HEXSPEC

This program is a hexadecimal machine-code loader that reads hex byte values entered by the user and POKEs them into memory. The user specifies a start and finish address, then enters each group of eight bytes as space-separated two-digit hex pairs (e.g., “C5 3A 00 42…”) per line. A running checksum total is accumulated and compared against a user-supplied value to catch data entry errors. Two DEF FN functions handle the hex conversion: `FN h$` converts a byte value to its two-character hex string, and `FN h` converts a single hex character back to its numeric nibble value.

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Program Analysis

Program Purpose

The program allows a user to manually enter machine code listings in hexadecimal format and write them directly into RAM. It processes eight bytes per line, accumulates a checksum, and prompts the user to confirm the total before advancing, providing a basic error-detection mechanism for hand-typed hex data.

Program Structure

Lines	Role
10–15	REM headers identifying the program
20–30	DEF FN definitions for hex conversion
40–50	User input: start and finish addresses
60–170	Main loop: prompt, input, decode, POKE, checksum verify

Hex Conversion Functions

Two complementary DEF FN functions handle the hex/decimal translation:

• FN h$(a) (line 20): Converts a byte value (0–255) to a two-character uppercase hex string. It splits the byte into high and low nibbles, adds 48 (ASCII ‘0’), and applies a +7 offset for values above 9 to bridge the gap between ‘9’ and ‘A’ in the character set.
• FN h(h$) (line 30): Converts a single hex character to its numeric nibble value. It reverses the encoding: subtracts 48 and conditionally subtracts 7 for letters A–F (characters greater than ‘9’).

Note that FN h$ is defined but never called in the main loop — only FN h is used to decode input characters.

Input Parsing

Each input line a$ is expected to contain eight two-digit hex values separated by single spaces, e.g. C5 3A 00 42 FF 01 7E 42. The inner loop (lines 80–120) processes the string by always reading characters 1 and 2, converting them via FN h(a$(1))*16+FN h(a$(2)), then slicing the string with LET A$=A$(4 TO ) to discard the consumed pair and its trailing space. This is a compact sliding-window parse technique common in Spectrum BASIC.

Checksum Mechanism

The variable tot (line 65) accumulates the sum of all eight decoded byte values. After the inner loop completes, the user is asked to INPUT t (line 140), supplying the expected total from the printed listing. If t <> tot, an error message is shown and control jumps back to line 62 to re-enter the same line. This is a simple additive checksum, not a CRC, but it is effective at catching most single-character transpositions.

Notable Techniques and Idioms

• The IF a$="END" THEN STOP guard at line 75 provides a clean exit before the full address range is exhausted.
• Mixed-case variable usage: the string is initially assigned to a$ (lowercase) but later manipulated as A$ (uppercase) at line 110, which are the same variable in Sinclair BASIC. This inconsistency is harmless but worth noting.
• Similarly, the inner loop counter is declared as b (line 80) but the NEXT statement references B (line 120) — again equivalent in Sinclair BASIC.
• The STEP 8 on the outer FOR loop (line 60) advances the address pointer by 8 after each full line is processed, matching the eight-bytes-per-line input format.

Bugs and Anomalies

• The comment at line 66 spells “hexadecimal” as “HEXIDECIMAL” — a common misspelling, not a code defect.
• If the finish address f is not a multiple of 8 bytes beyond s, the FOR n=s TO f STEP 8 loop may not reach exactly f, potentially leaving the last few bytes unloaded.
• There is no bounds checking on a$ length; if the user enters fewer than the expected 23 characters (8 pairs + 7 spaces), the A$(4 TO) slice at line 110 will eventually cause a BASIC error on the fifth or later iteration.
• FN h$ is defined but unused in the program’s logic, suggesting this loader may be a stripped-down version of a larger utility that also included a hex-dump display feature.