CHRs Gen

CHRs Gen is a character font generator that creates a modified TS 2068 character set in RAM starting at address 64000, intended for use with a printer (specifically a 2040-series printer). The program reads the ROM character definitions from addresses 15616–16383 using PEEK, applies a series of byte-value substitutions to thicken or restyle each character’s pixel rows, and POKEs the results into a 1536-byte block. System variable 23607 (CHARS) is manipulated to switch between the new font (249/0 combination) and the default ROM font (60), while 23609 controls the printer’s speed. After building the font, the program prints instructions, triggers two COPY operations to produce hardcopy output, then self-destructs with NEW.

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Program Structure

The program divides into four logical phases:

1. Initialization (lines 20–70): plays a startup beep fanfare, sets CLEAR 63999 to reserve high RAM, configures display attributes, and pokes printer speed into address 23609.
2. First font block (lines 80–270): iterates over ROM addresses 15616–16135 (the first 65 characters’ pixel rows), applies byte substitution rules, and POKEs results to 64000–64519.
3. Second font block (lines 280–510): iterates over ROM addresses 16136–16383 (remaining characters and UDGs), applies a separate set of substitution rules, and POKEs results starting at 64520.
4. Output and self-destruction (lines 520–750): switches CHARS pointer to the new font, prints and copies instructions twice (once with new font, once with standard font for comparison), then calls NEW.

Font Relocation Mechanism

The Spectrum’s system variable at address 23607 (CHARS, the low byte of the character set pointer) is the key to font switching. By default the ROM font lives at 15360; the system variable pair 23606/23607 holds a pointer offset by −256. Setting 23607 to 249 and 23606 to 0 (lines 520–530) points the system at address 64000−256 = 63744, making the new font active. Line 600 restores 23607 to 60, switching back to the ROM font for the comparison printout.

Byte Substitution Logic

Rather than storing a fully hand-crafted font, the program derives the new character bitmaps by transforming the existing ROM pixel rows. Each 8-bit value represents one horizontal pixel row of a character. The substitution tables in lines 130–240 and 310–490 map specific byte values to new ones, generally widening strokes or adding pixels to produce a bolder appearance suited to the 2040 printer’s output characteristics.

Notable substitution patterns:

• Line 130: 128 → 192 (adds a set bit, extends leftmost pixel)
• Line 190: 64 → 96, line 200: 32 → 48, line 210: 16 → 24 — a cascade that progressively doubles the rightmost set bit for thinner rows
• Lines 310–490 handle the upper character range with a denser substitution table covering values like 120→124, 56→60, 60→126, etc.

Line 250 includes a bounds guard: if b=0 or b>255 the POKE is skipped, avoiding writing a zero (blank row) that might have been an unmodified pass-through, and catching any arithmetic overflow — though overflow cannot actually occur here since no rule produces a value above 255.

Key BASIC Idioms

• VAL "number" is used throughout for numeric literals in loop bounds and CLEAR (lines 40, 80, 110, 290), a well-known technique to reduce the memory consumed by stored line data since short string tokens take less space than full numeric mantissas.
• The subroutine at line 740 prints a blank line and plays a descending beep sweep (FOR j=20 TO 30: BEEP .1,-j: NEXT j), acting as both an audio and visual separator between instruction sections.
• Line 640 ends with NEW, causing the program to erase itself after the second printout — the “self-destruct” described in the on-screen instructions.

Memory Map Summary

Address range	Purpose
15616–16135	ROM font, chars 32–96 (source, first block)
16136–16383	ROM font, chars 97–127 + UDGs (source, second block)
64000–64519	Modified font, first block (destination)
64520–64767	Modified font, second block (destination)
23606–23607	CHARS system variable (font pointer −256)
23609	Printer speed control

Anomalies and Observations

• The two font loops together cover 521 bytes (lines 80–270) and 248 bytes (lines 280–510), totaling 769 source bytes — but the destination block runs from 64000 to 64767, also 768 bytes (plus one, since a is incremented after the last POKE). This is consistent with the SAVE instruction on line 730 specifying length 1536 — twice 768 — implying UDG data from a second source would also be saved.
• The destination pointer a for the second block is initialized to 64520 (line 280) rather than 64521, which means the second loop’s output slightly overlaps with where the first loop’s pointer ended up, leaving one address (64520) written by both loops. This is likely a minor off-by-one in the design.
• The credit string on line 550 embeds an author name reference which is present in the program text but is noted here only as a structural observation about line content.