Apple ][ specific information for cc65

Oliver Schmidt

2014-04-10
An overview over the Apple ][ runtime system as it is implemented for the cc65 C compiler.

1. Overview

2. Binary format

3. Memory layout

4. Linker configurations

5. ProDOS 8 system programs

6. Platform specific header files

7. Loadable drivers

8. Limitations

9. Other hints

10. License


1. Overview

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

Please note that Apple ][ 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 file format generated by the linker for the Apple ][ target is a binary program with a 4 byte DOS 3.3 header containing the load address and load length. The default load address is $803.

AppleCommander 1.3.5 or later (available at http://applecommander.sourceforge.net/) includes the option -cc65 that allows to put binary files with a DOS 3.3 header onto disk images containing DOS 3.3 as well as ProDOS 8.

For ProDOS 8 system programs the load address is fixed to $2000 so there is no need for a header. Thus the linker configuration apple2-system.cfg for those programs omits the DOS 3.3 header. The right AppleCommander option to put system files without a header on a ProDOS 8 disk image is -p.

3. Memory layout

In the standard setup, cc65 generated programs use the memory from $803 to $95FF, so 35.5 KB of RAM are available.

Special locations:

Stack

The C runtime stack is located at HIMEM and grows downwards, regardless of how your linker config file is setup.

Heap

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

While running main() the Language Card bank 2 is enabled for read access. However while running module constructors/destructors the Language Card is disabled.

Enabling the Language Card allows to use it as additional memory for cc65 generated code. However code is never automatically placed there. Rather code needs to be explicitly placed in the Language Card either per file by compiling with --code-name LC or per function by enclosing in #pragma code-name (push, "LC") and #pragma code-name (pop). In either case the cc65 runtime system takes care of actually moving the code into the Language Card.

The amount of memory available in the Language Card for generated code depends on the linker configuration parameters. There are several usefull settings:

LC address: $D400, LC size: $C00

For plain vanilla ProDOS 8 which doesn't actually use the Language Card bank 2 memory from $D400 to $DFFF. This is the default setting.

LC address: $D000, LC size: $1000

For ProDOS 8 together with the function rebootafterexit(). If a program doesn't quit to the ProDOS 8 dispatcher but rather reboots the machine after exit then a plain vanilla ProDOS 8 doesn't make use of the Language Card bank 2 at all.

LC address: $D000, LC size: $3000

For plain vanilla DOS 3.3 which doesn't make use of the Language Card at all.

4. Linker configurations

The ld65 linker comes with a default config file for the Apple ][, which is used via -t apple2. The apple2 package comes with additional secondary linker config files, which are used via -t apple2 -C <configfile>.

4.1 default config file (apple2.cfg)

Default configuration for a binary program.

Parameters:

STARTADDRESS: Program start address

Default: $803. Use -S <addr> to set a different start address.

__EXEHDR__: Executable file header

Default: DOS 3.3 header (address and length). Use -D __EXEHDR__=0 to omit the header.

__STACKSIZE__: C runtime stack size

Default: $800. Use -D __STACKSIZE__=<size> to set a different stack size.

__HIMEM__: Highest usable memory address presumed at link time

Default: $9600. Use -D __HIMEM__=<addr> to set a different highest usable address.

__LCADDR__: Address of code in the Language Card

Default: $D400. Use -D __LCADDR__=<addr> to set a different code address.

__LCSIZE__: Size of code in the Language Card

Default: $C00. Use -D __LCSIZE__=<size> to set a different code size.

4.2 apple2-system.cfg

Configuration for a system program running on ProDOS 8 and using the memory from $2000 to $BEFF.

Parameters:

__STACKSIZE__: C runtime stack size

Default: $800. Use -D __STACKSIZE__=<size> to set a different stack size.

__LCADDR__: Address of code in the Language Card

Default: $D400. Use -D __LCADDR__=<addr> to set a different code address.

__LCSIZE__: Size of code in the Language Card

Default: $C00. Use -D __LCSIZE__=<size> to set a different code size.

4.3 apple2-overlay.cfg

Configuration for overlay programs with the up to nine overlays. The overlay files don't include the DOS 3.3 header. See samples/overlaydemo.c for more information on overlays.

Parameters:

STARTADDRESS: Program start address

Default: $803. Use -S <addr> to set a different start address.

__EXEHDR__: Executable file header

Default: DOS 3.3 header (address and length). Use -D __EXEHDR__=0 to omit the header.

__STACKSIZE__: C runtime stack size

Default: $800. Use -D __STACKSIZE__=<size> to set a different stack size.

__HIMEM__: Highest usable memory address presumed at link time

Default: $9600. Use -D __HIMEM__=<addr> to set a different highest usable address.

__LCADDR__: Address of code in the Language Card

Default: $D400. Use -D __LCADDR__=<addr> to set a different code address.

__LCSIZE__: Size of code in the Language Card

Default: $C00. Use -D __LCSIZE__=<size> to set a different code size.

__OVERLAYSIZE__: Size of code in the overlays

Default: $1000. Use -D __OVERLAYSIZE__=<size> to set a different code size.

4.4 apple2-asm.cfg

Configuration for a assembler programs which don't need a special setup.

Parameters:

STARTADDRESS: Program start address

Default: $803. Use -S <addr> to set a different start address.

__EXEHDR__: Executable file header

Default: No header. Use -u __EXEHDR__ apple2.lib to add a DOS 3.3 header (address and length).

5. ProDOS 8 system programs

ProDOS 8 system programs are always loaded to the start address $2000. For cc65 programs this means that the 6 KB from $800 to $2000 are by default unused. There are however several options to make use of that memory range.

5.1 LOADER.SYSTEM

The easiest (and for really large programs in fact the only) way to have a cc65 program use the memory from $800 to $2000 is to link it as binary (as opposed to system) program using the default linker configuration apple2.cfg with __HIMEM__ set to $BF00 and load it with the LOADER.SYSTEM utility. The program then works like a system program (i.e. quits to the ProDOS dispatcher).

Using LOADER.SYSTEM is as simple as copying it to the ProDOS 8 directory of the program to load under name <program>.SYSTEM as a system program. For example the program MYPROG is loaded by MYPROG.SYSTEM.

5.2 Heap

If the cc65 program can be successfully linked as system program using the linker configuration apple2-system.cfg, but uses the heap either explicitly or implicitly (i.e. by loading a driver) then the memory from $800 to $2000 can be added to the heap by calling _heapadd ((void *) 0x0800, 0x1800); at the beginning of main().

5.3 ProDOS 8 I/O buffers

ProDOS 8 requires for every open file a page-aligned 1 KB I/O buffer. By default these buffers are allocated by the cc65 runtime system on the heap using posix_memalign(). While this is generally the best solution it means quite some overhead for (especially rather small) cc65 programs which do open files but don't make use of the heap otherwise.

The apple2 package comes with the alternative ProDOS 8 I/O buffer allocation module apple2-iobuf-0800.o which uses the memory between $800 and the program start address for the 1 KB I/O buffers. For system programs (with start address $2000) this results in up to 6 I/O buffers and thus up to 6 concurrently open files.

While using _heapadd() as described in the section above together with the default I/O buffer allocation basically yields the same placement of I/O buffers in memory the primary benefit of apple2-iobuf-0800.o is a reduction in code size - and thus program file size - of more than 1400 bytes.

Using apple2-iobuf-0800.o is as simple as placing it on the linker command line like this:

cl65 -t apple2 -C apple2-system.cfg myprog.c apple2-iobuf-0800.o

6. Platform specific header files

Programs containing Apple ][ specific code may use the apple2.h header file.

6.1 Apple ][ specific functions

The functions listed below are special for the Apple ][. See the function reference for declaration and usage.

6.2 Hardware access

There's currently no support for direct hardware access. This does not mean you cannot do it, it just means that there's no help.

7. Loadable drivers

The names in the parentheses denote the symbols to be used for static linking of the drivers.

7.1 Graphics drivers

a2.lo.tgi (a2_lo_tgi)

This driver features a resolution of 40×48 with 16 colors.

The function tgi_apple2_mix() allows to activate 4 lines of text. The function clears the corresponding area at the bottom of the screen.

a2.hi.tgi (a2_hi_tgi)

This driver features a resolution of 280×192 with 8 colors and two hires pages. Note that programs using this driver will have to be linked with -S $4000 to reserve the first hires page or with -S $6000 to reserve both hires pages.

The function tgi_apple2_mix() allows to activate 4 lines of text. The function doesn't clear the corresponding area at the bottom of the screen.

In memory constrained situations the memory from $803 to $1FFF can be made available to a program by calling _heapadd ((void *) 0x0803, 0x17FD); at the beginning of main(). Doing so is beneficial even if the program doesn't use the the heap explicitly because loading the driver (and in fact already opening the driver file) uses the heap implicitly.

7.2 Extended memory drivers

a2.auxmem.emd (a2_auxmem_emd)

Gives access to 47.5 KB RAM (190 pages of 256 bytes each) on an Extended 80-Column Text Card.

Note that this driver doesn't check for the actual existence of the memory and that it doesn't check for ProDOS 8 RAM disk content!

7.3 Joystick drivers

a2.stdjoy.joy (a2_stdjoy_joy)

Supports up to two standard analog joysticks connected to the game port of the Apple ][.

7.4 Mouse drivers

a2.stdmou.mou (a2_stdmou_mou)

Driver for the AppleMouse II Card. Searches all Apple II slots for an AppleMouse II Card compatible firmware. The default bounding box is [0..279,0..191].

Programs using this driver will have to be linked with -S $4000 to reserve the first hires page if they are intended to run on an Apple ][ (in contrast to an Apple //e) because the AppleMouse II Card firmware writes to the hires page when initializing on that machine.

Note that the Apple ][ default mouse callbacks support text mode only.

7.5 RS232 device drivers

a2.ssc.ser (a2_ssc_ser)

Driver for the Apple II Super Serial Card. Supports up to 19200 baud, hardware flow control (RTS/CTS) and interrupt driven receives. Note that because of the peculiarities of the 6551 chip transmits are not interrupt driven, and the transceiver blocks if the receiver asserts flow control because of a full buffer.

The driver defaults to slot 2. Call ser_apple2_slot() prior to ser_open() in order to select a different slot. ser_apple2_slot() succeeds for all Apple II slots, but ser_open() fails with SER_ERR_NO_DEVICE if there's no SSC firmware found in the selected slot.

8. Limitations

8.1 DOS 3.3

Although the standard binaries generated by the linker for the Apple ][ generally run both on DOS 3.3 (with Applesoft BASIC) and on ProDOS 8 (with BASIC.SYSTEM) there are some limitations for DOS 3.3:

Disk File I/O

There's no disk file I/O support. Any attempt to use it yields an error with errno set to ENOSYS. This implicitly means that loadable drivers are in general not functional as they depend on disk file I/O. Therefore the statically linked drivers have to be used instead.

Interrupts

There's no interruptor support. Any attempt to use it yields the message 'FAILED TO ALLOC INTERRUPT' on program startup. This implicitly means that joystick, mouse and RS232 device drivers are not functional as they depend on interrupts.

8.2 Direct console I/O

Color

The Apple ][ has no color text mode. Therefore the functions textcolor(), bgcolor() and bordercolor() have no effect.

9. Other hints

9.1 Passing arguments to the program

Command line arguments can be passed to main() after BLOAD. Since this is not supported by BASIC, the following syntax was chosen:

]CALL2051:REM ARG1 " ARG2 IS QUOTED" ARG3 "" ARG5

  1. Arguments are separated by spaces.
  2. Arguments may be quoted.
  3. Leading and trailing spaces around an argument are ignored. Spaces within a quoted argument are allowed.
  4. The first argument passed to main is the program name.
  5. A maximum number of 10 arguments (including the program name) are supported.

9.2 Interrupts

The runtime for the Apple ][ uses routines marked as .INTERRUPTOR for ProDOS 8 interrupt handlers. Such routines must be written as simple machine language subroutines and will be called automatically by the interrupt handler code when they are linked into a program. See the discussion of the .CONDES feature in the assembler manual.

9.3 DIO

Drive ID

The function dio_open() has the single parameter device to identify the device to be opened. Therefore an Apple II slot and drive pair is mapped to that device according to the formula

device = slot + (drive - 1) * 8

so that for example slot 6 drive 2 is mapped to device 14.

Sector count

The function dio_query_sectcount() returns the correct sector count for all ProDOS 8 disks. However for any non-ProDOS 8 disk it simply always returns 280 (which is only correct for a 140 KB disk). This condition is indicated by the _oserror value 82.

9.4 Specifying file types for fopen

Explanation of File Types

ProDOS associates a file type and an auxiliary type with each file. These type specifications are separate from the file's name, unlike Windows which uses the file name's suffix (a.k.a. extension) to specify the file type. For example, .exe, .doc, or .bat. The ProDOS low-level Machine-Language Interface (MLI) functions for creating and opening files require these types to be specified. And if they don't match with the file being opened, the operation may fail.

In contrast, the ISO C function fopen() and the POSIX function open() have no parameter to specify either a file type or an auxiliary type. Therefore, some additional mechanism for specifying the file types is needed.

Specifying the File Type and Auxiliary Type

There are two global variables provided that allow the file type and auxiliary type to be specified before a call to fopen() or open(). They are defined in apple2_filetype.h:

  extern unsigned char _filetype;  /* Default: PRODOS_T_BIN */
  extern unsigned int _auxtype;    /* Default: 0            */
  

The header file apple2_filetype.h also defines many values that can be used to set these variables. It is included in apple2.h, which is in turn included in apple2enh.h. So it isn't necessary to include it directly. Just include one of apple2.h or apple2enh.h.

Example

A text file cannot be created with just the standard C functions because they default to the binary type PRODOS_T_BIN. The _filetype variable must be set to PRODOS_T_TXT to create a text file.

For a text file, _auxtype specifies the record length. A zero record length text file is referred to as a sequential text file. This is equivalent to text files on other operating systems, except that the line terminator is a carriage return instead of a line-feed (Linux/BSD/MacOS) or carriage return, line-feed pair (Windows).

The "sequential" text file terminology is in contrast to a "random-access" text file which would have a fixed-length, non-zero record length, so that the file position of any individual record can be calculated.

For this example, the _auxtype does not need to be set because it defaults to the desired value, which is zero. To be more explicit, _auxtype can also be set to PRODOS_AUX_T_TXT_SEQ which is defined as zero.

    #include <stdio.h>
    #include <string.h>
    #include <errno.h>
    #include <apple2.h>

    void main(void)
    {
        FILE *out;
        char *name = "MY.FAVS";

        /*-----------------------------*/

        _filetype = PRODOS_T_TXT;
        _auxtype  = PRODOS_AUX_T_TXT_SEQ;

        /*-----------------------------*/

        if ((out = fopen(name, "w")) != NULL) {
            fputs("Jorah Mormont\r", out);
            fputs("Brienne of Tarth\r", out);
            fputs("Daenerys Targaryen\r", out);
            fputs("Sandor Clegane\r", out);
            if (fclose(out) == EOF) {
                fprintf(stderr, "fclose failed for %s: %s", name, strerror(errno));
            }
        }
        else {
            fprintf(stderr, "fopen failed for %s: %s", name, strerror(errno));
        }
    }
  

10. 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.