Cassette Directory

This program catalogs the contents of a cassette tape by reading file headers directly from memory, displaying the file type (BASIC, Array, or Code), filename, byte length, and load/autostart address for each block found. It uses a machine code routine POKEd into RAM at address 65451 (near the top of a 64K address space) to intercept the tape-loading mechanism, with the header data examined at address 65475. The directory can be printed to a printer via channel 3, saved to tape as a character array, or reloaded for later review. A hex-string-to-machine-code loader at lines 9100–9110 converts two ASCII hex digits at a time into byte values using arithmetic on character codes, with a checksum validation step to catch transcription errors.

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Program Analysis

Program Structure

The program is organized around a menu loop at lines 100–130, with distinct functional modules reached via computed GO TO targets. The main tape-reading loop runs from lines 15–80, the printer toggle is at 200–240, the save routine at 300–360, the load routine at 600–690, and the machine code installer at 9000–9110. Lines 9900–9999 handle self-save/verify and cleanup.

Lines	Function
10–80	Tape header capture and display loop
100–130	Main menu and dispatch
200–240	Printer toggle
300–360	Save directory to tape
400–420	Keypress subroutine (wait-for-release, then wait-for-press)
500–510	Tape name/side initialisation
600–690	Load and review saved directory
9000–9110	Machine code hex-string installer with checksums
9990	Tape error handler
9998–9999	Self-save/verify sequence

Machine Code Usage

The installer routine at lines 9000–9110 encodes two separate Z80 routines as hex strings in a$. The first (40 bytes, 80 hex chars) is loaded at address 65451; RANDOMIZE USR 65451 at line 9020 executes it immediately as a test. The second routine (24 bytes, 48 hex chars) is also placed at 65451 (overwriting the first), and is the one used at runtime by line 20.

The hex-to-byte conversion in the subroutine at line 9110 uses the expression (CODE a$(x)-(48 AND CODE a$(x)<58)-(55 AND CODE a$(x)>64)) to convert each ASCII hex digit: subtracting 48 for digit characters (‘0’–’9’) and 55 for letter characters (‘A’–’F’), yielding values 0–15. Two such nibbles are combined with *16+.

Checksum validation at line 9100 sums the ASCII codes of all characters in a$. Expected values (4735 and 2887) are hard-coded, guarding against transcription errors when the program is entered by hand.

At line 9040, a secondary block of 8 bytes is POKEd into address 65286 using offsets read from a DATA statement, with a separate data checksum check (expected sum 133). The byte 254 is POKEd as a sentinel before the offset-addressed 255 bytes are placed.

Header Parsing at Runtime

After RANDOMIZE USR 65451 triggers the tape-reading machine code, line 20 reads the captured header from address ix=65475. The type byte at ix+1 is used to select the label string: 0=BASIC, 1 or 2=Array, 3=Code. The filename occupies 10 bytes at ix+2 to ix+11. Byte length is taken from a two-byte little-endian word at ix+12/ix+13, and the load/autostart parameter from ix+14/ix+15.

For array types (t=1 or t=2), line 60 reconstructs the variable name: it reads the byte at ix+15, subtracts 32 for a numeric array (t=1) or 96 for a string array (t=2), and appends $ only for string arrays (t=2), along with ().

Display Formatting with z$

The string z$ is constructed at line 10 as CHR$ 19+CHR$ 1+CHR$ 19+CHR$ 0, which represents ink colour control sequences (bright ink on, bright ink off). Slicing z$( TO 2) gives the “bright on” prefix and z$(3 TO ) gives the “bright off” suffix, so field labels are displayed in bright ink while values are in normal ink.

Key BASIC Idioms

• Conditional string concatenation using ("string" AND condition) — produces the string if true, empty string if false.
• Computed GO TO at line 130 using arithmetic on Boolean results of k$ comparisons to select the branch target.
• The keypress subroutine (lines 400–420) first waits for any currently held key to be released, then waits for a new keypress — a standard debounce idiom.
• POKE 23692,0 at line 70 resets the scroll counter (system variable SCRCT) to prevent the “scroll?” prompt interrupting output.
• INPUT INKEY$ at lines 360 and 600 clears any key already in the buffer before waiting for tape operations.
• ON ERR GO TO (rendered as { in the source) provides structured error handling throughout; ON ERR RESET (© ) at line 120 clears the error handler within the menu.

Directory Storage

Each parsed header entry is accumulated into b$ (line 70), separated by double carriage returns. The tape name and side, set in the subroutine at lines 500–510, are stored at the start of b$ followed by CHR$ 255 as a delimiter; k holds the length of this prefix. On save (line 330), b$ is copied into a DIM’d character array c$() matching its current length before being saved with SAVE … DATA c$(). On load, the delimiter is located by scanning for CHR$ 255 (lines 610–630) to recover k.

Self-Save Sequence

Line 9999 saves the BASIC program itself with SAVE "directory" LINE 9998 so it autoruns to line 9998 on load, then saves the machine code block at address 65253 (length 222 bytes) as a separate CODE file. Line 9998 then loads that CODE block back and jumps to the main menu. Line 9900 uses DELETE 9000,9988 to remove the installer lines before the self-save, keeping the runtime image clean.

Bugs and Anomalies

• At line 130, the arithmetic uses both addition and subtraction of line-number offsets in a single expression to select the menu branch. The expression relies on exactly one k$ comparison being true; if none match, the result is GO TO 120, repeating the keypress wait — a safe default.
• The GO TO 9988 at line 9080 targets a non-existent line; execution will fall through to line 9900, which clears and deletes the installer lines — an intentional technique.
• Line 9989 (GO TO 9999) appears to be reachable only if line 9900’s DELETE somehow did not reach line 9989 itself; in practice it serves as a forward jump past the error handler at 9990 into the self-save at 9999.