Chapter 7: Memory Map

The Commander X16 has 64 KB of ROM and 2,088 KB (2 MB1 + 40 KB) of RAM. Some of the ROM and RAM is always visible at certain address ranges, while the remaining ROM and RAM is banked into one of two address windows.

This is an overview of the X16 memory map:

Addresses Description
$0000-$9EFF Fixed RAM (40 KB minus 256 bytes)
$9F00-$9FFF I/O Area (256 bytes)
$A000-$BFFF Banked RAM (8 KB window into one of 256 banks for a total of 2 MB)
$C000-$FFFF Banked ROM (16 KB window into one of 32 banks for a total of 512 KB)

Banked Memory

The currently enabled RAM and ROM banks can be configured by writing to zero page locations 0 and 1:

Address Description
$0000 Current RAM bank (0-255)
$0001 Current ROM bank (0-31)

The currently set banks can also be read back from the respective memory locations. Both settings default to 0 on RESET. The upper three bits of location 1 are undefined.

ROM Allocations

This is the allocation of the banks of banked ROM:

Bank Name Description
0 KERNAL KERNAL operating system and drivers
1 KEYBD Keyboard layout tables
2 CBDOS The computer-based CBM-DOS for FAT32 SD cards
3 GEOS GEOS KERNAL
4 BASIC BASIC interpreter
5 MONITOR Machine Language Monitor
6 CHARSET PETSCII and ISO character sets (uploaded into VRAM)
7 CODEX CodeX16 Interactive Assembly Environment / Monitor
6-31 [Currently unused]

Important: The layout of the banks is still constantly changing.

RAM Contents

This is the allocation of fixed RAM in the KERNAL/BASIC environment.

Addresses Description
$0000-$00FF Zero page
$0100-$01FF CPU stack
$0200-$03FF KERNAL and BASIC variables, vectors
$0400-$07FF Available for machine code programs or custom data storage
$0800-$9EFF BASIC program/variables; available to the user

The $0400-$07FF can be seen as the equivalent of $C000-$CFFF on a C64. A typical use would be for helper machine code called by BASIC.

Zero Page

Addresses Description
$0000-$0001 Banking registers
$0002-$0021 16 bit registers r0-r15 for KERNAL API
$0022-$007F Available to the user
$0080-$009C Used by KERNAL and DOS
$009D-$00A8 Reserved for DOS/BASIC
$00A9-$00D3 Used by the Math library (and BASIC)
$00D4-$00FF Used by BASIC

Machine code applications are free to reuse the BASIC area, and if they don't use the Math library, also that area.

Banking

This is the allocation of banked RAM in the KERNAL/BASIC environment.

Bank Description
0 Used for KERNAL/CBDOS variables and buffers
1-255 Available to the user

(On systems with only 512 KB RAM, banks 64-255 are unavailable.)

During startup, the KERNAL activates RAM bank 1 as the default for the user.

I/O Area

This is the memory map of the I/O Area:

Addresses Description
$9F00-$9F0F VIA I/O controller #1
$9F10-$9F1F VIA I/O controller #2
$9F20-$9F3F VERA video controller
$9F40-$9F41 YM2151 audio controller
$9F42-$9F5F Reserved
$9F60-$9FFF External devices


  1. Current development systems have 2 MB of bankable RAM. Actual hardware is currently planned to have an option of either 512 KB or 2 MB of RAM.↩︎