ZX81 character set

The ZX81 character set is the character encoding used by the Sinclair Research ZX81 family of microcomputers including the Timex Sinclair 1000 and Timex Sinclair 1500. The encoding uses one byte per character for 256 code points. It has no relationship with previously established ones like ASCII or EBCDIC, but it is related though not identical to the character set of the predecessor ZX80.

Printable characters

The character set has 64 unique glyphs present at code points 0–63. With the most significant bit set the character is generated in inverse video; corresponding to code points 128–191. These 128 values are the only displayable ones allowed in the video memory (known as the display file). The remaining code points (64–127 and 192–255) are used as control characters such as 118 for newline, or uniquely to Sinclair BASIC for keywords, while some are unused.

The small effective range of only 64 unique glyphs precludes support for Latin lower case letters, and many symbols used widely in computing such as the exclamation point and the at sign. The lack of an apostrophe led some software authors to use a comma instead.

There are 11 block graphics characters, counting code point 0 which also doubles as space. Together with the 11 inverse video versions these 22 code points provide every combination of the character cell divided into 2×2 black-and-white block pixels for low-resolution 64×48 pixel graphics, or into 1×2 black, white or dithered gray wide block pixels for a 32×48 resolution. The 2×2 versions of these are also present in the Block Elements Unicode block.

Code point 11 is the double-quote (") symbol when used in the display file. The BASIC function CHR$ 192 prints as the same character but is shown as "" in BASIC source listings; it is used for including the literal " character in a string without conflict with the " string delimiter.^[1]

Changes from the ZX80

The character set in the ZX81 was derived from the ZX80 character set. They have mostly the same code points, e.g. for A-Z and 0-9, but the code points are different for the block graphics characters, the symbols ", -, +, *, /, =, >, <, and the BASIC keyword tokens (with many new added). There are also changes to the control characters and code point 1 is no longer an unprintable string terminator. The ZX81 8K BASIC ROM was also available as an upgrade for the ZX80, replacing its integer-only 4K BASIC ROM.^[2]

In the later Sinclair ZX Spectrum the entire character encoding was replaced with the ZX Spectrum character set, which is a derivative of ASCII and includes lower case letters and more.

System font

The ZX81 system font uses an 8×8 pixel-per-character grid where most glyphs fit in 6×6 pixels leaving two pixels horizontal and vertical space between rows and columns. This font was modified from the one in the ZX80's ROM which had slightly wider 7×6 pixel glyphs with only one pixel horizontal space between them. Some glyphs also received a different design in the ZX81 system font, noticeable on the *, the slashed and less rounded 0, and the less rounded $, C, G and J.

The ZX Spectrum uses the same font as the ZX81 but adds many characters including the lowercase Latin alphabet.

Layout

Legend:

Alphabetic   Control character   Numeric digit   Punctuation

Extended punctuation   Graphic character   International   Undefined

ZX81 character set^[1][3]

	_0	_1	_2	_3	_4	_5	_6	_7	_8	_9	_A	_B	_C	_D	_E	_F
0_	SP U+0020 0	U+2598 1	U+259D 2	U+2580 3	U+2596 4	U+258C 5	U+259E 6	U+259B 7	U+2592 8[lower-alpha 1]	9[lower-alpha 2]	10[lower-alpha 2]	" U+0022 11[lower-alpha 3]	£ U+00A3 12	$ U+0024 13	: U+003A 14	? U+003F 15
1_	( U+0028 16	) U+0029 17	> U+003E 18	< U+003C 19	= U+003D 20	+ U+002B 21	- U+002D 22	* U+002A 23	/ U+002F 24	; U+003B 25	, U+002C 26	. U+002E 27	0 U+0030 28	1 U+0031 29	2 U+0032 30	3 U+0033 31
2_	4 U+0034 32	5 U+0035 33	6 U+0036 34	7 U+0037 35	8 U+0038 36	9 U+0039 37	A U+0041 38	B U+0042 39	C U+0043 40	D U+0044 41	E U+0045 42	F U+0046 43	G U+0047 44	H U+0048 45	I U+0049 46	J U+004A 47
3_	K U+004B 48	L U+004C 49	M U+004D 50	N U+004E 51	O U+004F 52	P U+0050 53	Q U+0051 54	R U+0052 55	S U+0053 56	T U+0054 57	U U+0055 58	V U+0056 59	W U+0057 60	X U+0058 61	Y U+0059 62	Z U+005A 63
4_	RND 64[lower-alpha 4]	INKEY$ 65[lower-alpha 4]	PI 66[lower-alpha 4]
5_
6_
7_	UP 112	DOWN 113	LEFT 114	RIGHT 115	GRAPH ICS 116	EDIT 117	NEW LINE 118	RUB OUT 119	<span style="background: black; color: white; padding: 0 2px">K/<span style="background: black; color: white; padding: 0 2px">L mode 120	FUNC TION 121					number 126	cursor 127
8_	U+2588 128	U+259F 129	U+2599 130	U+2584 131	U+259C 132	U+2590 133	U+259A 134	U+2597 135	U+2592 136[lower-alpha 1]	137[lower-alpha 2]	138[lower-alpha 2]	" 139	£ 140	$ 141	: 142	? 143
9_	( 144	) 145	> 146	< 147	= 148	+ 149	- 150	* 151	/ 152	; 153	, 154	. 155	0 156	1 157	2 158	3 159
A_	4 160	5 161	6 162	7 163	8 164	9 165	A 166	B 167	C 168	D 169	E 170	F 171	G 172	H 173	I 174	J 175
B_	K 176	L 177	M 178	N 179	O 180	P 181	Q 182	R 183	S 184	T 185	U 186	V 187	W 188	X 189	Y 190	Z 191
C_	"" 192[lower-alpha 3]	AT 193[lower-alpha 4]	TAB 194[lower-alpha 4]		CODE 196[lower-alpha 4]	VAL 197[lower-alpha 4]	LEN 198[lower-alpha 4]	SIN 199[lower-alpha 4]	COS 200[lower-alpha 4]	TAN 201[lower-alpha 4]	ASN 202[lower-alpha 4]	ACS 203[lower-alpha 4]	ATN 204[lower-alpha 4]	LN 205[lower-alpha 4]	EXP 206[lower-alpha 4]	INT 207[lower-alpha 4]
D_	SQR 208[lower-alpha 4]	SGN 209[lower-alpha 4]	ABS 210[lower-alpha 4]	PEEK 211[lower-alpha 4]	USR 212[lower-alpha 4]	STR$ 213[lower-alpha 4]	CHR$ 214[lower-alpha 4]	NOT 215[lower-alpha 4]	** 216[lower-alpha 5]	OR 217[lower-alpha 4]	AND 218[lower-alpha 4]	<= 219[lower-alpha 6]	>= 220[lower-alpha 7]	<> 221[lower-alpha 8]	THEN 222[lower-alpha 4]	TO 223[lower-alpha 4]
E_	STEP 224[lower-alpha 4]	LPRINT 225[lower-alpha 4]	LLIST 226[lower-alpha 4]	STOP 227[lower-alpha 4]	SLOW 228[lower-alpha 4]	FAST 229[lower-alpha 4]	NEW 230[lower-alpha 4]	SCROLL 231[lower-alpha 4]	CONT 232[lower-alpha 4]	DIM 233[lower-alpha 4]	REM 234[lower-alpha 4]	FOR 235[lower-alpha 4]	GOTO 236[lower-alpha 4]	GOSUB 237[lower-alpha 4]	INPUT 238[lower-alpha 4]	LOAD 239[lower-alpha 4]
F_	LIST 240[lower-alpha 4]	LET 241[lower-alpha 4]	PAUSE 242[lower-alpha 4]	NEXT 243[lower-alpha 4]	POKE 244[lower-alpha 4]	PRINT 245[lower-alpha 4]	PLOT 246[lower-alpha 4]	RUN 247[lower-alpha 4]	SAVE 248[lower-alpha 4]	RAND 249[lower-alpha 4]	IF 250[lower-alpha 4]	CLS 251[lower-alpha 4]	UNPLOT 252[lower-alpha 4]	CLEAR 253[lower-alpha 4]	RETURN 254[lower-alpha 4]	COPY 255[lower-alpha 4]
	_0	_1	_2	_3	_4	_5	_6	_7	_8	_9	_A	_B	_C	_D	_E	_F

See also

• ZX80 character set
• ZX Spectrum character set
• ATASCII
• Atari ST character set
• PETSCII
• Extended ASCII

Notes

1. Because they are 50% gray rasters, code points 8 and 136 have the same appearance although every pixel is inverted. Both can be expressed as Unicode character U+2592 ("Medium shade") in the Block Elements Unicode block, but of course if both are converted to the same code point the conversion is non-reversible.
2. Not in the Block Elements Unicode block.
3. Code point 11 is the double-quote (") symbol when used in the video memory (called the display file). Code point 192, CHR$ 192, prints as the same character but shows as "" in BASIC listings and is used for including the literal " character in a string without conflict with the " string delimiter.^[1]
4. Sinclair BASIC tokenizes keywords into single-byte code points.
5. The raise to a power multi-character operator tokenized into a single-byte code point.
6. The greater than or equal to multi-character operator tokenized into a single-byte code point.
7. The less than or equal to multi-character operator tokenized into a single-byte code point.
8. The not equal sign multi-character operator tokenized into a single-byte code point.

References

1. Vickers, Steven (1981). Sinclair ZX81 BASIC Programming. Sinclair Research Ltd.
2. "8K BASIC ROM UPGRADE".
3. Wearmouth, Geoff. "An Assembly Listing of the Operating System of the ZX81 ROM". Archived from the original on August 15, 2015.