Chapter 8: Memory Map

The Commander X16 has 512 KB of ROM and 2,088 KB (2 MB¹ + 40 KB) of RAM with up to 3.5MB of RAM or ROM available to cartridges.

Some of the ROM/RAM is always visible at certain address ranges, while the remaining ROM/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 System ROM and Cartridge ROM/RAM (16 KB window into one of 256 banks, see below)

Banked Memory

Writing to the following zero-page addresses sets the desired RAM or ROM bank:

Address	Description
$0000	Current RAM bank (0-255)
$0001	Current ROM/Cartridge bank (ROM is 0-31, Cartridge is 32-255)

The currently set banks can also be read back from the respective memory locations. Both settings default to 0 on RESET.

ROM Allocations

Here is the ROM/Cartridge bank allocation:

Bank	Name	Description
0	KERNAL	KERNAL operating system and drivers
1	KEYBD	Keyboard layout tables
2	CBDOS	The computer-based CMDR-DOS for FAT32 SD cards
3	FAT32	The FAT32 driver itself
4	BASIC	BASIC interpreter
5	MONITOR	Machine Language Monitor
6	CHARSET	PETSCII and ISO character sets (uploaded into VRAM)
7	DIAG	Memory diagnostic
8	GRAPH	Kernal graph and font routines
9	DEMO	Demo routines
10	AUDIO	Audio API routines
11	UTIL	System Configuration (Date/Time, Display Preferences)
12	BANNEX	BASIC Annex (code for some added BASIC functions)
13-14	X16EDIT	The built-in text editor
15	BASLOAD	A transpiler that converts BASLOAD dialect to BASIC V2
16-31	–	[Currently unused]
32-255	–	Cartridge RAM/ROM

Important: The layout of the banks may still change.

Cartridge Allocation

Cartridges can use the remaining 32-255 banks in any combination of ROM, RAM, Memory-Mapped IO, etc. See Kevin’s reference cartridge design for ideas on how this may be used. This provides up to 3.5MB of additional RAM or ROM.

Important: The layout of the banks is not yet final.

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/CMDR-DOS 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.

Bank 0

Addresses	Description
$A000-$BEFF	System Reserved
$BF00-$BFFF	Parameter passing space

You can use the space at $0:BF00-0:$BFFF to pass parameters between programs. This space is initalized to zeroes, so you may use it however you wish.

The suggested use is to store a PETSCII string in this space and use semicolons to separate parameters. The string should be null terminated:

Example:

FRANK;3;BLUE\x00

A program that reads the parameters is responsible for resetting the data to zeroes, so that another program does not see unexpected data and malfunction.

I/O Area

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

Addresses	Description	Speed
$9F00-$9F0F	VIA I/O controller #1	8 MHz
$9F10-$9F1F	VIA I/O controller #2	8 MHz
$9F20-$9F3F	VERA video controller	8 MHz
$9F40-$9F41	YM2151 audio controller	2 MHz
$9F42-$9F5F	Unavailable	—
$9F60-$9F7F	Expansion Card Memory Mapped IO3	8 MHz
$9F80-$9F9F	Expansion Card Memory Mapped IO4	8 MHz
$9FA0-$9FBF	Expansion Card Memory Mapped IO5	2 MHz
$9FC0-$9FDF	Expansion Card Memory Mapped IO6	2 MHz
$9FE0-$9FFF	Cartidge/Expansion Memory Mapped IO7	2 MHz

Expansion Cards & Cartridges

Expansion cards can be accessed via memory-mapped I/O (MMIO), as well as I2C. Cartridges are essentially expansion cards which are housed in an external enclosure and may contain RAM, ROM and an I2C EEPOM (for save data). Internal expansion cards may also use the RAM/ROM space, though this could cause conflicts.

While they may be uncomon, since cartridges are essentially external expansion cards in a shell, that means they can also use MMIO. This is only necessary when a cartridge includes some sort of hardware expansion and MMIO was desired (as opposed to using the I2C bus). In that case, it is recommended cartridges use the IO7 range and that range should be the last option used by expansion cards in the system. MMIO is unneeded for cartridges which simply have RAM/ROM.

For more information, consult the Hardware section of the manual.

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