Target system specification

Prog8 targets the following hardware:

  • 8 bit MOS 6502/65c02/6510 CPU

  • 64 Kb addressable memory (RAM or ROM)

  • optional use of memory mapped I/O registers

  • optional use of system ROM routines

Currently these machines can be selected as a compilation target (via the -target compiler argument):

  • ‘c64’: the Commodore 64

  • ‘cx16’: the Commander X16

  • ‘c128’: the Commodore 128 (limited support)

  • ‘atari’: the Atari 800 XL (experimental support)

This chapter explains some relevant system details of the c64 and cx16 machines.


If you only use standard kernal and prog8 library routines, it is possible to compile the exact same program for both machines (just change the compilation target flag)!

Memory Model

Physical address space layout

The 6502 CPU can address 64 kilobyte of memory. Most of the 64 kilobyte address space can be used by Prog8 programs. This is a hard limit: there is no built-in support for RAM expansions or bank switching.

memory area





contains many sensitive system variables


Hardware stack

used by the CPU, normally not accessed directly


Free RAM or ROM

free to use memory area, often a mix of RAM and ROM

A few of these memory addresses are reserved and cannot be used for arbitrary data. They have a special hardware function, or are reserved for internal use in the code generated by the compiler:

reserved address

in use for


data direction (CPU hw)


bank select (CPU hw)


internal scratch variable


internal scratch variable

$fb - $fc

internal scratch variable

$fd - $fe

internal scratch variable

$fffa - $fffb

NMI vector (CPU hw)

$fffc - $fffd

RESET vector (CPU hw)

$fffe - $ffff

IRQ vector (CPU hw)

The actual machine will often have many other special addresses as well, For example, the Commodore-64 has:

  • ROMs installed in the machine: BASIC, kernal and character roms. Occupying $a000$bfff and $e000$ffff.

  • memory-mapped I/O registers, for the video and sound chips, and the CIA’s. Occupying $d000$dfff.

  • RAM areas that are used for screen graphics and sprite data: usually at $0400$07ff.

Prog8 programs can access all of those special memory locations but it will have a special meaning.

ZeroPage (“ZP”)

The ZeroPage memory block $02$ff can be regarded as 254 CPU ‘registers’, because they take less clock cycles to access and need fewer instruction bytes than accessing other memory locations outside of the ZP. Theoretically they can all be used in a program, with the follwoing limitations:

  • several addresses ($02, $03, $fb - $fc, $fd - $fe) are reserved for internal use

  • most other addresses will already be in use by the machine’s operating system or kernal, and overwriting them will probably crash the machine. It is possible to use all of these yourself, but only if the program takes over the entire system (and seizes control from the regular kernal). This means it can no longer use (most) BASIC and kernal routines from ROM.

  • it’s more convenient and safe to let the compiler allocate these addresses for you and just use symbolic names in the program code.

Prog8 knows what addresses are safe to use in the various ZP handling configurations. It will use the free ZP addresses to place its ZP variables in, until they’re all used up. If instructed to output a program that takes over the entire machine, (almost) all of the ZP addresses are suddenly available and will be used.

ZeroPage handling is configurable: There’s a global program directive to specify the way the compiler treats the ZP for the program. The default is to be reasonably restrictive to use the part of the ZP that is not used by the C64’s kernal routines. It’s possible to claim the whole ZP as well (by disabling the operating system or kernal). If you want, it’s also possible to be more restricive and stay clear of the addresses used by BASIC routines too. This allows the program to exit cleanly back to a BASIC ready prompt - something that is not possible in the other modes.

IRQs and the ZeroPage

The normal IRQ routine in the C-64’s kernal will read and write several addresses in the ZP (such as the system’s software jiffy clock which sits in $a0 - $a2):

$a0 - $a2; $91; $c0; $c5; $cb; $f5 - $f6

These addresses will never be used by the compiler for ZP variables, so variables will not interfere with the IRQ routine and vice versa. This is true for the normal ZP mode but also for the mode where the whole system and ZP have been taken over. So the normal IRQ vector can still run and will be when the program is started!


Directly Usable Registers

The hardware CPU registers are not directly accessible from regular Prog8 code. If you need to mess with them, you’ll have to use inline assembly. Be extra wary of the X register because it is used as an evaluation stack pointer and changing its value you will destroy the evaluation stack and likely crash the program.

The status register (P) carry flag and interrupt disable flag can be written via a couple of special builtin functions (set_carry(), clear_carry(), set_irqd(), clear_irqd()), and read via the read_flags() function.

The 16 ‘virtual’ 16-bit registers that are defined on the Commander X16 machine are not real hardware registers and are just 16 memory-mapped word values that you can access directly.

IRQ Handling

Normally, the system’s default IRQ handling is not interfered with. You can however install your own IRQ handler (for clean separation, it is advised to define it inside its own block). There are a few library routines available to make setting up C-64 60hz IRQs and Raster IRQs a lot easier (no assembly code required).

For the C64 these routines are:

c64.set_irq(uword handler_address, boolean useKernal)
c64.set_rasterirq(uword handler_address, uword rasterline, boolean useKernal)
c64.restore_irq()     ; set everything back to the systems default irq handler

And for the Commander X16:

cx16.set_irq(uword handler_address, boolean useKernal)          ; vsync irq
cx16.set_rasterirq(uword handler_address, uword rasterline)     ; note: disables kernal irq handler! sys.wait() won't work anymore
cx16.restore_irq()     ; set everything back to the systems default irq handler