28 CP437 glyphs in the 80x25 font differ from the IBM VGA ROM

## Summary

BlockZone is described in the README as a "faithful, pixel-perfect recreation of the original IBM VGA font." 228 of the 256 CP437 glyphs are byte-identical to the original ROM, but **28 glyphs differ**. Of these, 25 are off by a single-pixel position shift (horizontal or vertical) and 3 are reshaped (`0x7C |`, `0xF1 ±`, `0xFE ■`).

This affects the font embedded in `ansilove/ansilove` (`libansilove/src/fonts/font_pc_80x25.h`) and in ansilove.js (`FONT_PRESETS["80x25"]`), both of which carry the same bitmaps and therefore the same deviations.

## Reference used

The comparison reference is a font extracted from IBM hardware: [`spacerace/romfont`](https://github.com/spacerace/romfont), `font-bin/IBM_VGA_8x16.bin` (4096 bytes = 256 glyphs x 16 rows, 1 byte per row).

Supporting checks:

- The 28 deviations appear identically in both the C font (`font_pc_80x25.h`) and the JS font (`FONT_PRESETS["80x25"]`), confirming the discrepancy is in the font data, not the renderer.

- VileR's *Ultimate Oldschool PC Font Pack* (`Bm437_IBM_VGA8`) is byte-for-byte identical to the `romfont` dump (0 of 256 glyphs differ), giving an independent second source for the reference bitmaps.

- The font was also compared against 7 distinct VGA-clone ROMs (Trident, 3dfx/Riva, Tseng, ATI, AVGA2, SeaBIOS, Vanta). The IBM VGA ROM is the closest match at 228/256; every clone scores lower. The 28 deviations do not correspond to any of these hardware fonts.

## Question

Are these 28 glyphs intentionally redrawn (for example, re-centering narrow glyphs on the 9-wide cell, or deliberately filling the broken vertical bar), or are they unintended artifacts of recreating the font? The README's "pixel-perfect" claim implies the former is not intended. If a ROM-accurate font is the goal, the corrected bitmaps are included below.

## Reshaped glyphs

Three glyphs are not simple position shifts:

**`0x7C` `|` (vertical bar).** The ROM glyph is a broken bar with a gap mid-height; BlockZone renders it solid:

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   .........   <
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xF1` `±` (plus-minus).** The ROM bottom rule spans the full 8-px cell width; BlockZone's is 6 px:

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  ...##....   ...##....    
  ...##....   ...##....    
  .######..   .######..    
  ...##....   ...##....    
  ...##....   ...##....    
  .........   .........    
  .........   .........    
  .######..   ########.   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xFE` `■` (filled square).** The ROM glyph is 5 px wide; BlockZone's is 6 px:

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

## Full list of deviations

| Code | Char | Name | Pixels off | Difference |
|------|------|------|-----------|------------|
| `0x60` | ` | grave accent | 4 | shifted down 1 row |
| `0x66` | f | small f | 24 | shifted right 1 px |
| `0x76` | v | small v | 13 | shifted right 1 px |
| `0x7C` | \| | vertical bar | 2 | reshaped (see below) |
| `0x7E` | ~ | tilde | 14 | shifted down 1 row |
| `0x80` | Ç | C with cedilla | 16 | body/cedilla repositioned |
| `0x87` | ç | c with cedilla | 37 | shifted up 1 row |
| `0x8F` | Å | A with ring | 7 | ring + crossbar repositioned |
| `0x90` | É | E with acute | 21 | accent + body repositioned |
| `0x91` | æ | ae ligature | 2 | shifted right 1 px |
| `0x9B` | ¢ | cent sign | 12 | shifted right 1 px |
| `0x9F` | ƒ | florin | 12 | descender repositioned |
| `0xA6` | ª | feminine ordinal | 26 | shifted down 1 row |
| `0xA7` | º | masculine ordinal | 22 | shifted down 1 row |
| `0xAB` | ½ | one half | 9 | numerator shifted right 1 px |
| `0xAC` | ¼ | one quarter | 11 | numerator shifted right 1 px |
| `0xE3` | π | pi | 10 | shifted down 1 row |
| `0xE4` | Σ | sigma | 18 | shifted down 1 row |
| `0xE6` | µ | mu | 18 | shifted down 1 row |
| `0xE8` | Φ | phi | 16 | shifted down 1 row |
| `0xE9` | Θ | theta | 16 | shifted down 1 row |
| `0xF1` | ± | plus-minus | 2 | reshaped (see below) |
| `0xF5` | ⌡ | integral bottom | 8 | shifted down 1 row |
| `0xF6` | ÷ | division sign | 4 | dots repositioned |
| `0xFA` | · | middle dot | 4 | shifted up 1 row |
| `0xFC` | ⁿ | superscript n | 24 | shifted right 1 px |
| `0xFD` | ² | superscript 2 | 26 | shifted right 1 px |
| `0xFE` | ■ | filled square | 7 | reshaped (see below) |

All 228 remaining glyphs, including every box-drawing, block, and shading character, are identical to the ROM.

<details>
<summary>Side-by-side bitmaps for all 28 glyphs</summary>

`#` = pixel on, `.` = pixel off. Left column is BlockZone (9 px wide as stored), right column is the IBM VGA ROM. Differing rows are marked `<`.

**`0x60`  `  grave accent** (4 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   ..##.....   <
  ..##.....   ..##.....    
  ...##....   ...##....    
  ....##...   .........   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x66`  f  small f** (24 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  ...###...   ..###....   <
  ..##.##..   .##.##...   <
  ..##..#..   .##..#...   <
  ..##.....   .##......   <
  .####....   ####.....   <
  ..##.....   .##......   <
  ..##.....   .##......   <
  ..##.....   .##......   <
  ..##.....   .##......   <
  .####....   ####.....   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x76`  v  small v** (13 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  ##...##..   .##..##..   <
  ##...##..   .##..##..   <
  ##...##..   .##..##..   <
  ##...##..   .##..##..   <
  ##...##..   .##..##..   <
  .##.##...   ..####...   <
  ..###....   ...##....   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x7C`  |  vertical bar** (2 px, reshaped (see below))

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   .........   <
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x7E`  ~  tilde** (14 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .###.##..   .........   <
  ##.###...   .###.##..   <
  .........   ##.###...   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x80`  Ç  C with cedilla** (16 px, body/cedilla repositioned)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  ..####...   ..####...    
  .##..##..   .##..##..    
  ##....#..   ##....#..    
  ##.......   ##.......    
  ##.......   ##.......    
  ##.......   ##.......    
  ##.......   ##....#..   <
  ##....#..   .##..##..   <
  .##..##..   ..####...   <
  ..####...   ....##...   <
  ...##....   .....##..   <
  .###.....   .#####...   <
  .........   .........    
  .........   .........    
```

**`0x87`  ç  c with cedilla** (37 px, shifted up 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   ..####...   <
  .#####...   .##..##..   <
  ##...##..   .##......   <
  ##.......   .##......   <
  ##.......   .##..##..   <
  ##.......   ..####...   <
  ##...##..   ....##...   <
  .#####...   .....##..   <
  ...##....   ..####...   <
  .###.....   .........   <
  .........   .........    
  .........   .........    
```

**`0x8F`  Å  A with ring** (7 px, ring + crossbar repositioned)

```
      blockzone    ibm vga rom
  ..###....   ..###....    
  .##.##...   .##.##...    
  ..###....   ..###....    
  ...#.....   .........   <
  ..###....   ..###....    
  .##.##...   .##.##...    
  ##...##..   ##...##..    
  #######..   ##...##..   <
  ##...##..   #######..   <
  ##...##..   ##...##..    
  ##...##..   ##...##..    
  ##...##..   ##...##..    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x90`  É  E with acute** (21 px, accent + body repositioned)

```
      blockzone    ibm vga rom
  ....##...   ...##....   <
  ...##....   ..##.....   <
  .........   .##......   <
  #######..   .........   <
  .##..##..   #######..   <
  .##...#..   .##..##..   <
  .##.#....   .##......   <
  .####....   .#####...   <
  .##.#....   .##......   <
  .##...#..   .##......   <
  .##..##..   .##..##..    
  #######..   #######..    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x91`  æ  ae ligature** (2 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  ###.##...   ##..##...   <
  ..##.##..   .###.##..   <
  ..##.##..   ..##.##..    
  .######..   .######..    
  ##.##....   ##.##....    
  ##.##....   ##.##....    
  .##.###..   .##.###..    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x9B`  ¢  cent sign** (12 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  ...##....   ...##....    
  ...##....   ...##....    
  .#####...   ..####...   <
  ##...##..   .##..##..   <
  ##.......   .##......   <
  ##.......   .##......   <
  ##.......   .##......   <
  ##...##..   .##..##..   <
  .#####...   ..####...   <
  ...##....   ...##....    
  ...##....   ...##....    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0x9F`  ƒ  florin** (12 px, descender repositioned)

```
      blockzone    ibm vga rom
  .........   .........    
  ....###..   ....###..    
  ...##.##.   ...##.##.    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  .######..   .######..    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ##.##....   ...##....   <
  .###.....   ...##....   <
  .........   ##.##....   <
  .........   .###.....   <
  .........   .........    
  .........   .........    
```

**`0xA6`  ª  feminine ordinal** (26 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   ..####...   <
  ..####...   .##.##...   <
  .##.##...   .##.##...    
  .##.##...   ..#####..   <
  ..#####..   .........   <
  .........   .######..   <
  .######..   .........   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xA7`  º  masculine ordinal** (22 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   ..###....   <
  ..###....   .##.##...   <
  .##.##...   .##.##...    
  .##.##...   ..###....   <
  ..###....   .........   <
  .........   .#####...   <
  .#####...   .........   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xAB`  ½  one half** (9 px, numerator shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .##......   ##.......   <
  ###......   ##.......   <
  .##...#..   ##....#..   <
  .##..##..   ##...##..   <
  .##.##...   ##..##...   <
  ...##....   ...##....    
  ..##.....   ..##.....    
  .##......   .##......    
  ##.###...   ##.###...    
  #....##..   #....##..    
  ....##...   ....##...    
  ...##....   ...##....    
  ..#####..   ..#####..    
  .........   .........    
  .........   .........    
```

**`0xAC`  ¼  one quarter** (11 px, numerator shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .##......   ##.......   <
  ###......   ##.......   <
  .##...#..   ##....#..   <
  .##..##..   ##...##..   <
  .##.##...   ##..##...   <
  ...##....   ...##....    
  ..##.....   ..##.....    
  .##..##..   .##..##..    
  ##..###..   ##..###..    
  #..##.#..   #..####..   <
  ..######.   ..#####..   <
  .....##..   .....##..    
  .....##..   .....##..    
  .........   .........    
  .........   .........    
```

**`0xE3`  π  pi** (10 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   #######..   <
  #######..   .##.##...   <
  .##.##...   .##.##...    
  .##.##...   .##.##...    
  .##.##...   .##.##...    
  .##.##...   .##.##...    
  .##.##...   .##.##...    
  .##.##...   .##.##...    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xE4`  Σ  sigma** (18 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  #######..   .........   <
  ##...##..   #######..   <
  .##......   ##...##..   <
  ..##.....   .##......   <
  ...##....   ..##.....   <
  ...##....   ...##....    
  ..##.....   ..##.....    
  .##......   .##......    
  ##...##..   ##...##..    
  #######..   #######..    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xE6`  µ  mu** (18 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .##..##..   <
  .##..##..   .##..##..    
  .##..##..   .##..##..    
  .##..##..   .##..##..    
  .##..##..   .##..##..    
  .##..##..   .#####...   <
  .##..##..   .##......   <
  .#####...   .##......   <
  .##......   ##.......   <
  .##......   .........   <
  ##.......   .........   <
  .........   .........    
```

**`0xE8`  Φ  phi** (16 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .######..   .........   <
  ...##....   .######..   <
  ..####...   ...##....   <
  .##..##..   ..####...   <
  .##..##..   .##..##..    
  .##..##..   .##..##..    
  .##..##..   .##..##..    
  ..####...   ..####...    
  ...##....   ...##....    
  .######..   .######..    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xE9`  Θ  theta** (16 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  ..###....   .........   <
  .##.##...   ..###....   <
  ##...##..   .##.##...   <
  ##...##..   ##...##..    
  #######..   ##...##..   <
  ##...##..   #######..   <
  ##...##..   ##...##..    
  ##...##..   ##...##..    
  .##.##...   .##.##...    
  ..###....   ..###....    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xF1`  ±  plus-minus** (2 px, reshaped (see below))

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  ...##....   ...##....    
  ...##....   ...##....    
  .######..   .######..    
  ...##....   ...##....    
  ...##....   ...##....    
  .........   .........    
  .........   .........    
  .######..   ########.   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xF5`  ⌡  integral bottom** (8 px, shifted down 1 row)

```
      blockzone    ibm vga rom
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ...##....    
  ...##....   ##.##....   <
  ##.##....   ##.##....    
  ##.##....   ##.##....    
  ##.##....   .###.....   <
  .###.....   .........   <
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xF6`  ÷  division sign** (4 px, dots repositioned)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   ...##....   <
  ...##....   ...##....    
  .........   .........    
  .######..   .######..    
  .........   .........    
  ...##....   ...##....    
  .........   ...##....   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xFA`  ·  middle dot** (4 px, shifted up 1 row)

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  ...##....   .........   <
  .........   ...##....   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xFC`  ⁿ  superscript n** (24 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  .##.##...   ##.##....   <
  ..##.##..   .##.##...   <
  ..##.##..   .##.##...   <
  ..##.##..   .##.##...   <
  ..##.##..   .##.##...   <
  ..##.##..   .##.##...   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xFD`  ²  superscript 2** (26 px, shifted right 1 px)

```
      blockzone    ibm vga rom
  .........   .........    
  ..####...   .###.....   <
  .##..##..   ##.##....   <
  ....##...   ..##.....   <
  ...##....   .##......   <
  ..##..#..   ##..#....   <
  .######..   #####....   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

**`0xFE`  ■  filled square** (7 px, reshaped (see below))

```
      blockzone    ibm vga rom
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .######..   .#####...   <
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
  .........   .........    
```

</details>


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28 CP437 glyphs in the 80x25 font differ from the IBM VGA ROM #2

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Reshaped glyphs

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Code	Char	Name	Pixels off	Difference
`0x60`	`	grave accent	4	shifted down 1 row
`0x66`	f	small f	24	shifted right 1 px
`0x76`	v	small v	13	shifted right 1 px
`0x7C`	\|	vertical bar	2	reshaped (see below)
`0x7E`	~	tilde	14	shifted down 1 row
`0x80`	Ç	C with cedilla	16	body/cedilla repositioned
`0x87`	ç	c with cedilla	37	shifted up 1 row
`0x8F`	Å	A with ring	7	ring + crossbar repositioned
`0x90`	É	E with acute	21	accent + body repositioned
`0x91`	æ	ae ligature	2	shifted right 1 px
`0x9B`	¢	cent sign	12	shifted right 1 px
`0x9F`	ƒ	florin	12	descender repositioned
`0xA6`	ª	feminine ordinal	26	shifted down 1 row
`0xA7`	º	masculine ordinal	22	shifted down 1 row
`0xAB`	½	one half	9	numerator shifted right 1 px
`0xAC`	¼	one quarter	11	numerator shifted right 1 px
`0xE3`	π	pi	10	shifted down 1 row
`0xE4`	Σ	sigma	18	shifted down 1 row
`0xE6`	µ	mu	18	shifted down 1 row
`0xE8`	Φ	phi	16	shifted down 1 row
`0xE9`	Θ	theta	16	shifted down 1 row
`0xF1`	±	plus-minus	2	reshaped (see below)
`0xF5`	⌡	integral bottom	8	shifted down 1 row
`0xF6`	÷	division sign	4	dots repositioned
`0xFA`	·	middle dot	4	shifted up 1 row
`0xFC`	ⁿ	superscript n	24	shifted right 1 px
`0xFD`	²	superscript 2	26	shifted right 1 px
`0xFE`	■	filled square	7	reshaped (see below)

28 CP437 glyphs in the 80x25 font differ from the IBM VGA ROM #2

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