Hex Dec Bin

This program is a number base converter that translates values between decimal (0–65535), hexadecimal (4-digit), and binary (19-character with spaces) representations. It uses subroutines at lines 15, 210, 300, and 400 for binary-to-decimal, decimal-to-hex, hex-to-decimal, and binary-to-decimal conversions respectively, storing intermediate results in string arrays D$(1,19) and B$(1,4). The binary display uses a 19-character format with spaces at positions 5, 10, and 15 (skipped in loops via conditional NEXT I), representing a 16-bit value split into four nibbles. Output is presented using SCROLL and AT positioning on the lower display lines, and the program supports a COPY command for hard-copy output via a ZX printer.

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Program Analysis

Program Structure

The program is organized as a set of subroutines and a main input/output loop. The entry point is line 500, which presents a menu. Four subroutines handle the core conversions:

Lines	Role
15–60	Decimal → Binary: converts VAL C$ into D$(1,19) using successive halving
210–255	Decimal → Hex: converts VAL C$ into B$(1,4) using base-16 division
300–340	Hex → Decimal: reconstructs a decimal value from B$(1,4) and stores it in C$
400–440	Binary → Decimal: converts D$(1,19) back to a decimal value in C$

The main flow branches at lines 525–540 depending on input mode (H/D/B), and all paths converge at line 700 for output display.

Binary Representation Format

The 16-bit binary value is stored in a 19-character string D$(1,19). Positions 5, 10, and 15 are intentionally skipped in both the encoding loop (lines 25–55) and the decoding loop (lines 410–430) using IF I=5 OR I=10 OR I=15 THEN NEXT I. This means those positions act as spacers (likely spaces), visually grouping the 16 bits into four nibbles: 0000 0000 0000 0000. Binary input (line 620) requires the user to enter exactly this 19-character format including the spaces.

Hex Digit Encoding

In the decimal-to-hex subroutine (lines 230–235), digits 0–9 are stored directly as their string character, but hex digits A–F (values 10–15) are stored using CHR$ (A+28). On the ZX81/TS1000 character set, CHR$ 38 through CHR$ 43 correspond to printable characters — this is an unconventional encoding that avoids alphabetic characters. The hex-to-decimal subroutine at lines 300–325 compensates by pre-assigning A=10 through F=15, then using VAL B$(1,J) to evaluate each nibble, relying on the BASIC interpreter to resolve those single-letter variable names.

Key BASIC Idioms

• LET D$(1)=Z$ (line 622) and LET B$(1)=Q$ (line 560) assign an entire string to a dimensioned string array row — a compact ZX81 idiom for bulk string assignment.
• VAL C$ is used throughout as the shared numeric pipeline between subroutines, with C$ acting as a global numeric register in string form.
• FAST/SLOW mode switching (lines 564, 592, 623 / line 700) brackets the computation subroutines to speed up processing while keeping display readable.
• SCROLL is used repeatedly at lines 710–740 to scroll the single-line ZX81 display area, simulating multi-line output on the bottom line.

Input Validation

Each input mode includes basic validation. Hex input (line 556) checks for exactly 4 characters, then (lines 561–563) checks each character’s CODE is within the valid range (28–43), rejecting out-of-range values by re-looping to line 555. Decimal input (line 591) rejects values above 65535. Binary input (line 621) requires exactly 19 characters. None of these checks verify individual binary digit values (only ‘0’ or ‘1’), so non-binary characters in binary input would corrupt the result silently.

Output Display

The output section (lines 700–780) uses a sequence of SCROLL and PRINT AT 20,0 calls to build up a multi-line summary on the display. Line 730 shows the decimal value decomposed as 256 * high_byte + low_byte = total, computed inline using VAL C$. The user can then enter another number in any base, request a hard copy with COPY, or stop with S.

Bugs and Anomalies

• The hex validation at line 562 uses CODE B$(1,I)>43 OR CODE B$(1,I)<28. Character codes 28–37 represent digits 0–9 and codes 38–43 represent the non-standard A–F substitutes. However, this range also permits some non-hex characters that happen to fall within 28–43.
• Line 591 checks IF VAL C$>65535 THEN GOTO 585 but does not validate that C$ is actually numeric, so non-numeric decimal input would cause a BASIC error rather than a graceful re-prompt.
• After the binary input path (lines 610–630), execution falls through to line 700 directly, while the hex and decimal paths use explicit GOTO 700 and GOTO 700 — but the binary path at line 630 has no GOTO 700, relying on fall-through from line 630 to 700. This works only because there are no intervening lines between 630 and 700.