Template:Short description Template:Lead too short Template:Use dmy dates Template:Infobox character encoding
PETSCII (PET Standard Code of Information Interchange), also known as CBM ASCII, is the character set used in Commodore Business Machines' 8-bit home computers.
This character set was first used by the PET from 1977, and was subsequently used by the CBM-II, VIC-20, Commodore 64, Commodore 16, Commodore 116, Plus/4, and Commodore 128. However, the Amiga personal computer family instead uses standard ISO/IEC 8859-1.
HistoryEdit
The character set was largely designed by Leonard Tramiel (the son of Commodore CEO Jack Tramiel) and PET designer Chuck Peddle.<ref>Template:Cite journal</ref><ref name="On the Edge">Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The graphic characters of PETSCII were one of the extensions Commodore specified for Commodore BASIC when laying out desired changes to Microsoft's existing 6502 BASIC to Microsoft's Ric Weiland in 1977.<ref name="peddlevid">{{#invoke:citation/CS1|citation |CitationClass=web }} (mirror)</ref> The VIC-20 used the same pixel-for-pixel font as the PET, although the characters appeared wider due to the VIC's 22-column screen. The Commodore 64, however, used a slightly re-designed, heavy upper-case font, essentially a thicker version of the PET's, in order to avoid color artifacts created by the machine's higher resolution screen. The C64's lowercase characters are identical to the lowercase characters in the Atari 8-bit computers font (released 2.75 years earlier).
Peddle claims the inclusion of card suit symbols was spurred by the demand that it should be easy to write card games on the PET (as part of the specification list he received).<ref name="On the Edge"/>
SpecificationsEdit
"Unshifted" PETSCII is based on the 1963 version of ASCII (rather than the 1967 version, which most if not all other computer character sets based on ASCII use). It has only uppercase letters, an up-arrow Template:Angbr instead of caret Template:Angbr at 0x5E and a left-arrow Template:Angbr instead of an underscore Template:Angbr at 0x5F. In all versions except the original Commodore PET, it also has a British pound sign Template:Angbr instead of the backslash Template:Angbr at 0x5C. Other characters added in ASCII-1967 (lowercase letters, the grave accent, curly braces, vertical bar, and tilde) do not exist in PETSCII. Codes 0xA0–0xDF are allotted to CBM-specific block graphics characters—horizontal and vertical lines, hatches, shades, triangles, circles and card suits.
PETSCII also has a "shifted" mode (also called "business mode"), which changes the uppercase letters at 0x41–0x5A to lowercase, and changes the graphics at 0xC1–0xDA to uppercase letters. Upper- and lower-case are swapped from where ASCII has them. The mode is toggled by holding one of the SHIFT keys and then pressing and releasing the Commodore key. The shift can be done by POKEing location 59468 with the value 14 to select the alternative set or 12 to revert to standard. On the Commodore 64, the sets are alternated by flipping bit 2 of the byte 53272. On some models of PET, this can also be achieved via special control code PRINT CHR$(14) which adjust the line spacing as well as changing the character set; the POKE method is still available and does not alter the line spacing.<ref name="petfaq">{{#invoke:citation/CS1|citation
|CitationClass=web
}}</ref>
Included in PETSCII are cursor and screen control codes, such as {HOME}, {CLR}, {RVS ON}, and {RVS OFF} (the latter two activating/deactivating reverse-video character display). The control codes appeared in program listings as reverse-video graphic characters, although some computer magazines, in their efforts to provide more clearly readable listings, pretty-printed the codes using their actual names in curly braces, like the above examples. This is unambiguous as PETSCII has no curly brace characters.
Different mappings are used for storing characters (the "interchange" mapping, as used by CHR$()) and displaying characters (the "video" mapping). For example, to display the characters "@ABC" on screen by directly writing into the screen memory, one would POKE the decimal values 0, 1, 2, and 3 rather than 64, 65, 66, and 67.<ref name="Aivosto"/><ref name="L2/19-025 ReadMe.txt"/>
The keyboard by default provides access to the lower half of the code page. Pressing Shift and a key gives the corresponding upper half code point. Some PETSCII code points cannot be printed and are only used for keyboard input (e.g. F1, RUN/STOP).
Character setEdit
Template:Contains special characters
The tables below represent the "interchange" PETSCII encoding, as used by CHR$().
Control characters are defined in the ranges 0x00–0x1F and 0x80–0x9F, although which control characters are defined and what they are defined as varies between systems. The tables below exclude control characters—the encoding of control characters in discussed in § Control characters.
The ranges 0x60–0x7F and 0xE0–0xFF are duplicate ranges, although what they duplicate varies between systems. On the Commodore PET, they duplicate 0x20–0x3F and 0xA0–0xBF, respectively; on the Commodore VIC-20, 64, 16, and 128 they duplicate 0xC0–0xDF and 0xA0–0xBF, respectively.<ref name="Aivosto"/> While these characters are visually duplicates, they are semantically different; for example, on the Commodore PET, code points 0x2C and 0x6C both produce a comma character, but only 0x2C functions as a delimiter between input fields.<ref>Template:Cite mailing list</ref>
Graphic characters are mostly identical across systems, with the exceptions of 0x5C (which is \ on the Commodore PET, and £ on other systems), 0xDE (which is U+1FB95 CHECKER BOARD FILL on the Commodore PET and VIC-20, and U+1FB96 INVERSE CHECKER BOARD FILL on other systems), and the range 0x60–0x7F (which duplicates a different range on Commodore PET). Additionally, in Commodore PET 2001's shifted character set, uppercase and lowercase letters are swapped relative to other systems'.
Unicode equivalentsEdit
PETSCII characters are represented in the Unicode standard in various blocks:
- Basic Latin (U+0000 - U+007F)
- Latin-1 Supplement (U+0080 - U+00FF)
- Greek and Coptic (Template:Unichar)
- General Punctuation (Template:Unichar)
- Arrows (two arrows, U+2190 and U+2191)
- Box Drawing (U+2500 - U+257F)
- Block Elements (U+2580 - U+259F)
- Geometric Shapes (U+25A0 - U+25FF)
- Miscellaneous Symbols (U+2600 - U+26FF)
- Dingbats (Template:Unichar)
- Symbols for Legacy Computing (U+1FB00 - U+1FBFF, first added in 2020 as part of Unicode 13.0)<ref name="L2/19-025">Template:Citation</ref>
StandardEdit
The following tables represent the PETSCII encoding used on the Commodore VIC-20, 64, 16, and 128.
UnshiftedEdit
ShiftedEdit
Commodore PETEdit
UnshiftedEdit
ShiftedEdit
Control charactersEdit
While the graphic characters were mostly shared between Commodore systems, the control characters frequently varied. The follow table describes what the control characters represent on the Commodore PET 2001, Commodore PET 8032, VIC-20, Commodore 64, Commodore 16, Commodore 128 (40 and 80 column modes).
| Hex | Decimal | PET 2001 | PET 8032 | VIC-20 | C64 | C16 | C128 (40 col) | C128 (80 col) |
|---|---|---|---|---|---|---|---|---|
| 00 | 0 | colspan=7 Template:N/a | ||||||
| 01 | 1 | colspan=7 Template:N/a | ||||||
| 02 | 2 | colspan=6 Template:N/a | UNDERLINE ON | |||||
| 03 | 3 | STOP | ||||||
| 04 | 4 | colspan=7 Template:N/a | ||||||
| 05 | 5 | colspan=2 Template:N/a | WHITE | |||||
| 06 | 6 | colspan=7 Template:N/a | ||||||
| 07 | 7 | Template:N/a | BELL | colspan=3 Template:N/a | BELL | |||
| 08 | 8 | colspan=2 Template:N/a | LOCK CASE | colspan=2 Template:N/a | ||||
| 09 | 9 | Template:N/a | TAB | UNLOCK CASE | TAB | |||
| 0A | 10 | colspan=5 Template:N/a | LINE FEED | |||||
| 0B | 11 | colspan=5 Template:N/a | UNLOCK CASE | |||||
| 0C | 12 | colspan=5 Template:N/a | LOCK CASE | |||||
| 0D | 13 | RETURN | ||||||
| 0E | 14 | Template:N/a | LOWER CASE | |||||
| 0F | 15 | Template:N/a | SET WINDOW TOP | colspan=4 Template:N/a | FLASH ON | |||
| 10 | 16 | colspan=7 Template:N/a | ||||||
| 11 | 17 | CURSOR DOWN | ||||||
| 12 | 18 | REVERSE ON | ||||||
| 13 | 19 | HOME | ||||||
| 14 | 20 | DEL | ||||||
| 15 | 21 | Template:N/a | KILL LINE | colspan=5 Template:N/a | ||||
| 16 | 22 | Template:N/a | ERASE TO RIGHT | colspan=5 Template:N/a | ||||
| 17 | 23 | colspan=7 Template:N/a | ||||||
| 18 | 24 | colspan=3 Template:N/a | TAB SET/CLEAR | |||||
| 19 | 25 | Template:N/a | SCROLL UP | colspan=5 Template:N/a | ||||
| 1A | 26 | colspan=7 Template:N/a | ||||||
| 1B | 27 | Template:N/a | ESC | colspan=2 Template:N/a | ESC | |||
| 1C | 28 | colspan=2 Template:N/a | RED | |||||
| 1D | 29 | CURSOR RIGHT | ||||||
| 1E | 30 | colspan=2 Template:N/a | GREEN | |||||
| 1F | 31 | colspan=2 Template:N/a | BLUE | |||||
| 80 | 128 | colspan=7 Template:N/a | ||||||
| 81 | 129 | colspan=3 Template:N/a | ORANGE | DARK PURPLE | ||||
| 82 | 130 | colspan=3 Template:N/a | FLASH ON | Template:N/a | UNDERLINE OFF | |||
| 83 | 131 | colspan=2 Template:N/a | RUN | |||||
| 84 | 132 | colspan=3 Template:N/a | FLASH OFF | colspan=2 Template:N/a | ||||
| 85 | 133 | colspan=2 Template:N/a | F1 | |||||
| 86 | 134 | colspan=2 Template:N/a | F3 | |||||
| 87 | 135 | Template:N/a | DOUBLE BELL | F5 | ||||
| 88 | 136 | colspan=2 Template:N/a | F7 | |||||
| 89 | 137 | Template:N/a | TAB SET/CLEAR | F2 | ||||
| 8A | 138 | colspan=2 Template:N/a | F4 | |||||
| 8B | 139 | colspan=2 Template:N/a | F6 | |||||
| 8C | 140 | colspan=2 Template:N/a | F8 | HELP | F8 | |||
| 8D | 141 | SHIFT + RETURN | ||||||
| 8E | 142 | Template:N/a | UPPER CASE | |||||
| 8F | 143 | Template:N/a | SET WINDOW END | colspan=4 Template:N/a | FLASH OFF | |||
| 90 | 144 | colspan=2 Template:N/a | BLACK | |||||
| 91 | 145 | CURSOR UP | ||||||
| 92 | 146 | REVERSE OFF | ||||||
| 93 | 147 | CLEAR | ||||||
| 94 | 148 | INST | ||||||
| 95 | 149 | Template:N/a | INSERT LINE ABOVE | Template:N/a | BROWN | DARK YELLOW | ||
| 96 | 150 | Template:N/a | ERASE TO LEFT | Template:N/a | PINK | YELLOW-GREEN | PINK | |
| 97 | 151 | colspan=3 Template:N/a | DARK GRAY | PINK | DARK GRAY | DARK CYAN | ||
| 98 | 152 | colspan=3 Template:N/a | MEDIUM GRAY | BLUE-GREEN | MEDIUM GRAY | |||
| 99 | 153 | Template:N/a | SCROLL DOWN | Template:N/a | LIGHT GREEN | LIGHT BLUE | LIGHT GREEN | |
| 9A | 154 | colspan=3 Template:N/a | LIGHT BLUE | DARK BLUE | LIGHT BLUE | |||
| 9B | 155 | colspan=3 Template:N/a | LIGHT GRAY | LIGHT GREEN | LIGHT GRAY | |||
| 9C | 156 | colspan=2 Template:N/a | PURPLE | |||||
| 9D | 157 | CURSOR LEFT | ||||||
| 9E | 158 | colspan=2 Template:N/a | YELLOW | |||||
| 9F | 159 | colspan=2 Template:N/a | CYAN | |||||
The colors of the VIC-20 and C64/128 are listed in the VIC-II article.
Base 128Edit
{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= Template:Ambox }} Out of PETSCII's first 192 codes, there are 128 graphic characters: 32–127 and 160–192. This permits "base128"-style encodings in DATA statements, or perhaps between PETSCII-speaking machines. This can also include control characters, which are visible when quoted, although which control characters are defined varies between systems.
The primary application for a "Base 128" encoding is in DATA statements in Commodore BASIC. Binary data can be stored with relatively low overhead, allowing one character of data to encode seven bits of data. On a standard 80-character line, typically four characters are used for the line number, and two characters for the abbreviated DATA statement (dA). Since the comma and colon are significant to BASIC, a quote character is also needed, leaving 73 characters for data. At seven bits per character, one DATA line could store 511 bits of binary data, for 79% efficiency. If three-digit line numbers are used, efficiency increases to 80%. If two-digit line numbers are used, efficiency is 82%.
| Line Numbers | Data chars per Line | Bits per Line | Efficiency | Max. Lines | Max. Total Data Bytes |
|---|---|---|---|---|---|
| 1-9<ref group="lower-alpha" name="GOTO"/> | 76 | 532 | 0.83125 | 9 | 598 |
| 10-99 | 75 | 525 | 0.820312 | 90 | 5,906 |
| 100-999 | 74 | 518 | 0.809375 | 900 | 58,275 |
| 1000-9999 | 73 | 511 | 0.7984375 | 9,000 | 574,875 |
| 10000-65535<ref group="lower-alpha" name="off-by-one"/> | 72 | 504 | 0.7875 | 55,536<ref group="lower-alpha" name="off-by-one"/> | 3.5 MB (approx.) |
<references group="lower-alpha"> <ref name="GOTO">Assume line 0 is a GOTO.</ref> <ref name="off-by-one">Maximum line number is probably off-by-one.</ref> </references>
For storing binary data in Commodore BASIC, it appears that two- or three-digit line numbers are typically the best choice.
Base 164Edit
164 PETSCII characters are representable in quoted strings; theoretically, then, Base 164 is possible. This adds in the color values, the function keys, and cursor controls.
See alsoEdit
ReferencesEdit
External linksEdit
- PETSCII to Unicode mapping and a TrueType font using that mapping
- Typography in 8 bits: System fonts
- Online PETSCII-art editor
- PETSCII-art