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 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.
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 so the BASIC function
CHR$(1) returns a null string;
CHR$(212) translates to the printable " character.
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. 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.
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 $, C, G and J.
- The ZX80 Operating Manual has an error and shows the > and < symbols in the wrong order.
- Code point 1 is the double-quote (") symbol when used in the video memory (called the display file), but it is used internally as the string terminator character so
CHR$(1)returns a null string. Code point 212,
CHR$(212), translates to the printable " character.
- Because they are 50% gray rasters, code points 9 and 137 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.
- Not in the Block Elements Unicode block.
- Sinclair BASIC tokenizes keywords into single-byte code points.
- The raise to a power multi-character operator tokenized into a single-byte code point.
- Wearmouth, Geoff. "An Assembly Listing of the ROM of the Sinclair ZX80". Archived from the original on August 15, 2015.
- Davenport, Hugo (1980). A Course in BASIC Programming - Sinclair ZX80 Operating Manual. Science of Cambridge Ltd. pp. 75–77.
- Davenport, Hugo (1980). A Course in BASIC Programming - Sinclair ZX80 Operating Manual. Science of Cambridge Ltd. pp. 116–117.
- "8K BASIC ROM UPGRADE".
- Sinclair ZX80 Character Set