64 Column

This program demonstrates a 64-column text display mode for the TS2068, achieved by loading and executing machine code routines stored at address 55296. The machine code provides four entry points: a setup routine at 55296, a clear-screen routine at 55338, a character-print routine at 55375, and a hardware scroll routine at 55470. The BASIC shell implements a simple interactive typewriter loop, tracking cursor position in column and row variables (stored as parameters at addresses 23748–23750) and calling the appropriate machine code routine via RANDOMIZE USR. The OUT 255,62 instruction switches the display hardware into the 64-column mode, while OUT 255,0 restores the standard display on exit. The program saves itself along with 306 bytes of machine code starting at 55296.

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Program Structure

The program is divided into four logical regions:

1. Lines 1–14: REM documentation blocks describing the program’s origin and purpose.
2. Lines 20–80: Initialization — sets display attributes, clears memory above 55295, loads machine code from tape, and calls the setup entry point via RANDOMIZE USR 55296.
3. Lines 1000–1210: The interactive typewriter loop, reading keystrokes and dispatching to machine code print and scroll routines.
4. Lines 2000–2090: A subroutine that initializes named address variables and switches the display hardware into 64-column mode.
5. Line 9998: SAVE commands for the BASIC program and the machine code block.

Machine Code Integration

The machine code is loaded from a separate CODE file onto address 55296 (just above the CLEAR boundary at 55295). Four entry points are identified by the subroutine at line 2000 and stored in BASIC variables:

Variable	Address	Purpose
setup	55296	Initialize 64-column display hardware
clrs	55338	Clear the 64-column screen
print	55375	Print one character at (row, col)
scroll	55470	Hardware scroll the 64-column display up one line

Parameters are passed to the machine code via three fixed RAM locations: col (23748), row (23749), and data (23750). The BASIC program POKEs the desired character code and cursor coordinates into these addresses before each RANDOMIZE USR print call. The total machine code block is 306 bytes, as confirmed by the SAVE statement on line 9998.

Hardware Display Switching

The OUT 255,62 call in line 2080 switches the display hardware into 64-column mode. On exit (when the backtick key is pressed), OUT 255,0 restores the normal display mode before halting. This port-based switching is central to the enhanced display capability being demonstrated.

Typewriter Loop Logic

The main input loop (lines 1020–1210) implements a simple typewriter with the following behavior:

• Waits for a keypress using a busy-poll on INKEY$ (line 1020).
• Backtick (`) exits to normal mode (line 1030).
• Enter (code 13) advances to the next row without printing (line 1040 → 1110).
• Characters outside the printable range 32–127 are ignored (line 1050).
• After printing, the column counter increments; wrapping at 64 columns advances the row (lines 1100–1110).
• When row 24 is exceeded, the machine code scroll routine is called and the row is reset to 23 (lines 1120–1140).
• A debounce loop at line 1200 waits for the key to be released before returning for the next keypress, preventing character repetition.

Key BASIC Idioms

• RANDOMIZE USR is used throughout as the standard method to call machine code routines from BASIC, discarding the return value safely.
• Address variables (col, row, data, print, scroll, etc.) are initialized once in the subroutine at line 2000 and reused throughout, avoiding repeated numeric literals and making the code easier to maintain.
• CLEAR 55295 both clears variables and raises RAMTOP to protect the machine code area from being overwritten by the BASIC system.

Notable Anomalies

Line numbers 1000 and 2080 call GO SUB 2000 and OUT 255,62 respectively: the initial call to GO SUB 2000 at line 1000 sets up variables and activates 64-column mode, but a second path exists at line 80 that calls RANDOMIZE USR 55296 (the setup entry point) directly before the subroutine is called, meaning setup is effectively invoked twice on first run. This is harmless but slightly redundant. Additionally, line 14 is a REM crediting the typist, placed among other REM lines but using an out-of-sequence line number, suggesting it was inserted after the original REM block was written.