Conversion

This program is a unit-conversion utility offering eleven conversion types, including temperature (Celsius to Fahrenheit), length (centimeters to inches), and several pharmacist/apothecary-related measurements such as drams, grains, and milligrams. The main menu is displayed at lines 30–80, and user selection dispatches via a chain of IF/GO SUB statements to individual subroutines starting at line 210. Each conversion subroutine uses FLASH 1/FLASH 0 to highlight the result value on screen. Lines 770–820 reveal a hidden developer utility, accessible only by running those lines directly, that provides renumber, rem-kill, delete, and variable-list functions by calling machine code routines via RANDOMIZE USR with addresses computed using a bitmask technique.

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Program Analysis

Program Structure

The program divides cleanly into four regions:

1. Menu display (lines 20–80): Sets up the display and prints all eleven conversion choices.
2. Dispatch (lines 90–200): Reads the user’s choice into A and routes to the appropriate subroutine via a chain of IF … THEN GO SUB statements, falling through to GO TO 30 if no valid option is entered.
3. Conversion subroutines (lines 210–750): Each subroutine follows the same pattern: CLS, print a heading, INPUT a value, compute the result, print it with FLASH highlighting, then call the shared “press any key” subroutine at line 250 before RETURNing.
4. Hidden developer utilities (lines 760–820): A SAVE/verify block and a developer toolkit that is never reached during normal execution.

Shared “Press Any Key” Subroutine

Line 250 acts as a shared tail-end subroutine called by every conversion routine via GO SUB 250. It prints a prompt at the bottom of the screen using PRINT AT 21,0, waits for a keypress with PAUSE 0, clears the screen, and returns. The subsequent RETURN in the calling subroutine (e.g., line 300’s GO SUB 250: RETURN) correctly unwinds both stack frames.

FLASH Highlighting of Results

Each conversion result is bracketed with FLASH 1 and FLASH 0 inline within a single PRINT statement, so only the numeric answer blinks on screen while the surrounding label text remains static. This is a straightforward but effective way to draw the user’s eye to the converted value.

Hidden Developer Toolkit (Lines 770–820)

These lines are unreachable during normal program flow — line 760 is a SAVE command and line 200 loops back to 30 — so the toolkit can only be invoked by manually running from line 770 with RUN 770 or GO TO 770. It offers four built-in utility functions:

• Renumber — machine code at address 64048
• Remkill — machine code at address 64841
• Delete (range) — machine code at address 65017
• Variable list — machine code at address 65184

The target address is selected using a bitmask technique on line 770: LET zzz=(64048 AND fff=1)+(64841 AND fff=2)+(65017 AND fff=3)+(65184 AND fff=4). In Sinclair BASIC, a boolean expression evaluates to 1 (true) or 0 (false), so multiplying the address by the boolean effectively selects exactly one address while zeroing out the rest. IF NOT zzz THEN GO TO 770 rejects invalid input.

Lines 810–820 pass parameters to the machine code via POKEs into system variables at addresses 23299–23302, then invoke the routine with RANDOMIZE USR zzz. The variable-list function uses PRINT USR zzz: STOP instead, suggesting it returns a value rather than performing an in-place operation.

There is a minor bug on line 810: the first POKE calculates the low byte of fff as fff - 265 * PEEK 23300 instead of the correct fff - 256 * PEEK 23300. This off-by-nine error would corrupt the parameter for the renumber start line when fff >= 256.

Bugs and Anomalies

Line	Issue
40, 260, 270, 290	Consistent misspelling: “CENTEMETERS” should be “CENTIMETERS”.
490	The drams-to-milliliters subroutine prints the result labeled “MILLIGRAMS” instead of “MILLILITERS”.
810	Low-byte calculation uses multiplier 265 instead of 256, corrupting the parameter when fff ≥ 256.
30–80	The menu heading and option 8 spell it “FARENHEIT”; lines 210 and 240 also use “FARENHEIT” — correct spelling is “FAHRENHEIT”.

Key BASIC Idioms

• Chained IF A=n THEN GO SUB … without ELSE — standard dispatch pattern in Sinclair BASIC, which has no SELECT CASE.
• Boolean arithmetic for address selection: (address AND condition) used as a multiply-by-boolean.
• PAUSE 0 as an indefinite wait for any keypress (line 250).
• Inline FLASH attribute changes within a single PRINT to selectively highlight output.