c64-petscii

name

c64-petscii

description

The three C64 character-code systems and putting text on screen: PETSCII (what PRINT and CHR$/ASC use), screen/display codes (a DIFFERENT mapping you POKE into screen RAM at $0400), and ASCII; plus cursor/color/reverse control codes and building text UIs with PETSCII box/line graphics. Use this skill WHEN asked "what CHR$ code clears the screen", "why does POKE 1024,1 show an A", "PETSCII vs screen codes", "how do I print in red", "draw a box/menu in PETSCII", "color code for cyan", or "POKE to screen RAM". Pairs with c64-keyboard, c64-graphics, and c64-memory-map.

C64 Character Codes: PETSCII, Screen Codes, ASCII

Three different code systems share the 0–255 range. Mixing them up is the #1 text bug on the C64. Keep them straight:

System	Used by	Range	"A" is	Notes
PETSCII	PRINT, CHR$(), ASC(), the keyboard, files	0–255	65	Commodore's ASCII variant; includes control + color codes
Screen (display) codes	what's stored in screen RAM $0400+ (POKE/PEEK)	0–255	1	indexes the character ROM; no control codes
ASCII	true ASCII (printers, modems)	0–127	65	C64 PETSCII ≈ ASCII for digits/punct, differs for letters & ctrl

The core gotcha: PRINT CHR$(n) uses PETSCII; POKE 1024+offset,n uses screen codes. They are not the same number for the same glyph. CHR$(65) prints "A"; POKE 1024,65 shows "▌"(spade-ish) — to get "A" in the top-left you POKE 1024,1. Letters: PETSCII "A"=65, screen-code "A"=1.

Two ways to put a character on screen

REM PRINT path — uses PETSCII, honors control codes, scrolls
PRINT CHR$(147);"HELLO"        : REM 147 = clear screen, then text

REM POKE path — uses SCREEN codes, no scrolling, set color separately
POKE 1024,8 : POKE 55296,1     : REM screen code 8 = "H", color 1 = white

• Screen RAM default: $0400–$07E7 (1024–2023), 40×25 = 1000 cells. Offset of row r, col c = r*40 + c.
• Color RAM: $D800–$DBE7 (55296–56295), one nibble per cell (0–15). A POKEd character is invisible if its color matches the background — always set color RAM too. (PRINTing a char sets color from the current cursor color.)
• Reverse video of any screen code: add 128. Codes 128–255 are the reversed images of 0–127.

PETSCII control codes (CHR$ values) — cheat-sheet

These do something when PRINTed; embed them in string constants (in quote mode they show as reversed glyphs — see c64-keyboard).

CHR$	Effect	CHR$	Effect
5	white	13	RETURN (CR)
8 / 9	disable / enable SHIFT+C= charset switch	14	switch to lower case
17	cursor down	145	cursor up
29	cursor right	157	cursor left
19	HOME	147	CLR (clear + home)
18	RVS ON (reverse)	146	RVS OFF
20	DEL	148	INST
34	" quote	141	shifted RETURN
142	switch to upper case/graphics	133–140	f1,f3,f5,f7,f2,f4,f6,f8

Color codes

PETSCII (for PRINT CHR$(n)): 144 black, 5 white, 28 red, 159 cyan, 156 purple, 30 green, 31 blue, 158 yellow, 129 orange, 149 brown, 150 lt red, 151 grey1, 152 grey2, 153 lt green, 154 lt blue, 155 grey3.

Color-RAM / VIC color nibble values (0–15, used by POKE and registers like $D020/$D021): 0 black, 1 white, 2 red, 3 cyan, 4 purple, 5 green, 6 blue, 7 yellow, 8 orange, 9 brown, 10 lt red, 11 grey1, 12 grey2, 13 lt green, 14 lt blue, 15 grey3. (Same 16 colors, two numbering schemes.)

PRINT CHR$(28);"RED TEXT";CHR$(5);"WHITE AGAIN"
PRINT CHR$(18);"REVERSED";CHR$(146);"NORMAL"

Two character sets (uppercase/graphics vs lowercase)

Only one is active at a time:

• Set 1 = upper case + graphics (default at power-on). SHIFT gives the graphic on the right of the key.
• Set 2 = lower case + upper case. SHIFT gives upper case.
• Switch by SHIFT+C= keys, or from BASIC: POKE 53272,21 → upper/graphics, POKE 53272,23 → lower/upper. ($D018 selects the char-ROM base.)
• The C= key with a graphic key gives the left-front graphic in either set.

Building a TUI / box with PETSCII

PETSCII has dedicated line- and box-drawing glyphs (horizontal/vertical lines, corners, T-pieces). You place them either by PRINTing the character (type it on a real keyboard, or CHR$ its PETSCII value) or by POKEing its screen code into screen RAM. Look up the exact screen code for each graphic in the SET 1 column of references/screen-display-codes.md — the etext chart shows the POKE value beside each glyph. Reverse-space (a SPACE screen code + 128) makes solid color blocks for bars and backgrounds.

Pattern for a POKE-drawn frame (substitute the line/corner screen codes you read off the chart for HL horizontal, VL vertical, and the four corner values):

10 PRINT CHR$(147)                         : REM clear
20 V=1024:C=55296                          : REM screen / color RAM bases
30 FOR I=1 TO 38:POKE V+I,HL:POKE C+I,1:NEXT          : REM top edge
40 FOR I=1 TO 38:POKE V+24*40+I,HL:POKE C+24*40+I,1:NEXT : REM bottom edge
50 FOR R=1 TO 23                                       : REM side edges
60 POKE V+R*40,VL    :POKE C+R*40,1
70 POKE V+R*40+39,VL :POKE C+R*40+39,1
80 NEXT

Always POKE color RAM alongside, or the glyph is invisible. For the PRINT approach, position with HOME + cursor-down/right control codes inside a string, then PRINT the box characters and reverse-video bars.

Quick answers

• "Clear the screen": PRINT CHR$(147) (or POKE the screen RAM with 32s).
• "Print a quote mark": PRINT CHR$(34).
• "Why does POKE 1024,1 give 'A'?": 1 is the screen code for A (POKE path), not PETSCII 65 (PRINT path).
• "Make text reverse": PRINT CHR$(18) … CHR$(146), or add 128 to a screen code.
• "Read what's on screen at top-left": PRINT PEEK(1024) → a screen code.

How to read the references

• references/screen-display-codes.md — PRG App B verbatim: the full SET 1 / SET 2 / POKE table mapping every glyph to its screen code 0–127 (128–255 = reversed), plus the POKE-to-screen / color-RAM worked example. Read when POKEing characters into screen RAM or decoding a PEEK.
• references/ascii-and-chr-codes.md — PRG App C verbatim: the PRINT CHR$(X) / ASC table for all 0–255 PETSCII values, including every control and color code and the f-keys. Read when building strings for PRINT, decoding a GET, or needing a control/color CHR$ number.

(The User's Guide App E/F cover the same two tables; their content is folded into these files rather than duplicated.)

Cross-links

• The physical keys that emit these PETSCII codes, charset switching with C=+SHIFT, and quote/insert-mode behavior → c64-keyboard.
• Where screen RAM lives, how to relocate it, and VIC bank/charset selection at $D018 → c64-graphics and c64-memory-map.