---
title: "Large Characters"
id: 54984
type: "computer_media"
slug: "large-characters"
url: "http://localhost/computer_media/large-characters/"
markdown_url: "http://localhost/computer_media/large-characters.md"
published_at: "2024-06-10T01:44:45+00:00"
modified_at: "2026-03-30T21:43:26+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2019/02/20230809-041332.jpg"
excerpt: "Watch every printable character from the ROM font rendered as a giant 8×8 pixel grid, one at a time, with clever bit-extraction arithmetic."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 2068"
    slug: "ts2068"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts2068/"
model:
  - name: "Timex/Sinclair 2068"
    slug: "ts-2068"
    taxonomy: "model"
    url: "http://localhost/model/ts-2068/"
genre:
  - name: "Demo"
    slug: "demo"
    taxonomy: "genre"
    url: "http://localhost/type/demo/"
media_contents:
  - id: 54964
    title: "ISTUG Public Domain Library 6"
    type: "computer_media"
    url: "http://localhost/computer_media/istug-public-domain-library-6/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/06/SCR-20260329-ntbh.png"
media_type_tags: "Demo"
---

This program displays each printable ASCII character (CHR$ 32 through CHR$ 126) rendered as a large 8×8 pixel grid using a custom UDG character. It reads the ROM character set by locating the font address via system variables at 23606–23607, then uses bit-shifting arithmetic to extract each pixel bit from the font bytes. A single UDG (char 144) is defined with a border/box pattern from the DATA statement at line 100, and its inverse (CHR$ 143, i.e., 144−1) is used to represent set pixels while CHR$ 144 represents clear pixels. The program pauses for a keypress between each character display, stepping through all 96 printable characters before stopping.

***

## Program Analysis

### Program Structure

The program is organized into three functional phases:

1. **Initialization (lines 5–10):** Defines UDG ‘A’ (char 144) from inline DATA and locates the ROM character set base address.
2. **Display loop (lines 15–65):** Iterates over all 96 printable characters (CHR$ 32–126), rendering each as a large pixel grid.
3. **Data (line 100):** Eight bytes defining a bordered rectangle UDG pattern (`255,129,129,129,129,129,129,255`).

### UDG Setup

Line 5 uses `RESTORE` followed by a `FOR` loop to `POKE` all 8 bytes of the DATA at line 100 into the address returned by `USR CHR$ 144` (i.e., `USR "\a"`), defining UDG ‘A’. The pattern `255,129,129,129,129,129,129,255` creates a hollow box — a full top row, full bottom row, and only the outermost bits set in middle rows.

### Font Address Lookup

Line 10 computes the character set base address using system variables:

- `PEEK 23606 + 256 * PEEK 23607` reads the two-byte system variable CHARS (address &5C36), which points 256 bytes before the font data.
- Adding 256 corrects for this offset, giving `c` as the true start of the font bitmap data.

This technique works whether the font is in ROM or has been relocated to RAM.

### Pixel Extraction and Rendering

For each character `j` and each row `p` (0–7), the program fetches the font byte at `c + p + j*8` (line 25). The inner loop (lines 35–50) extracts bits from least-significant to most-significant using integer division:

- `LET x = INT(byte/2)` — integer right-shift by one.
- `LET bit = byte - 2*x` — isolates the LSB (0 or 1).
- `LET byte = x` — advances to the next bit.

The extracted `bit` is used to select either CHR$ 143 (set pixel, `144−1`) or CHR$ 144 (clear pixel, `144−0`). Because bits are extracted LSB-first but printed left-to-right starting at column 8 down to column 1 (via `AT p, 9-l`), the pixel order is naturally reversed back to the correct left-to-right visual orientation.

### Display Layout

| Screen element | Position |
| --- | --- |
| Large pixel grid | Rows 0–7, columns 1–8 |
| Raw byte values | Rows 0–7, column 15 |
| CHR$ label and character | Row 4, column 20 |
| “Press any key” prompt | Row 15, column 0 |

### Notable Techniques

- Using `CHR$ (144 - bit)` elegantly selects between two UDG states with a single expression rather than an `IF` branch.
- The `PAUSE 0` on line 60 (conditioned on `j <> 95`) waits for a keypress on every character except the last, avoiding a redundant pause before `STOP`.
- The font lookup via CHARS system variable is portable and will correctly follow any relocated or custom font.

### Potential Anomalies

- Line 60 uses `PAUSE 0` alone without reading `INKEY$`; `PAUSE 0` on the Spectrum/TS2068 waits until any key is pressed, so this is valid and sufficient.
- The UDG box pattern is defined but only its code-point relationship (`144 - bit`) matters; the actual visual appearance of the box/blank pixels is a design choice and not central to correctness.
- Line 10 adds 256 to the CHARS pointer. The CHARS system variable holds a value 256 less than the font start, so this addition is correct and intentional.

## Source Code

```
    1 REM LARGE CHARACTERS
    5 RESTORE : FOR j=0 TO 7: READ a: POKE USR CHR$ 144+j,a: NEXT j
   10 LET c=PEEK 23606+256*PEEK 23607+256
   15 FOR j=0 TO 95: CLS : PRINT AT 4,20;"CHR$ ";j+32;" ";CHR$ (j+32)
   20 FOR p=0 TO 7
   25 LET byte=PEEK (c+p+j*8)
   30 PRINT AT p,15;byte
   35 FOR l=1 TO 8
   40 LET x=INT (byte/2): LET bit=byte-2*x: LET byte=x
   45 PRINT AT p,9-l;CHR$ (144-bit)
   50 NEXT l
   55 NEXT p
   60 IF j<>95 THEN PRINT AT 15,0;"Press any key for next character": PAUSE 0
   65 NEXT j: STOP 
  100 DATA 255,129,129,129,129,129,129,255
 9998 SAVE "LARGE CHAR" LINE 0
```