PETSCII (PET Standard Code of Information Interchange), also known as CBM ASCII, is the variation of the ASCII character set used in Commodore Business Machines (CBM)'s 8-bit home computers, starting with the PET from 1977 and including the VIC-20, C64, Plus/4, C16 and C128.
The character set was largely designed by Leonard Tramiel (the son of Commodore CEO Jack Tramiel) and PET designer Chuck Peddle. 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). The VIC-20 used the same pixel-for-pixel font as the PET (although the characters appeared wider due to the 22-column screen). The Commodore 64, however, used a slightly re-designed, heavy upper-case font that was different from the font on the PET. This font was referred to as the "Atari font". It has been suggested that Commodore simply copied and modified this font from the Atari ROM images produced by the Commodore Semiconductor Group i.e. MOS Technology. (See the article on the Atari 8-bit family, and compare the screenshots to the ones on this page.)





PETSCII is based on the 1963 version of ASCII (rather than the 1967 version, which most if not all other character sets based on ASCII use). As such, PETSCII has only uppercase letters (in its unshifted mode, that is; see below), an up-arrow (↑) instead of a caret (^) in position $5E and a left-arrow (←) instead of an underscore ( _ ) in position $5F. In unshifted mode, codes $60–$7F and $A0–$FF are allotted to CBM-specific block graphics characters (horizontal and vertical lines, hatches, shades, triangles, circles and card suits). Ranges $00–$1F and $80–$9F have control codes. The characters in positions $60–$7F repeat themselves in positions $C0–$DF, as do $A0–$BE in $E0–$FE, and $7E in $FF.

The Commodore PET's lack of a programmable bitmap-mode for computer graphics as well as it having no redefinable character set capability, may be one of the reasons PETSCII was developed; by creatively using the well thought-out block graphics, a higher degree of sophistication in screen graphics is attainable than by using plain ASCII's letter/digit/punctuation characters. In addition to the relatively diverse set of geometrical shapes that can thus be produced, PETSCII allows for several grayscale levels by its provision of differently hatched checkerboard squares/half-squares. Finally, the reverse-video mode (see below) is used to complete the range of graphics characters, in that it provides mirrored half-square blocks.

PETSCII also has a shifted mode, in which the range $41–$5A has lowercase letters (instead of uppercase in unshifted mode), and the range $61–$7A (and its duplicate $C1–$DA) has uppercase letters (instead of the block graphics). This is in reverse to ASCII-1967, so any text transfer between an 8-bit Commodore machine and one that uses standard ASCII would result in reverse-case text upon arrival to the destination. Thus, like for other computers based on non-standard-ASCII character sets, software conversion is needed when exchanging text files and/or telecommunicating with standard ASCII systems. The other ranges are unchanged in shifted mode.

Included in PETSCII are cursor and screen control codes, such as , , , and (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, like the above examples. The screen control codes were essentially similar to escape codes for text based computer terminals.

As indicated above, PETSCII provides for shifting between the power-on default (unshifted) uppercase+graphics character set and the alternative (shifted) lower+uppercase set (where the shifted set contains a subset of the block graphic characters of the unshifted set). The shift between modes is done via special control codes. Thus, screen editor state changes, rather than the employment of separate ASCII codes, are used to choose between single-case (all capitals) and dual case. In the VIC-20, C64, and later machines (not including the CBM business computers), color codes supplement the other screen control codes. (The colors of the VIC-20 and C64/128 are listed in the C64 article.)