The Screen Is Memory

Everything so far has used PRINT, which follows a cursor down the screen. Now meet the C64's defining idea: the screen is a block of memory. Each of its 1000 cells (40 across, 25 down) is one byte in RAM, starting at address 1024. Write a number into that byte with POKE, and the matching character appears in that cell — instantly, wherever it is, with no cursor involved. This is how the C64 does graphics: there are no drawing keywords; you write to the screen's memory directly.

Milestone 1 — one character, placed by hand

10 PRINT CHR$(147)
20 POKE 1024,1

Line 10 clears the screen; line 20 pokes the number 1 into address 1024 — the top-left cell. RUN it:

Why does 1 mean A? The number you poke is a screen code, the C64's internal character number — and it is not the same as PETSCII or ASCII. In screen codes, @ is 0, A is 1, B is 2, and so on. (The character appears in light blue because that's the default colour already sitting in the screen's colour memory — that memory is the next unit's subject.)

Milestone 2 — a row, by counting addresses

Cells sit in memory one after another: 1024, 1025, 1026… across the top row. A loop can walk them, poking a rising screen code into each:

10 PRINT CHR$(147)
20 FOR I=0 TO 9
30 POKE 1024+I,1+I
40 NEXT I

POKE 1024+I, 1+I writes screen code 1+I into cell 1024+I. RUN it for the alphabet's first ten letters, laid down by address:

Milestone 3 — anywhere, by row and column

The screen is 40 cells wide, so the cell at row R, column C is at address 1024 + R*40 + C. That one formula reaches any cell on the screen:

10 PRINT CHR$(147)
20 LET R=12
30 LET C=20
40 POKE 1024+R*40+C,42

RUN it to drop a * (screen code 42) at row 12, column 20 — the middle of the screen:

That 1024 + R*40 + C is one of the most useful lines on the C64: it turns a row and a column into a memory address. Every character you place in a game goes through it.

When it doesn't work

• Nothing appeared. The character may be there but invisible — its colour cell matches the background. After a clear the default colour shows; if a cell stays hidden, set its colour too (next unit).
• The character is wrong. You used a PETSCII or ASCII code, not a screen code. A is screen code 1, not 65. Letters are 1–26; digits 48–57; * is 42.
• ?ILLEGAL QUANTITY ERROR. A POKE value must be 0–255, and the address 0–65535. Check your R*40+C didn't run past the screen (max cell is 1024+999).

Before and after

You started placing text with PRINT and a cursor, and finished writing characters straight into the screen's memory — one cell, a row, then any cell by formula. The idea underneath, and the heart of this whole primer: the C64 screen is memory at 1024, and POKE writes to it directly. No drawing keywords — just addresses.

Try this

• Your initials, centred. Work out the address for the middle row and poke two or three screen codes side by side.
• A border line. Loop C from 0 to 39 and poke the same code along row 0.
• Read it back. PRINT PEEK(1024) after poking — PEEK reads a byte where POKE writes one. You'll lean on PEEK for input later.

What you've learnt

• The C64 screen is memory starting at address 1024 — 40×25 cells, one byte each.
• POKE address, code writes a screen code into a cell; the character appears there.
• Screen codes are the C64's own numbering — A is 1, not 65.
• 1024 + R*40 + C is the address of the cell at row R, column C.

What's next

Your characters all came out light blue — the one colour already in the screen's colour memory. In Unit 10 we write to that memory too, at 55296, and the border and background registers — and the screen finally has colour you choose.