ZX80 character set

The ZX80 character set is the character encoding used by the Sinclair Research ZX80 microcomputer with its original 4K BASIC ROM. 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 successor ZX81.

The Sinclair ZX80 character set rendered in the system font.

Printable characters
Screenshot of a ZX80 4K BASIC program that demonstrates all code points including BASIC keywords and nonprintable characters, the latter rendered as question marks except CHR$(1) rendered as a null string. Therefore the top line covers the 33 code points 0–32, the following 5 lines cover 32 each, and the last lines with keywords cover yet fewer.

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 or Sinclair BASIC 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 or the at sign.

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 1 is the double-quote (") character when used in the display file, but uniquely to the ZX80 it is used internally as the string terminator character[1] so the BASIC function CHR$(1) returns a null string;[2] CHR$(212) translates to the printable " character.[3]

Changes in the ZX81

The 8K BASIC ROM of the follow-up ZX81 model was also available as an upgrade for the ZX80, replacing its integer-only 4K BASIC ROM.[4] It introduced the modified ZX81 character set which has 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.

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 ZX80 system font uses an 8×8 pixel-per-character grid where most glyphs fit in 7×6 pixels leaving one pixel horizontal space between them. This font was modified in the ZX81's ROM to slightly narrower 6×6 pixel glyphs with two pixels horizontal space between them, which improved the look of single inverted characters by showing inverted pixels on both sides. 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.

Character set
ZX80 character set[1][3][lower-alpha 1]
_0 _1 _2 _3 _4 _5 _6 _7 _8 _9 _A _B _C _D _E _F
0_
0		 SP
U+0020		 "[lower-alpha 2]
U+0022
U+258C
U+2584
U+2598
U+259D
U+2596
U+2597
U+259E		 [lower-alpha 3]
U+2592
U+1FB8F
U+1FB8E		 £
U+00A3		 $
U+0024		 :
U+003A		 ?
U+003F
1_
16		 (
U+0028		 )
U+0029		 -
U+002D		 +
U+002B		 *
U+002A		 /
U+002F		 =
U+003D		 >
U+003E		 <
U+003C		 ;
U+003B		 ,
U+002C		 .
U+002E		 0
U+0030		 1
U+0031		 2
U+0032		 3
U+0033
2_
32		 4
U+0034		 5
U+0035		 6
U+0036		 7
U+0037		 8
U+0038		 9
U+0039		 A
U+0041		 B
U+0042		 C
U+0043		 D
U+0044		 E
U+0045		 F
U+0046		 G
U+0047		 H
U+0048		 I
U+0049		 J
U+004A
3_
48		 K
U+004B		 L
U+004C		 M
U+004D		 N
U+004E		 O
U+004F		 P
U+0050		 Q
U+0051		 R
U+0052		 S
U+0053		 T
U+0054		 U
U+0055		 V
U+0056		 W
U+0057		 X
U+0058		 Y
U+0059		 Z
U+005A
4_
64		                                
5_
80		                                
6_
96		                                
7_
112		 UP
 		 DOWN
 		 LEFT
 		 RIGHT
 		 HOME
 		 EDIT
 		 newline
 		 rubout
 		                
8_
128
U+2588		 "
 
U+2590
U+2580
U+259F
U+2599
U+259C
U+259B
U+259A		 [lower-alpha 3]
U+1FB90
U+1FB91
U+1FB92		 £
 		 $
 		 :
 		 ?
 
9_
144		 (
 		 )
 		 -
 		 +
 		 *
 		 /
 		 =
 		 >
 		 <
 		 ;
 		 ,
 		 .
 		 0
 		 1
 		 2
 		 3
 
A_
160		 4
 		 5
 		 6
 		 7
 		 8
 		 9
 		 A
 		 B
 		 C
 		 D
 		 E
 		 F
 		 G
 		 H
 		 I
 		 J
 
B_
176		 K
 		 L
 		 M
 		 N
 		 O
 		 P
 		 Q
 		 R
 		 S
 		 T
 		 U
 		 V
 		 W
 		 X
 		 Y
 		 Z
 
C_
192		                                
D_
208		         "[lower-alpha 2]
 		 THEN[lower-alpha 4]
 		 TO[lower-alpha 4]
 		 ;
 		 ,
 		 )
 		 (
 		 NOT[lower-alpha 4]
 		 -
 		 +
 		 *
 		 /
 
E_
224		 AND[lower-alpha 4]
 		 OR[lower-alpha 4]
 		 **[lower-alpha 5]
 		 =
 		 >[lower-alpha 1]
 		 <[lower-alpha 1]
 		 LIST[lower-alpha 4]
 		 return[lower-alpha 4]
 		 CLS[lower-alpha 4]
 		 DIM[lower-alpha 4]
 		 SAVE[lower-alpha 4]
 		 FOR[lower-alpha 4]
 		 go to[lower-alpha 4]
 		 POKE[lower-alpha 4]
 		 input[lower-alpha 4]
 		 randomise[lower-alpha 4]
 
F_
240		 LET[lower-alpha 4]
 		     NEXT[lower-alpha 4]
 		 print[lower-alpha 4]
 		   NEW[lower-alpha 4]
 		 RUN[lower-alpha 4]
 		 STOP[lower-alpha 4]
 		 continue[lower-alpha 4]
 		 IF[lower-alpha 4]
 		 go sub[lower-alpha 4]
 		 LOAD[lower-alpha 4]
 		 clear[lower-alpha 4]
 		 REM[lower-alpha 4]
 		  

  Letter   Number   Punctuation   Symbol   Other   Undefined

Notes
  1. The ZX80 Operating Manual has an error and shows the > and < symbols in the wrong order.[5]
  2. Code point 1 is the double-quote (") symbol when used in the video memory (called the display file), but it is used internally as the string terminator character[1] so CHR$(1) returns a null string. Code point 212, CHR$(212), translates to the printable " character.
  3. Because they are 50% gray rasters, code points 9 and 137 have the same appearance although every pixel is inverted.
  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.

References
  1. Wearmouth, Geoff. "An Assembly Listing of the ROM of the Sinclair ZX80". Archived from the original on August 15, 2015.
  2. Davenport, Hugo (1980). A Course in BASIC Programming - Sinclair ZX80 Operating Manual. Science of Cambridge Ltd. pp. 75–77.
  3. Davenport, Hugo (1980). A Course in BASIC Programming - Sinclair ZX80 Operating Manual. Science of Cambridge Ltd. pp. 116–117.
  4. "8K BASIC ROM UPGRADE".
  5. Sinclair ZX80 Character Set

See also
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