c64-graphics

name

c64-graphics

description

C64 screen graphics from the programmer's view: the five VIC-II display modes (standard/multicolor text, ECM, standard/multicolor bitmap), custom 8x8 character sets, screen/color/character memory layout, video bank selection, and smooth scrolling. Use this skill WHEN you see "how do I define custom characters", "turn on multicolor mode", "POKE 53272 / 53265 / 53270 / 53281", "$D011 $D016 $D018 bitmap mode", "switch VIC-II video banks", "where is screen RAM / color RAM", "set the bitmap base", or "smooth scroll the screen". Pairs with c64-sprites, c64-vic-ii (raster timing, full chip spec), and c64-memory-map.

C64 Graphics (VIC-II display modes)

Programming the C64 screen: text and bitmap modes, custom charsets, the screen/color/char memory map, video banks, and smooth scrolling. Hardware sprites (MOBs) are the separate c64-sprites skill. For cycle-exact raster timing, bad lines, the invalid modes, and the full register spec, use c64-vic-ii; for ROM/RAM/IO banking under the VIC see c64-memory-map.

The VIC-II (6567 NTSC / 6569 PAL) lives at $D000–$D02E (53248–53294). A display is 40 columns x 25 rows of text, or 320 x 200 dots in bitmap mode.

The five display modes and their control bits

Three bits across two registers select the mode. ECM=$D011 bit6, BMM (bitmap) =$D011 bit5, MCM (multicolor)=$D016 bit4.

Mode	ECM	BMM	MCM	Resolution / cell	Colors per cell
Standard text	0	0	0	40x25 chars, 8x8	1 (per-cell from color RAM) + shared background
Multicolor text	0	0	1	8x8, dots 2 wide	3 shared ($D021/$D022/$D023) + 1 per-cell
Extended bg (ECM)	1	0	0	40x25, 8x8	1 fg + 1 of 4 backgrounds ($D021–$D024) per char
Standard bitmap	0	1	0	320x200	2 per 8x8 cell, both from screen RAM nibbles
Multicolor bitmap	0	1	1	160x200	4 per 8x8 cell

$D011 = 53265, $D016 = 53270. These are read-modify-write registers — always preserve the other bits:

POKE 53265,PEEK(53265)OR 64    : REM ECM on  ($D011 bit6)
POKE 53265,PEEK(53265)OR 32    : REM bitmap on ($D011 bit5)
POKE 53270,PEEK(53270)OR 16    : REM multicolor on ($D016 bit4)
POKE 53270,PEEK(53270)AND 239  : REM multicolor off

Assembly equivalents (set bitmap mode, keep other bits):

    LDA $D011
    ORA #$20        ; set BMM (bit 5)
    STA $D011

ECM and MCM together (ECM+MCM, or ECM+BMM) select invalid/illegal modes that blank to black — documented in c64-vic-ii.

Memory map: screen RAM, color RAM, character memory

What	Default address	Notes
Screen RAM (char codes)	$0400–$07E7 (1024–2023), 1000 bytes	one screen-code byte per cell
Color RAM	$D800–$DBE7 (55296–56295), 1000 nibbles	4-bit only, fixed location, never moves
Character ROM	$D000–$DFFF	2 sets (uppercase/gfx + lowercase), under the I/O
Character generator base	default $1000 (ROM image)	set by $D018
Border color	$D020 (53280)
Background color #0	$D021 (53281)	screen background

Color RAM is only the low 4 bits at $D800 — you cannot store 8-bit values there, and it is the same physical RAM regardless of video bank. Background colors #1–#3 are $D022/$D023/$D024 (used by multicolor text and ECM).

$D018 — screen + character memory pointer (53272)

$D018 bits 7-4 : VM13-VM10  screen RAM base   = (bits 7-4) * $0400  within bank
$D018 bits 3-1 : CB13-CB11  char memory base  = (bits 3-1) * $0800  within bank
                 (in bitmap mode, bit 3 alone selects the $2000 bitmap base)

POKE 53272,(PEEK(53272)AND240)OR A   : REM A picks SCREEN base, hi nibble
POKE 53272,(PEEK(53272)AND15) OR A   : REM A picks CHAR base, low nibble

Char-memory A values (add the bank base): 0=$0000, 2=$0800, 4=$1000 ROM image (default), 6=$1800 ROM image, 8=$2000, 10=$2800, 12=$3000, 14=$3800. POKE 53272,(PEEK(53272)AND240)+12 is the standard "put my charset at 12288" move. POKE 53272,21 restores the normal C64 setup.

Custom (programmable) characters

A character is 8x8 = 8 bytes (1 byte per row, MSB = leftmost pixel). A full set is 256 chars x 8 = 2K (2048 bytes), and must start on a 2K boundary (the $D018 low-nibble values above).

Steps in BASIC:

1. Lower the top of BASIC to protect the charset (e.g. POKE 52,48:POKE 56,48:CLR to reserve from 12288 up).
2. Point VIC at it: POKE 53272,(PEEK(53272)AND240)+12 (charset at 12288).
3. Copy any ROM chars you want to keep (the char ROM is under I/O — see Gotchas), then POKE your 8-byte patterns: char N's rows live at base + N*8 .. +7.

Compute a row byte from 8 pixels: b = b1*128 + b2*64 + ... + b8*1.

Video bank selection (CIA2 $DD00)

The VIC sees only 16K at a time. Bits 0–1 of CIA #2 port A ($DD00, 56576) select which 16K bank — but the bits are inverted (value 3 = bank 0):

$DD00 bits 1,0	Bank	VIC range
11 (POKE val 3)	0	$0000–$3FFF (default)
10 (val 2)	1	$4000–$7FFF
01 (val 1)	2	$8000–$BFFF
00 (val 0)	3	$C000–$FFFF

POKE 56578,PEEK(56578)OR 3              : REM make $DD00 bits 0,1 outputs
POKE 56576,(PEEK(56576)AND 252)OR A     : REM A = value from table above

The character ROM image is only visible to the VIC in banks 0 and 2 (at $1000 and $9000 respectively). In banks 1 and 3 you must supply your own charset (or copy the ROM into bank RAM). All VIC base addresses ($D018, sprite pointers) are relative to the selected bank's start.

Bitmap mode specifics

Standard hi-res bitmap: 8000 bytes (320x200 / 8). Color comes from screen RAM (NOT color RAM): upper nibble = "1" pixel color, lower nibble = "0" pixel color, per 8x8 cell.

Multicolor bitmap (160x200, 2-bit pixels) color sources per cell:

00  Background color #0 ($D021)
01  upper 4 bits of screen memory
10  lower 4 bits of screen memory
11  the color-RAM nibble ($D800+)

Set bitmap base with $D018 bit 3 (e.g. POKE 53272,PEEK(53272)OR 8 puts the bitmap at offset $2000 in the bank). You must clear the 8K bitmap yourself before drawing.

Smooth scrolling

The fine-scroll registers are the low 3 bits of $D011 (vertical) and $D016 (horizontal); they shift the whole screen 0–7 pixels. To scroll new data on you shrink the visible area so the covered edge has room:

• 38-column mode: $D016 bit 3 = 0 (POKE 53270,PEEK(53270)AND247) for horizontal scroll — still 40 cols in RAM, only 38 shown.
• 24-row mode: $D011 bit 3 = 0 (POKE 53265,PEEK(53265)AND247) for vertical.

POKE 53270,(PEEK(53270)AND 248)+X   : REM X = 0..7 horizontal fine scroll
POKE 53265,(PEEK(53265)AND 248)+Y   : REM Y = 0..7 vertical fine scroll

Loop the fine value 0→7, then do a full character-cell shift of screen RAM (in machine language for speed) and reset the fine register. Exact raster-stable scrolling lives in c64-vic-ii.

Quick answers

• Default screen RAM? $0400 (1024). Color RAM? $D800 (55296), 4-bit.
• Change border / background? POKE 53280,c / POKE 53281,c, c = 0–15.
• Where's the charset normally? Char ROM imaged at $1000 in bank 0; the default $D018 is $15 (21), and its char-base field selects that $1000 image (bit 0 is unused).
• Why did my chars vanish after switching banks? The ROM charset image only exists in banks 0 and 2; supply your own in banks 1/3.

Gotchas

• Color RAM is 4-bit nibbles at $D800 and is fixed — it does not move with the screen or the bank. Don't expect to read back a full byte.
• The character ROM is "under" the I/O at $D000–$DFFF — the source notes it "begins at location 53248 ($D000) when the I/O is switched off." To copy it from the CPU you must bank the I/O block out via the $0001 processor port (and disable interrupts while it's out); see c64-memory-map for the exact banking POKEs. The VIC always sees the ROM image at $1000/$9000 in banks 0/2 regardless of CPU banking.
• $DD00 bank bits are inverted (3 = bank 0). And you must set them as outputs in $DD02 first.
• Always read-modify-write $D011 and $D016 — they pack mode bits, the fine-scroll bits, the 24/25-row and 38/40-col bits, and raster bit 8 together.
• Sprite pointers sit at the end of screen RAM ($07F8 for the default screen); moving the screen moves them too — see c64-sprites.

How to read the references

• references/graphics-programming.md — the Programmer's Reference Guide Chapter 3 graphics half, verbatim: every mode, full char-memory and bank tables, the worked custom-character and bitmap BASIC programs, and the smooth- scroll walkthrough. Read this for the authoritative POKE values and listings.
• references/color-graphics-tutorial.md — the User's Guide Chapter 5 gentle tutorial (PRINTing colors, CHR$ color codes, POKEing the screen, bouncing-ball example). Read this when you want the beginner-level framing.
• references/screen-color-memory-map.md — App D screen and color memory maps with the address-of-each-row diagrams and the 16 color codes. Read when you need the exact address of a screen/color cell.
• references/vic-register-map.md — App G VIC register map ($D000–$D02E) as a convenience copy. The deep, cycle-exact reference is c64-vic-ii; start there for anything timing-related.