PC Screen Font (PSF) is a bitmap font format currently employed by the Linux kernel for console fonts. Documentation of the PSF file format can be found within the source code of the Linux kbd utilities. [1] The format is described by the University Eindhoven. [2]
Every PSF file begins with a header. There are two header types: PSF1 and PSF2. All multibyte integers are in least significant byte order (little endian).
| Length | Description | Notes |
|---|---|---|
| 2 | Magic bytes | Always 36 04 |
| 1 | PSF Font mode | Various font flags, see font modes |
| 1 | Glyph size | Glyph size in bytes, 8 bit unsigned integer. For psf1, the character size always equals the glyph height |
| Length | Description | Notes |
|---|---|---|
| 4 | Magic bytes | Always 72 b5 4a 86 |
| 4 | Version | 32 bit unsigned integer, currently always 0 |
| 4 | Header Size | 32 bit unsigned integer, size of the header in bytes (usually 32) |
| 4 | Flags | 32 bit unsigned integer, see Font flags |
| 4 | Length | 32 bit unsigned integer, number of glyphs |
| 4 | Glyph size | 32 bit unsigned integer, number of bytes per glyph |
| 4 | Height | 32 bit unsigned integer, height of each glyph |
| 4 | Width | 32 bit unsigned integer, width of each glyph |
All psf1 glyphs are 8 pixels wide.
The font mode in a psf1 header is an 8 bit unsigned integer containing various flags about the font
| Value (hex) | Name | Meaning |
|---|---|---|
0x01 | PSF1_MODE512 | If this bit is set, the font face will have 512 glyphs. If it is unset, then the font face will have just 256 glyphs. |
0x02 | PSF1_MODEHASTAB | If this bit is set, the font face will have a unicode table. |
0x04 | PSF1_MODESEQ | Equivalent to PSF1_MODEHASTAB |
The font flags field in a psf2 header is a 32 bit unsigned integer containing various flags about the font. There is currently only one flag.
| Value (hex) | Name | Meaning |
|---|---|---|
| 0x00000001 | PSF2_HAS_UNICODE_TABLE | If this bit is set, the font face will have a unicode table |
The actual glyph data immediately follows the header. Each bit in each glyph represents one pixel in the font: 0 for undrawn, 1 for drawn. Each row of each glyph is padded to a whole number of bytes. For example, a 12x12 font would have 2 bytes per row. The letter 'A' in a 12x12 PSF bitmap may look like this:
padding Font data | +----------+ +--+ 000001100000 0000 000011110000 0000 000110011000 0000 001100001100 0000 011000000110 0000 110000000011 0000 111111111111 0000 111111111111 0000 110000000011 0000 110000000011 0000 110000000011 0000 110000000011 0000
with twelve bits of actual data and padding to fill each row to the nearest byte. Rows are stored left-most column first.
If a PSF file contains a unicode table, then every glyph has an entry in the unicode table, with the first glyph corresponding to the first entry in the table, the second glyph the second entry, and so on. The format of an entry in the unicode table depends on the type of the PSF header.
Entries in the unicode table of a PSF1 file are represented as a series of 16 bit little-endian unsigned integers ending in 0xffff. The first integers in the entry contain a unicode character represented by the corresponding glyph. These unicode characters continue until the integer 0xfffe is encountered. Then, starting from the 0xfffe value, rather than representing individual unicode characters, the numbers represent series of unicode characters that correspond to the glyph, starting a new series with every 0xfffe encountered, and stopping once 0xffff is encountered. For example, the following series of 16 bit little-endian unsigned integers would be interpreted in the following way:
0xdead 0xbeef 0xfffe 0x3141 0x5926 0xfffe 0x1234 0x5678 0xffff
Whenever either U+dead, U+beef, the sequence U+3141 U+5926, or the sequence U+1234 U+5678 is seen, display the glyph corresponding to this unicode table entry.
Entries in the unicode table of a PSF2 file are the same as those in a PSF1 file, except unicode characters are represented in UTF-8, unicode sequences begin with the one byte 0xfe (rather than the two byte value 0xfffe), and unicode entries end with the one byte 0xff (rather than the two byte value 0xffff). Neither the values 0xfe nor 0xff can occur in valid UTF-8 sequences.