Programming Utilities

This ZX81/TS1000 BASIC utility provides three number-base conversion tools for machine code programmers: hexadecimal-to-decimal, decimal-to-hexadecimal, and a two-byte (LSB/MSB) splitter. The hex-to-decimal converter manually parses each character of an input string using ZX81 character codes (digits map to codes 28–37, letters A–F to 38–43), then multiplies each nibble by the appropriate power of 16. The two-byte converter exploits the ZX81’s RAND statement, which stores its argument in the SEED system variable at addresses 16434–16435, to extract the least-significant and most-significant bytes of any 16-bit value without machine code. The program is explicitly designed to be renumbered and embedded into other programs using the Delphic Toolkit.

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Program Structure

The program begins at line 9000, consistent with its stated purpose of being appended to existing programs and renumbered. It is organised into clearly delineated subroutines and sections:

1. Main menu (9035–9115): Displays five options and branches on numeric input.
2. Hex-to-Dec converter (9120–9295): String parsing loop with character-code arithmetic.
3. Dec-to-Hex converter (9305–9450): Successive division by powers of 16 with a lookup string.
4. 2-Byte converter (9480–9645): Uses RAND and PEEK to split a 16-bit value.
5. Shared subroutines: Border-drawing (9650–9695), scroll-clear (9455–9475), field-clear (9630–9645), and error display (9700–9720).
6. Save and exit (9725–9730).

Hex-to-Decimal Converter

The converter at lines 9215–9275 manually parses a hex string entered as H$. It iterates from the most-significant to least-significant character, converting each to a numeric nibble via ZX81 character codes:

Character type	Code range	Operation	Result range
Digits 0–9	28–37	C = C - 28	0–9
Letters A–F	38–43	C = C - 28 (yields 10–15)	10–15

There is a subtle bug at line 9255: after the first IF C>=28 AND C<=37 subtracts 28, the condition at line 9260 checks IF C>=38 AND C<=43, which can never be true after subtraction. However, line 9255 also has an erroneous GOTO 9270 guard (line 9255 does not branch, unlike 9260). In practice, letters A–F (codes 38–43) fall through correctly: the first IF at 9250 does not fire (38>37), so control reaches 9260 where subtraction of 28 yields 10–15. The logic accidentally works despite the redundant second condition check on line 9255.

Decimal-to-Hex Converter

This section (9350–9445) uses the string A$="0123456789ABCDEF" as a nibble lookup table. The 16-bit decimal value Q is decomposed by successive integer division:

• D = INT(Q/4096) — most-significant nibble
• E = INT(R/256) — second nibble
• H = INT(R/16) — third nibble
• L = R - 16*H — least-significant nibble

Each nibble indexes into A$ with A$(D+1 TO D+1), exploiting ZX81 BASIC’s one-based string slicing. Input of 123456 is used as a magic number to switch to the hex-to-dec converter (line 9385), and 0 returns to the main menu.

There is a bug at line 9395: GOTO 400 (without the 9000-block prefix) is almost certainly a typographical error for GOTO 9400. In context the intent is to re-prompt after a bounds error, but line 400 does not exist, which would cause an error at runtime for out-of-range values.

Two-Byte Converter

The most elegant technique in the program appears at lines 9610–9620. Rather than implementing bit-shifting arithmetic, the converter exploits the ZX81 system variable SEED (used by RAND) stored at addresses 16434 (LSB) and 16435 (MSB):

• RAND N — stores N as a 16-bit integer into SEED.
• PEEK 16434 — reads the least-significant byte.
• PEEK 16435 — reads the most-significant byte.

This avoids any division or masking arithmetic and produces correct results for any value 1–65535. Zero is treated as an abort condition (line 9575) and redirects to the main menu.

Display and UI Techniques

The border-drawing subroutine at 9650–9695 uses PLOT to draw a rectangular frame: vertical sides by iterating Y from 0 to 43, and three horizontal lines (top, a mid-divider at Y=32, and bottom at Y=0). The % % sequences in PRINT statements produce flashing/inverse text effects by using ZX81 character alternation — here they animate the prompt text by printing the string twice in a short FOR loop (lines 9170–9205, 9525–9560), toggling between the percent-decorated version and a plain version to simulate flashing without FLASH.

The scroll-clearing subroutine at 9455–9475 clears result lines by printing spaces over rows 10–18, then resets the row counter A to 10. This prevents results from scrolling off the decorated border.

Notable Anomalies

• Line 9395 references GOTO 400 rather than GOTO 9400 — a clear typo that would cause a runtime error for out-of-range decimal inputs.
• The FAST/SLOW pairs around the border subroutine calls (e.g. lines 9140/9150, 9500/9510) draw the border quickly in FAST mode then return to SLOW for user interaction — a standard ZX81 speed optimisation idiom.