GNU Pascal (GPC) is compatible to version 7 of Borland Pascal (BP) to a large extent and comes with portable replacements of the BP standard units.
However, BP is a 16-bit compiler while GPC is a 32/64-bit compiler, so the size of the Integer type, for instance, is 16 bits in BP, but at least 32 bits in GPC. If a BP program has been designed with portability in mind from the ground up, it may work with GPC without any change. Programs which rely on byte order, on the internals or sizes of data types or which use unportable things like interrupts and assembler code, will need to be changed. The following section lists the possible problems with solutions.
The GPC Run Time System (RTS) is fairly complete, and you can use all libraries written for GNU C from GNU Pascal, so there is much less need to use unportable constructs than there was in BP. (For example, BP's Turbo Vision library uses assembler to call a local procedure through a pointer. With GPC you can do this in Pascal just as with global procedures.) Please do not throw away the advantage of full portability by sticking to those workarounds.
We have successfully ported real-world projects (with several 10000s of lines) from BP to GPC, so this is possible for you, too.