits are encountered.