Porting the Unix rtl to a new target
Very global instructions to porting FPC to a *nix clone on a supported architecture:
Setting up a new target in the compiler
(typically only done when the RTL already works, and the first program runs, using a similar target)
A few things must be done for a new target: (ARCH-OS combination, assuming ARCH is supported)
- Define enumerations (tsystem) for this target in systems.pas. Only append, changing the order breaks compability.
- A record with some parameters for the ARCH-OS combination must be defined in compiler/systems/i_*. If you only ported the OS to a new architecture, you can add it to the existing i_OS file.
- An object that handles calling the linker must be registered. These objects reside in compiler/systems/t_*. Reuse an existing one, or create a new one. Registering must be done in $ARCH/cputarg.pas file, by including the t_ unit. Also don't forget to review the initialisation code of the t_ unit if you adapt an existing one.
- the startupcode of systems.pas must be adapted to select your target as default, under ifdef for the default OS-ARCH.
If you get stuck on some linking problem, try to link a simple C program with some verbosity options turned on. (either static or linking to libc, depending on what you are trying).
A more detailed list of needed changes inside the compiler can be found at Porting Free Pascal.
Porting the RTL
Porting the Unix rtl is typically more work then the stuff above.
Typically it boils down to take the closest relative (rtl/freebsd/, and start checking and converting)
There are a few typical problem points:
- ftruncate, lseek, mmap and other calls that have 64-bits arguments and returnvalues. The exact handling and calling conventions of these changes per unix flavour and even per architecture.
- The startup code must be adapted. Twice. One for linking without libc (prt0), one for with libc (cprt0). Others (like gprt0) are only for profiling and basically a slightly modded cprt0
- signal to runtime error procedure must be reviewed. The way to get access to the registerstate is highly arch + OS dependant. The texception* tests in the testsuite are good tests for this.
The startup code mods are typically done by compiling the crt0 sources with -S to generate assembler AND compare with a objdumped small static binary. (the latter will reveal stuff like ELF identifiers), and checking with a comparable FPC existing startup code.
Typical modifications both prt0 and cprt0.as:
- reserve room for a few variables (FPC_*, operatingsystem*)
- fixup copro status word (x86)
- move argc,argv,envp to FPC namespace.
- Empty framepointer before calling main (xorl %ebp,%ebp)
- beware of ELF identifiers.
prt0 can strip the whole libc init/finalisation, cprt0 must preserve it.