PETSCII

PETSCII (PET Standard Code of Information Interchange), also known as CBM ASCII, is the 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.

History

The character set was largely designed by Leonard Tramiel (the son of Commodore CEO Jack Tramiel) and PET designer Chuck Peddle. 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.^[1] 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 800's system font (released several 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).

Specifications

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, in the VIC-20 and C64 version, 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 holding one of the SHIFT keys and then press and release the Commodore key. Regardless of whether the chip has undergone this graphic "shift", 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, as with 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 C64 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.^[2] 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 VIC-II article.)

Codepage layout

Since not all of the characters encoded by PETSCII are 'graphic' (i.e., 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.^[3]

The actual character generator ROM used a different set of assignments. 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.

PETSCII (Commodore 64)

	_0	_1	_2	_3	_4	_5	_6	_7	_8	_9	_A	_B	_C	_D	_E	_F
0_	0	1	2	RUN/ STOP 3	4	WHT 5	6	7	SHIFT DISABLE 8	SHIFT ENABLE 9	10	11	12	CR 000D 13	TEXT MODE 14	15
1_	16	DOWN 17	RVS ON 18	HOME 19	DEL 007F 20	21	22	23	24	25	26	27	RED 28	RIGHT 29	GRN 30	BLU 31
2_	SP 0020 32	! 0021 33	" 0022 34	# 0023 35	$ 0024 36	% 0025 37	& 0026 38	' 0027 39	( 0028 40	) 0029 41	* 002A 42	+ 002B 43	, 002C 44	- 002D 45	. 002E 46	/ 002F 47
3_	0 0030 48	1 0031 49	2 0032 50	3 0033 51	4 0034 52	5 0035 53	6 0036 54	7 0037 55	8 0038 56	9 0039 57	: 003A 58	; 003B 59	< 003C 60	= 003D 61	> 003E 62	? 003F 63
4_	@ 0040 64	A a 0041 65	B b 0042 66	C c 0043 67	D d 0044 68	E e 0045 69	F f 0046 70	G g 0047 71	H h 0048 72	I i 0049 73	J j 004A 74	K k 004B 75	L l 004C 76	M m 004D 77	N n 004E 78	O o 004F 79
5_	P p 0050 80	Q q 0051 81	R r 0052 82	S s 0053 83	T t 0054 84	U u 0055 85	V v 0056 86	W w 0057 87	X x 0058 88	Y y 0059 89	Z z 005A 90	[ 005B 91	£ 005C 92	] 005D 93	↑ 005E 94	← 005F 95
6_	━ 0060 96	♠ A 0061 97	│ B 0062 98	━ C 0063 99	� D 0064 100	� E 0065 101	� F 0066 102	� G 0067 103	� H 0068 104	╮ I 0069 105	╰ J 006A 106	╯ K 006B 107	� L 006C 108	╲ M 006D 109	╱ N 006E 110	� O 006F 111
7_	� P 0070 112	● Q 0071 113	� R 0072 114	♥ S 0073 115	� T 0074 116	╭ U 0075 117	╳ V 0076 118	○ W 0077 119	♣ X 0078 120	� Y 0079 121	♦ Z 007A 122	┼ 007B 123	� 007C 124	│ 007D 125	π ▒ 007E 126	◥ � 007E 127
8_	128	ORG 129	130	131	132	F1 133	F3 134	F5 135	F7 136	F2 137	F4 138	F6 139	F8 140	LF 000A 141	GRAPHICS 142	143
9_	BLK 144	UP 145	RVS OFF 146	CLR 147	INS 148	BRN 149	LT RED 150	GRAY1 151	GRAY2 152	LT GRN 153	LT BLU 154	GRAY3 155	PUR 156	LEFT 157	YEL 158	CYN 159
A_	SHIFT SP 160	▌ 161	▄ 162	▔ 163	▁ 164	▏ 165	▒ 166	▕ 167	� 168	◤ � 169	� 170	├ 171	� 172	└ 173	┐ 174	▂ 175
B_	┌ 176	┴ 177	┬ 178	┤ 179	▎ 180	▍ 181	� 182	� 183	� 184	▃ 185	� ✓ 186	� 187	� 188	┘ 189	� 190	� 191
C_	━ 0060 192	♠ A 0061 193	│ B 0062 194	━ C 0063 195	� D 0064 196	� E 0065 197	� F 0066 198	� G 0067 199	� H 0068 200	╮ I 0069 201	╰ J 006A 202	╯ K 006B 203	� L 006C 204	╲ M 006D 205	╱ N 006E 206	� O 006F 207
D_	� P 0070 208	● Q 0071 209	� R 0072 210	♥ S 0073 211	� T 0074 212	╭ U 0075 213	╳ V 0076 214	○ W 0077 215	♣ X 0078 216	� Y 0079 217	♦ Z 007A 218	┼ 007B 219	� 007C 220	│ 007D 221	π ▒ 007E 222	◥ � 007E 223
E_	CMDR SP 224	▌ 225	▄ 226	▔ 227	▁ 228	▏ 229	▒ 230	▕ 231	� 232	◤ � 233	� 234	├ 235	� 236	└ 237	┐ 238	▂ 239
F_	┌ 240	┴ 241	┬ 242	┤ 243	▎ 244	▍ 245	� 246	� 247	� 248	▃ 249	� � 250	� 251	� 252	┘ 253	� 254	π ▒ 255

Some PETSCII Codes cannot be printed and are only used for Keyboard input (e.g. F1, RUN/STOP).

PET 2001 keyboard layout, illustrating PETSCII graphics characters

base128

Out of PETSCII's first 192 codes, 164 have visible representations (for the control codes, these are visible when quoted): 5 (white), 17-20 and 28-31 (colors and cursor controls), 32-90 (ASCII equivalent), 91-127 (graphics), 129 (orange), 133-140 (function keys), 144-159 (colors and cursor controls), and 160-192 (graphics). This theoretically permits encodings, such as base128, between PETSCII-speaking machines; for example, 33-127, 160-192.

See also

• ATASCII
• Atari ST character set
• ZX Spectrum character set
• Extended ASCII
• Text semigraphics

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. A Conversation with Chuck Peddle, Bil Herd, Jeri Ellsworth - part 3 (2009 videoconference, 06:30)
2. THE COMMODORE PET COMPUTER / FREQUENTLY ASKED QUESTIONS FILE - VERSION 1.7 ( Updated 25 November 2000) BY LARRY ANDERSSON, COMMODORE COLLECTOR AND PET ENTHUSIAST
3. Commodore Trivia Edition #26 Answers for February 1996

External links

• PETSCII character map, part 1, part 2, part 3 (JPEG)
• An attempt at PETSCII to Unicode mapping, unshifted, shifted
• Commodore 128 PETSCII control characters
• Typography in 8 bits: System fonts
• Online PETSCII-art editor