# C64 BASIC Memory Format Reference
## Memory Layout
### BASIC Program Area
- **BASIC program start**: $0801 (2049 decimal)
- **End of BASIC pointer**: $002D-$002E (low byte, high byte)
- **Start of variables pointer**: $002F-$0030 (should match end of BASIC)
### Input/Control Areas
- **Keyboard buffer**: $0277-$0280 (10 bytes max)
- **Keyboard buffer count**: $00C6
## BASIC Line Structure
Each BASIC line has the following format:
```
[Next Line Ptr Lo] [Next Line Ptr Hi] [Line # Lo] [Line # Hi] [Tokens...] [00]
```
| Field | Size | Description |
|-------|------|-------------|
| Next Line Pointer | 2 bytes | Address of next line (little-endian) |
| Line Number | 2 bytes | Line number (little-endian), e.g., 10 = $0A $00 |
| Tokens/Content | Variable | Tokenized BASIC code |
| Terminator | 1 byte | $00 marks end of line |
## CRITICAL: End of Program Structure
### ⚠️ How the $00 $00 End Marker Works
The `$00 $00` end marker is a **null link pointer for a non-existent next line**. It is NOT the link pointer OF the last line.
**BASIC reads link pointers FIRST, before line content.** When BASIC encounters `$00 $00` as a link pointer, it stops immediately—it never reads whatever follows.
### Correct Structure:
```
Last actual line:
[Link Ptr → end marker location] [Line #] [Content] [00 terminator]
End marker (immediately after):
[00 00] ← This is where the last line's link pointer points to
```
### Visual Example:
```
$0810: 18 08 ← Last line's link pointer, points to $0818
$0812: 14 00 ← Line number (20)
$0814: 89 20.. ← Content (GOTO 10)
$0817: 00 ← Line terminator
$0818: 00 00 ← END MARKER (null link pointer)
↑
Last line's link pointer points HERE
$081A: ... ← End-of-BASIC pointer ($002D) points HERE
```
### Common Bug (INCORRECT):
```
$0810: 00 00 ← WRONG! BASIC sees this and stops immediately!
$0812: 14 00 ← Line 20 is never read
$0814: 89 20.. ← Content is never read
```
**Why this is wrong:** BASIC reads the link pointer at $0810 first. It sees `$00 $00` and thinks "end of program" before ever looking at the line number or content.
## Line Pointer Calculation Details
Calculate the next line pointer by counting ALL bytes in the current line:
```
Line structure:
2 bytes: next line pointer
2 bytes: line number
N bytes: tokenized content
1 byte: $00 terminator
Total: 5 + N bytes
Next address = Current address + (5 + N)
```
**Example:**
```
Line at $0801:
$0801-$0802: $0F $08 (next line pointer)
$0803-$0804: $0A $00 (line 10)
$0805: $97 (POKE token)
$0806: $20 (space)
$0807-$080B: $35 $33 $32 $38 $30 ("53280")
$080C: $2C (comma)
$080D: $30 ("0")
$080E: $00 (terminator)
Total bytes: 14 ($0E)
Next line: $0801 + $0E = $080F
So the pointer at $0801-$0802 should be $0F $08 (little-endian)
```
## Setting End-of-BASIC Pointer
After writing the complete program (including the `$00 $00` end marker), update $002D-$002E to point to the byte AFTER the end marker:
```
Last line's $00 terminator at $0817
End marker ($00 $00) at $0818-$0819
End-of-BASIC = $081A (byte after end marker)
Set $002D-$002E to $1A $08 (little-endian)
```
Also set $002F-$0030 (start of variables) to the same value.
## Complete Multi-Line Example
Program:
```basic
10 POKE 53280,0
20 GOTO 10
```
Memory layout:
```
Address Bytes Description
------- ---------- -----------
$0801 0F 08 Line 10 link → $080F
$0803 0A 00 Line number 10
$0805 97 POKE token
$0806 20 space
$0807 35 33 32 "532"
$080A 38 30 "80"
$080C 2C comma
$080D 30 "0"
$080E 00 Line 10 terminator
$080F 18 08 Line 20 link → $0818 (where end marker will be)
$0811 14 00 Line number 20
$0813 89 GOTO token
$0814 20 space
$0815 31 30 "10"
$0817 00 Line 20 terminator
$0818 00 00 END OF PROGRAM (null link pointer)
$081A -- End-of-BASIC pointer points here
```
Hex string: `0F080A00972035333238302C300018081400892031300000`
## Non-Tokenized Characters (PETSCII)
Characters that are NOT keywords are stored as PETSCII values:
| Character | PETSCII | Notes |
|-----------|---------|-------|
| Space | $20 | Required after many tokens |
| Comma | $2C | NOT $AC (multiply token) |
| Colon | $3A | Statement separator |
| Semicolon | $3B | Used in PRINT |
| Equals | $3D | In some contexts (LET) |
| 0-9 | $30-$39 | Digits stored as characters |
| A-Z | $41-$5A | Variables and string literals |
### Number Storage
**CRITICAL**: Numbers in BASIC code are stored as PETSCII character strings, NOT as binary values.
Examples:
- `10` → $31 $30 (two bytes: "1" "0")
- `255` → $32 $35 $35 (three bytes: "2" "5" "5")
- `53280` → $35 $33 $32 $38 $30 (five bytes: "5" "3" "2" "8" "0")
Do NOT convert numbers to binary. Store them as ASCII/PETSCII digit characters.
