Amiga

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This article applies to Amiga only.

See also: Multiplatform Programming Guide

This page is about the so called 'classic Amiga' version of Free Pascal, which means Motorola 680x0 CPU based systems running AmigaOS 3.x and below. For MorphOS, AROS and AmigaOS4 versions, see the relevant pages.

Please note, that the information detailed here is specific to version Free Pascal 2.x and above, and does *not* apply to the old 0.99.x and 1.0.x versions of Free Pascal for Amiga, still available on various Amiga download sites.

After a long break, Motorola 68k CPU series and classic Amiga is supported again in Free Pascal 3.0. Please note that to successfully use the compiler natively, a very fast 68k system is required, at least expanded with a Motorola 68060 processor and 128MB of memory or more and a Hard Disk controller with DMA support, or an emulator with a fast JIT-compiler like WinUAE, or a next-gen Amiga-compatible system. Large parts of the classic Amiga support was developed under MorphOS. Cross compiling from other architectures and operating systems is supported.

Identification

To identify classic Amiga exclusively during compile-time, use {$IFDEF AMIGA68K}. To identify classic Amiga or AmigaOS4, use {$IFDEF AMIGA}. To identify any Amiga-like system including AROS and MorphOS, use {$IFDEF HASAMIGA}. This is similar to HASUNIX which is defined across Unix systems. Please note that HASAMIGA define is only available in Free Pascal 3.0 and up, and AMIGA68K is only available in Free Pascal 3.0.2 and up.

Stack

Under Amiga, the default stack is set to 256 KiB. Stack size can be set directly by the programmer with a compiler switch (-Cs), or with the {$MEMORY} directive. The memory area for the stack is dynamically allocated on program startup, so if you set this value too high, in low memory situations your program may exit immediately on startup without any further notice.

Please note that increasing stack size with the 'stack' utility will *NOT* work for your app, because the RTL startup code will replace the existing stack with the size specified by the directive, using the exec.library/StackSwap function.

SysCalls

Introduction to SysCalls

Free Pascal supports generating Amiga-style library calls. You don't need to use additional hand-written assembly to call any library function. However, you must declare every function you're going to use in the following way:

 Var 
   my_LibBase: Pointer;
 
 function my_OldCall(param1: LongInt location 'd0',
                     param2: LongInt location 'd1'): LongInt; 
          SysCall my_LibBase 1234; { Amiga-style call }

Where my_LibBase is the library base returned by exec's OpenLibrary() call, and 1234 is the call offset. Please note that offset values in Free Pascal must be specified as positive values, and not negative as shown in the Amiga SDK. my_LibBase can be a typed pointer, a dword or a void pointer. It's recommended to use PLibrary or the library's own type if one exists or possible.

Other hints about libraries and Syscalls

MOST IMPORTANT: While creating your own interface units for libraries, you must open the libraries you're going to use explicitly before using any function from them. Don't forget to close all libraries before you exit. The interface units bundled with the compiler will auto open and close the respective libraries (unless otherwise noted).

Naming conventions

This section describes the differences between the official Amiga SDK, which is for C and assembly language, and Amiga-specific units in FPC RTL.

Constants

System constants are named exactly like in Amiga SDK.

Structures

System structures are named similar to those in the Amiga SDK, but follow the Pascal convention about type naming, so:

  • structure names has a T before the name
  • each type has a pointer to the type with P before the name.

The following example should make things trivial to understand:

 struct Task is equal to TTask
 struct Task * is equal to PTask

Alignment

Record elements are aligned to WORD (2 bytes) under Amiga by default. Use {$PACKRECORDS 4} if you need DWORD aligned structures. For byte aligned records, use a packed record.

Threading

Threading is supported on Amiga since Free Pascal 3.0, using the AThreads unit. Read there for possible caveats and unsupported features.

Things good to know

Here are a few hints, which I think are good to remember, when using FPC on Amiga.

  • A filesystem which supports file name length above 30 characters is highly recommended. Lack of it may cause unexpected results or compilation failures, especially in case of large projects like the compiler itself.
  • On program exit, System unit will close all files opened via its own functions, so it's recommended to use them in most cases.
  • Do not mix FPC and OS file functions on a single file. It may work in some cases, but that's purely a coincidence. It's bad programming and can stop working anytime without notice.
  • On program exit, the heap manager will free all memory allocated via its own functions. It uses a memory pool to avoid fragmentation. It also makes debugging easier when using heaptrc unit. Using OS memory functions directly is not recommended, unless you explicitly require memory areas which should stay in memory after your process has exited (like pointers passed to other processes).
  • Do not mix FPC and OS memory functions on a pointer. It won't work and will only cause crashes and/or memory leaks.

Linker

Some version of Free Pascal on Amiga, including latest SVN trunk versions default to vlink by Frank Wille as the default linker, when running natively on Amiga. The cross-compilers still default to GNU ld. vlink is open source, and it is available here. Binaries are available as part of the vbcc compiler package.

You can change the linker back to GNU ld by adding -XV- argument when compiling. For cross compilers -XV argument enables vlink.

More information

More information regarding Free Pascal and Amiga can be read on the Free Pascal 4 Amiga wiki. This wiki also contains links to (unofficial) nightly downloads and other (additional) information for Amiga.