Programming the MEGA65 — Appendices

Oct 4, 2026 · Updated Oct 11, 2026 · @Craig Taylor

Appendix A: Screen Codes and Colours

Screen codes

Screen codes are the values stored in screen memory, and are different from PETSCII. These are the codes of the upper-case character set; codes 128 to 255 are the same characters in reverse video.

To convert a letter's PETSCII code to its screen code in the upper-case set, subtract 64: PETSCII 65, A, is screen code 1. Digits and most punctuation have the same value in both. Screen codes 256 and above are used only in the VIC-IV's two-byte character modes; Chapter 12 describes them.

Colours

The first 16 palette entries are the C64's colours, used by COLOR, BORDER, BACKGROUND and colour RAM. CTRL with 1 to 8 types the first eight, and MEGA with 1 to 8 the second eight.

Entries 16 to 31 hold an alternate "rainbow" set, selected for text with ESC S and returned with ESC U, and entries 32 to 255 are free for programs to define with PALETTE.

Appendix B: PETSCII Codes

PETSCII is the character code used by PRINT, CHR$, ASC, GET and the keyboard. Codes 32 to 95 follow ASCII closely; the rest hold control codes and graphics.

In the lower-case set, 65-90 are lower-case letters and 193-218 are capitals.

Control codes

Printing these codes performs an action rather than showing a character. They can be typed into strings in quote mode, or produced with CHR$.

These are the C64's control codes, which the MEGA65 keeps; PRINT CHR$(27); "X" sends an escape sequence, as Chapter 2 describes. The MEGA65 also defines further codes, such as for tabs and flashing text; the Book's character appendix lists them all.

Appendix C: Number Conversion

Each hexadecimal digit stands for four binary digits, so conversion between the two needs only this table:

$D0 is therefore %11010000, and %10100101 is $A5.

Converting between hex and decimal

To convert hex to decimal, multiply each digit by its place value and add:

So $D020 is 13 × 4096 + 0 × 256 + 2 × 16 + 0 = 53,280. In BASIC 65 the conversions are built in: PRINT $D020 gives 53280, PRINT HEX$(53280) gives D020, PRINT %1010 gives 10, and PRINT STRBIN$(10) gives the binary digits.

Useful values

Appendix D: 45GS02 Opcode Map

The row gives the opcode's high digit and the column its low digit: LDA # is in row A, column 9, so its opcode is $A9. Mode abbreviations are those of Chapter 10: bp is base page, (bp),Z indirect indexed by Z, rel16 a 16-bit branch, and (d,SP),Y stack-relative. The 45GS02's flat and quad instructions are prefixed forms of these opcodes, as Chapter 10 describes. This map follows the cc65 CPU reference and the Book's 4510/45GS02 opcode table (MEGA65 Book, Appendix L). Opcode $62 uses the cc65 name RTN; the Book writes the same instruction as RTS #$nn.

Columns 0 to 7

Columns 8 to F

BBR and BBS take a base page address and an 8-bit branch offset.

Appendix E: Key Registers

This is a quick reference to the registers used in this book. The MEGA65 Book's register appendices document every register and bit. Most of these need the VIC-IV (MEGA65) I/O personality.

Appendix F: Converting C64 and C65 Programs

Many C64 programs run unchanged in C64 mode. Converting one to MEGA65 mode lets it use the MEGA65's speed, memory and graphics, but several differences need attention.

BASIC programs

Machine-language programs

• KERNAL calls through the standard jump table at $FF81-$FFF3 work unchanged, as Chapter 11 describes.
• Zero page and system RAM are laid out differently in MEGA65 mode; code that uses C64 zero-page locations or vectors other than the documented ones needs changing, or a private base page.
• Memory map. Code that relies on the C64's $01 banking should set it explicitly, and must allow for MAP and $D030 as Chapter 5 describes.
• Timing. Cycle-counted code runs at 1 MHz only in C64 mode or when the CPU is slowed; at full speed it needs raster synchronisation instead.
• STZ. 65C02 code that uses STZ to store zero needs Z held at zero, as Chapter 7 explains.
• I/O personality. C64 code expects the VIC-II personality; MEGA65 code that writes VIC-IV registers must knock first.

C65 programs

C65 BASIC 10 programs are close relatives of BASIC 65 and often run with small changes, mostly to graphics commands, which changed between BASIC 10 and BASIC 65. C65 machine language runs on the 45GS02, which executes every 4510 instruction.

Appendix G: The Built-in Monitor

The machine-language monitor is entered with MONITOR from BASIC, by a BRK instruction, or by holding RUN/STOP while pressing reset. It accepts 32-bit addresses and treats every number as hexadecimal unless it has a prefix: + decimal, & octal, % binary, ' a PETSCII character (MEGA65 Book, Appendix O).

Memory and registers shown by M and R can be changed by moving the cursor onto the display, typing over the values and pressing RETURN, as in other Commodore monitors. Chapter 7 walks through a first program entered with the monitor, and Chapter 8 covers debugging with it, with BRK breakpoints, and with Matrix Mode.