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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchOn a Commodore 64, PETSCII is the character and control-code system used by BASIC and the KERNAL; screen codes are the values the VIC-II reads to choose glyphs from character memory. A third value, the color code, controls the character’s color. Keeping those three jobs separate explains why a BASIC character value cannot always be written directly to screen memory—and how built-in and custom PETSCII-style graphics appear on screen.
How does the Commodore 64 character set work?
In standard character mode, each screen position has a screen-memory value identifying a glyph, while color memory stores its color separately. The VIC-II uses the screen code to find the corresponding pattern in character memory and displays that pattern in the selected position.
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A standard glyph is an 8-by-8 grid represented by eight bytes, one for each row. As the Commodore 64 Programmer’s Reference Guide puts it, “Each character is formed in an 8 by 8 grid of dots, where each dot may be either on or off.” Within each byte, the bits indicate which dots in that row are on.
The guide describes the character-pattern address calculation as CHARACTER ADDRESS = SCREEN CODE*8 + (CHARACTER SET*2048) + (BANK*16384). This expresses how the screen code, selected character set, and VIC-II memory bank determine where the pattern is found. The screen code chooses a glyph; it is not itself the bitmap or its color.
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What is the difference between PETSCII and screen codes?
PETSCII is the character and control-code scheme used by the C64’s KERNAL. BASIC operations such as PRINT, CHR$(), GET, and INPUT, along with keyboard and file operations, work with PETSCII values. Some values represent printable characters; others are control codes that can change such things as color, cursor position, reverse video, or character-set mode. A PETSCII byte therefore does not always mean “print this glyph.”
Screen codes are different: they are values stored in screen RAM that tell the VIC-II which glyph to display. For example, uppercase A is PETSCII 65, but its screen code is 1 in the uppercase/graphics character set. Spaces, digits, and some punctuation share values across the schemes, which can make them seem interchangeable until code handles letters or graphics.
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| Value type | What it does | Where you encounter it |
|---|---|---|
| PETSCII | Represents characters and control actions. | BASIC and KERNAL character input and output, including PRINT and CHR$(). |
| Screen code | Selects a glyph for a screen position. | Screen RAM read by the VIC-II. |
| Color code | Sets the character’s color independently of glyph selection. | Color memory or relevant video registers. |
Reverse video illustrates the distinction. With BASIC output, a PETSCII control code changes the editor’s reverse-video state. With a direct screen-memory write, the value must instead be the screen code for the reversed glyph in the relevant character set. The two routes produce related visible results but use different representations.
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How do I get PETSCII graphics on a C64?
For ordinary BASIC output, use PETSCII: a PRINT statement can send a character or control code, and CHR$(n) lets you specify a value numerically. For direct placement in screen RAM, use the screen code for the glyph you want in the active character set. Do not assume a PETSCII value is also the correct screen-memory value; uppercase A is 65 versus screen code 1 in uppercase/graphics mode.
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Choose the built-in character set that provides the alphabet and symbols your display needs. The C64 has two main sets:
| Character set | Alphabet | Graphics | How it affects the display |
|---|---|---|---|
| Uppercase/graphics | Uppercase letters | Includes graphic and block symbols useful for text-mode pictures, borders, and interfaces. | Displays uppercase glyphs and its available graphics. |
| Lowercase/uppercase | Lowercase and uppercase letters | Fewer graphic symbols than the uppercase/graphics set. | Displays both letter cases, with a different selection of glyphs. |
The system or video configuration can select the active set, and BASIC control codes are also commonly used to switch modes. Switching changes which glyphs the VIC-II displays for the relevant screen codes; it does not make PETSCII and screen codes the same encoding.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do custom character graphics work?
The VIC-II can use character patterns in RAM instead of the standard patterns held in character-generator ROM. A program can define its own 8-by-8 glyphs by supplying the corresponding row bytes, then configure video memory so the VIC-II reads those patterns as its character data.
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Custom character data takes the place of the standard character set for that configuration. If a display needs standard letters or other built-in symbols alongside custom shapes, the program must copy the needed standard glyph patterns into the RAM character set as well. The guide’s graphics chapter, “Programming Graphics — Standard Character Mode” and “Character Definitions”, explains the standard mode and programmable character shapes.
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Further reading
The Commodore 64 Programmer’s Reference Guide graphics chapter is the primary manual reference for character patterns and standard character mode. For explicit PETSCII, screen-code, and color-code mappings, consult Appendix G — PETSCII, Screen Codes & Color Codes; check that any chart you use identifies the character set it depicts, since the same screen codes can show different glyphs in different sets.
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