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.
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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$(65)
prints "A"; POKE 1024,65 shows "▌"(spade-ish) — to get "A" in the top-left you
. Letters: PETSCII "A"=65, screen-code "A"=1.
POKE 1024,1
Two ways to put a character on screen
REM PRINT path — uses PETSCII, honors control codes, scrolls
PRINT CHR$(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).
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.
