Nintendo Entertainment System specific information for cc65

Ullrich von Bassewitz,
Stefan A. Haubenthal

2014-04-12
An overview over the NES runtime system as it is implemented for the cc65 C compiler.

1. Overview

2. Binary format

3. Memory layout

4. Platform specific header files

5. Loadable drivers

6. Limitations

7. Other hints

8. License


1. Overview

This file contains an overview of the NES runtime system as it comes with the cc65 C compiler. It describes the memory layout, NES specific header files, available drivers, and any pitfalls specific to that platform.

Please note that NES specific functions are just mentioned here, they are described in detail in the separate function reference. Even functions marked as "platform dependent" may be available on more than one platform. Please see the function reference for more information.

2. Binary format

The standard binary output format generated by the linker for the NES target is a machine language program with an INES cartridge header. It is of course possible to change this behaviour by using a modified startup file and linker config.

3. Memory layout

cc65 generated programs with the default setup run with the I/O area and a CHR bank enabled, which gives a usable memory range of $8000 - $FFF3. All boot ROM entry points may be called directly without additional code.

Special locations:

Text screen

The text screen is located at VRAM $2000.

Stack

The C runtime stack is located at $7FFF and growing downwards.

Heap

The C heap is located at the end of the program and grows towards the C runtime stack.

4. Platform specific header files

Programs containing NES specific code may use the nes.h header file.

4.1 NES specific functions

4.2 Hardware access

The following pseudo variables declared in the nes.inc include file do allow access to hardware located in the address space.

PPU

The PPU defines allow access to the PPU chip.

APU

The APU defines allow access to the APU chip.

5. Loadable drivers

All drivers must be statically linked because no file I/O is available. The names in the parentheses denote the symbols to be used for static linking of the drivers.

5.1 Graphics drivers

nes-64-56-2.tgi (nes_64_56_2)

This driver features a resolution of 64×56 with 2 colors using the CHR bank.

5.2 Extended memory drivers

No extended memory drivers are currently available for the NES.

5.3 Joystick drivers

nes-stdjoy.joy (nes_stdjoy)

A joystick driver for the standard four buttons joypad is available.

The generic interface doesn't export the start and select buttons. To test for those, use the defines in nes.h instead of the generic masks.

Example:

if (joy_read(0) & KEY_A)

5.4 Mouse drivers

No mouse drivers are currently available for the NES.

5.5 RS232 device drivers

No serial drivers are currently available for the NES.

6. Limitations

6.1 Disk I/O

The existing library for the NES doesn't implement C file I/O. There are no hacks for the read() and write() routines.

To be more concrete, this limitation means that you cannot use any of the following functions (and a few others):

7. Other hints

8. License

This software is provided 'as-is', without any expressed or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.