Synertek Systems Sym-1 specific information for cc65

Wayne Parham

An overview over the Sym-1 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 Sym-1 runtime system as it comes with the cc65 C compiler. It describes the memory layout, Sym-1 specific header files, available drivers, and any pitfalls specific to the platform.

Please note that Sym-1 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 output format generated by the linker for the Sym-1 target is a raw binary BIN file, which is essentially a memory image. You can convert this to a HEX file using BIN2HEX, which is a popular open-source conversion utility program. A HEX file has ASCII representations of the hexadecimal byte values of the machine-language program. So the HEX file can be transferred to the Sym-1 using the RS-232 terminal port, just as if the machine-code was entered by hand. Enter 'm 200' in the monitor and start the HEX file transfer.

Included with this distribution is a 4k configuration file and a 32k config file. The Sym-1 on-board memory is limited to 4 kbytes but system memory can be increased to 32 kbytes of contiguous RAM with aftermarket add-on boards. So choose the config file that matches your system configuration before compiling and linking user programs.

3. Memory layout

The ROMs and I/O areas are defined in the configuration files, as are most of the entry points for useful subroutines in the Sym-1 monitor ROM. cc65 generated programs compiled and linked using 4k config run in the memory range of $200 - $0FFF. The 32k config expands this range to $7FFF. Memory above 32k can be used to extend the heap, as described below. The starting memory location and entry point for running the program is $200, so when the program is transferred to the Sym-1, it is executed by typing 'g 200'. The system returns control back to the monitor ROM when the program terminates, providing the '.' prompt.

Special locations:

Text screen

Conio support is not currently available for the Sym-1. But stdio console functions are available.


The C runtime stack is located at $0FFF on 4kb Syms, or at $7FFF for 32kb systems. The stack always grows downwards.


The C heap is located at the end of the program and grows towards the C runtime stack. Extended memory can be added to the heap, as described below.

4. Platform specific header files

Programs containing Sym-1 code may use the sym1.h header file. See the header file for more information.

4.1 Hardware access

The pseudo variables declared in the include file allow access to hardware located in the address space. See the include file for more information.

5. Loadable drivers

5.1 Graphics drivers

No graphics drivers are currently available for the Sym-1.

5.2 Extended memory drivers

There are no extended memory drivers for the Sym-1. However, there is a way to access memory beyond the 32kb boundary, if extended memory is physically present in the system. See the example program, symExtendedMemory, in the samples directory.

5.3 Joystick drivers

No joystick driver is currently available for the Sym-1.

5.4 Mouse drivers

No mouse drivers are currently available for the Sym-1.

5.5 RS232 device drivers

No communication port drivers are currently available for the Sym-1. It has only the "master console" e.g. stdin and stdout.

6. Limitations

6.1 Disk I/O

The existing library for the Sym-1 doesn't implement C file I/O.

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

7. Other hints

7.1 sym1.h

This header exposes Sym-specific I/O functions that are useful for reading and writing its ports and front panel. See the sym1.h include file for a list of the functions available.

Limited memory applications

As stated earlier, there are config files for 4kb and 32kb systems. If you have 32kb RAM, then you will probably want to use the sym1-32k configuration, but if not - if you are using the sym1-4k configuration - then you may want to use functions like getchar, putchar, gets and puts rather than functions like scanf and printf. Printf, for example, requires about 1KB because it needs to know how to process all the format specifiers.

Using extended memory

Memory may be physically present that is addressed at locations above the monitor ROM at $8000. This extended memory is accessible by adding to the heap, as described in the symExtendedMemory sample program.

Sample programs

All the samples will run on the "stock" 4kb Sym-1, except for symIO and symNotepad, which require 32kb. Additionally, symExtendedMemory shows how to access memory above 32kb, so it expects more than 32kb. These sample programs can be found in the samples/sym1 directory:

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.