TechDraw Jr 1.3

TechDraw Jr 1.3 is a loader and configuration utility for a technical drawing application, responsible for bootstrapping the main machine code module (“dcode”) and setting up printer and interface options. The program supports three storage devices — cassette tape, A&J Microdrive, and JLO disk drive — detected and selected at runtime, with the device type stored via a POKE/PEEK trick to survive across LOAD operations. Printer configuration covers six 80-column dot-matrix models (Epson, Gemini, Memotech, Panasonic, Seikosha, Prowriter) paired with five Centronics interface options (Aerco, Tasman, A&J, FDD-RS232), each configured by calling specific machine code routines via RANDOMIZE USR. The FDD-RS232 interface path additionally uses channel formatting and OPEN/CLOSE stream operations, and the main drawing engine is launched by jumping to the machine code entry point at address 50530.

---

Program Structure

The program is organized into several distinct functional phases executed in sequence:

1. Initialization and machine code load (lines 10–52): Sets display attributes, uses a POKE/PEEK idiom to preserve the device type across a LOAD, then loads the “dcode” machine code binary.
2. Device selection menu (lines 52–64): Presents a three-option storage device menu and calls machine code initialization routines for the chosen device.
3. Main menu (lines 70–98): Offers Run, Customize Printer, and Customized Backup options.
4. Printer and interface configuration (lines 202–920): Two-level menus select the printer model and Centronics interface, each dispatching to a machine code setup routine.
5. Backup save routines (lines 9910–9930): Save both the BASIC loader and the machine code block back to the selected storage device.

Device Persistence Trick

Line 10 uses a classic technique to preserve state across a LOAD operation: POKE 65535,dev writes the current device number to the top of RAM before loading, then LET dev=PEEK 65535 reads it back immediately after. On first run, dev is uninitialized (evaluates to 0), so the POKE stores 0, which is harmless. This allows the program to remember which device was used to load “dcode” without requiring a separate data file.

Machine Code Integration

The program makes heavy use of RANDOMIZE USR to call machine code subroutines embedded in the loaded “dcode” block. Key entry points include:

Address	Purpose
55259	Initialize A&J Microdrive I/O
55235	Initialize JLO disk I/O
55291	Initialize cassette tape I/O
50530	Launch main TechDraw drawing engine
63012–63042	Centronics interface setup (various)
63024–63039	Printer model initialization (various)

The machine code block is loaded to address 30036 and spans 35400 bytes, occupying a substantial portion of the address space above the BASIC program (which is CLEARed to 30035).

Printer Configuration Architecture

Printer setup follows a two-level dispatch pattern. Lines 202–240 select the printer model and jump to a model-specific stub (lines 400–920), each of which calls a machine code routine to configure that printer’s escape sequences, then falls through to line 300. Line 300 then presents the Centronics interface menu, dispatching to the appropriate interface machine code routine at lines 382–389. This separates printer-model concerns from interface-hardware concerns cleanly.

The REM statements at lines 400, 500, 600, 700, 800, and 900 contain embedded control characters (character codes 20, 1, and 0) that appear to be section labels or flags embedded in the token stream, possibly used by an external tool or editor.

FDD-RS232 Special Handling

The FDD-RS232 interface path (line 389) is notably more complex than the others. After calling its machine code initializer at 63042, it displays a parameter settings header with INVERSE 1, manipulates stream 3 with POKE 23729,255 and CLOSE #*3/POKE 23729,0 to reset the channel system, then uses FORMAT and OPEN #*3 to attach the RS-232 channel. The variable o (uninitialized, so 0) is passed as the OPEN parameter.

Key BASIC Idioms

• LET k=CODE INKEY$ at line 57, with a busy-wait loop at line 58, is used to poll for a keypress and validate it against ASCII codes 49–51 (‘1’–’3’).
• Line 82 uses IF i>90 THEN LET i=i-32 to convert lowercase ASCII to uppercase, enabling case-insensitive menu input without a separate function.
• The backup save routines at lines 9910–9930 are only reachable via GO TO 9900 from line 90, keeping them well out of the normal flow. Note that line 9900 itself is absent — the GO TO targets a non-existent line, so execution falls through to line 9910, which is a deliberate technique.
• Multiple PRINT ,,,,, constructs are used to push output down the screen without AT coordinates, relying on the comma separator to advance print positions.

Anomalies and Notes

• Line 44 (cassette fallback load) has no GO TO 52 after it, unlike lines 42 and 43. This is intentional — execution naturally falls through to line 52.
• The linefeed configuration at lines 392–393 POKEs either 0 or 10 into address 63122, directly patching the machine code’s linefeed byte rather than using a BASIC variable.
• Line 380 validates the interface selection with IF p<1 OR p>5 THEN GO TO 379, but the INPUT at line 379 would cause an error if a non-numeric value is entered, as there is no error trapping.
• The “Gorrilla Banana” printer name at line 224 is a misspelling of “Gorilla Banana,” a known third-party printer brand of the era.