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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, CBM-II, Plus/4, C16, C116 and C128. [1]

Contents

History

The character set was largely designed by Leonard Tramiel (the son of Commodore CEO Jack Tramiel) and PET designer Chuck Peddle. 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, essentially a thicker version of the PET's, in order to avoid color artifacts created by the machine's higher resolution screen.

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).[2]

Specifications

C64 startup screen with shifted and unshifted modes of PETSCII, and the two characters from ASCII-1963.
PETSCII Chart as displayed on the C64 in shifted and unshifted modes. (Not shown are control codes, as well as characters in the $C0-$FF range, which are the standard uppercase keycodes returned from the keyboard, and which are mirrored to the range $60-7F)

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). Assuming the graphics mode is unshifted, PETSCII has only uppercase letters in its powerup state, an up-arrow ( ↑ ) instead of a caret ( ^ ) in position $5E and a left-arrow ( ← ) instead of an underscore ( _ ) in position $5F. Also, the backslash ( \ ) in position $5C is occupied by a British pound sign ( £ ). 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 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 text mode, in which lowercase letters occupy the range $41–$5A, and uppercase letters occupy the range $C1–$DA. The text mode is not available at powerup, but must be actuated by pressing the SHIFT and CMDR keys simultaneously. Regardless of whether the chip has undergone a graphic "shift" (by holding the SHIFT key down and pressing the CMDR key simultaneously), there are block graphic characters in the range of $E0-FF. This serves to distinguish PETSCII from those kinds of ASCII that go back no farther than ASCII-1967, so any text transfer between an 8-bit Commodore machine and one that uses 1967-derived ASCII would result in text where uppercase letters appear to be lowercase, and lowercase letters uppercase. There is no easy Boolean operation to change these cases to the proper case. 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; this means that the other characters added in ASCII-1967 besides lowercase letters — i.e. the grave accent, curly braces, vertical bar, and tilde — do not exist in PETSCII.

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, like the above examples. Such names were commonly enclosed in curly braces in the listings. This prevented ambiguity, since, as mentioned, PETSCII had no curly brace characters. 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 by POKEing location 59468 with the value 14 to select the alternative set or 12 to revert to standard. 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.[1] 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.)

Code table

Since not all of the characters encoded by PETSCII are 'graphic' (i.e., not control codes) and not all of them have a corresponding Unicode representation, they cannot be portably displayed in a web browser. The following table shows the glyphs for PETSCII graphic characters where there is a corresponding Unicode glyph, and the Unicode replacement character U+FFFD (�) otherwise. Control characters and other non-printing characters are represented by abbreviations for their names. Where a particular code point encodes both a shifted and unshifted character, both characters are shown, with the unshifted character on the left. Row and column headings indicate the hexadecimal digit combinations to produce the eight-bit code value; e.g., the letter L is at code value 4C.

Note that the table below is for the Commodore 64. Other Commodore machines used slightly different versions of PETSCII, which used different control characters and in some cases different graphic characters. For example, on the Commodore 128 $07 was the bell control character, and on CBM machines prior to the VIC-20, characters $2C and $6C both produced a comma character, albeit with slightly different semantics.[2]

The actual character generator ROM used a different set of assignments. For example, to display the characters "@ABC" on screen by directly POKEing the screen memory, one would POKE the decimal values 0, 1, 2, and 3 rather than 64, 65, 66, and 67.

PETSCII (Commodore 64)
x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 xA xB xC xD xE xF
0x unused WHT unused SHIFT DISABLE SHIFT ENABLE unused CR Text mode unused
1x unused DOWN RVS ON HOME DEL unused RED RIGHT GRN BLU
2x SP ! " # $ % & ' ( ) * + , - . /
3x 0 1 2 3 4 5 6 7 8 9 : ; < = > ?
4x @ A a B b C c D d E e F f G g H h I i J j K k L l M m N n O o
5x P p Q q R r S s T t U u V v W w X x Y y Z z [ £ ]
6x ♠ A │ B ━ C � D � E � F � G � H ╮ I ╰ J ╯ K � L ╲ M ╱ N � O
7x � P ● Q � R ♥ S � T ╭ U ╳ V ○ W ♣ X � Y ♦ Z π ▒ ◥ �
8x unused ORG unused F1 F3 F5 F7 F2 F4 F6 F8 LF Graphics unused
9x BLK UP RVS OFF CLR INS BRN LT RED GRAY1 GRAY2 LT GRN LT BLU GRAY3 PUR LEFT YEL CYN
Ax SHFT_SP ◤ �
Bx � ✓
Cx ♠ A │ B ━ C � D � E � F � G � H ╮ I ╰ J ╯ K � L ╲ M ╱ N � O
Dx � P ● Q � R ♥ S � T ╭ U ╳ V ○ W ♣ X � Y ♦ Z π ▒ ◥ �
Ex CMDR_SP ◤ �
Fx � ✓ π ▒


PET 2001 keyboard layout, illustrating PETSCII graphics characters.


See also

Notes

  1. ^  The Amiga home/personal computer family uses standard ISO-8859-1.
  2. ^  see On The Edge by Brian Bagnall, ISBN 0-9738649-0-7, page 43, 54-55.

References

  1. ^ THE COMMODORE PET COMPUTER / FREQUENTLY ASKED QUESTIONS FILE - VERSION 1.7 ( Updated 11/25/2000) BY LARRY ANDERSSON, COMMODORE COLLECTOR AND PET ENTHUSIAST
  2. ^ Commodore Trivia Edition #26 Answers for February 1996

External links

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