IBM uses the zSeries designation to indicate an implementation of the system architecture that includes the System/360 (1964), System/370 (1970), System/390 (1990) and others.
The zSeries CPU uses a proprietary CISC architecture unrelated to other processors such as the PowerPC which is used in IBM's iSeries (AS/400) and pSeries (RS/6000) systems. Note that these designations are basically marketing terms and as such are somewhat fluid.
The original S/360 architecture had 32-bit integer registers and a 24-bit address space. This has been extended first to support a 31-bit address space and later to support 64-bit registers and address space. In addition many models support paged/expanded memory.
In the 1990s Linux was ported onto the S/390, almost invariably running as a guest/virtualised operating system in the context of a "traditional" host OS. In addition GCC was ported, the paper below discussing some of the problems that were encountered.
Notable points from this paper are that older versions of the S/390 and its predecessors had two significant limitations that were tolerable when the systems were programmed in assembler but caused significant problems for automatic code generation:
- Literals had to be in tables rather than inline. Tables were limited to 4K.
- There were no PC-relative jumps.
These limitations were likely to be particularly severe if a compiler was translating machine-generated source, where functions/procedures might be very large.
More recent versions of the S/390, probably the G3 manufactured after September 1996, enhance the 32-bit instruction set to allow inline literals and PC-relative jumps. These restrictions do not exist on more recent implementations of the architecture, e.g. the 64-bit zSeries systems, note that GCC v4 and Linux 2.6 appear to assume that the hardware is at least G5 i.e. no older than 2000.
It is possible to simulate a 32- or 64-bit system using the Hercules emulator, and IBM makes machine time available to developers porting code to their systems.
Whether this is relevant to FPC/Lazarus is arguable since the architecture is already supported on Linux by GCC Pascal, Pascal-XSC, and on other operating systems (as a commercial product) by at least IBM Pascal/VS and/or VS Pascal. Paul Robinson (see below) also points out that Lazarus might be irrelevant on this platform since many facilities provided as standard on PCs and workstations have no counterpart on "classic" IBM operating systems.
Paul Robinson, Lead Programmer and Chief Cook and Bottle Washer for Viridian Development Corporation is creating a cross-compiler for this architecture, and is documenting in real time what's involved. There is a link to his current work at the bottom of this document.
Refer to discussion threads in the fpc-devel mailing list  which includes discussion of the desirability of this port (unanimously agreed to be a good idea), selection of target hardware and operating system, and problems which might be caused by the EBCDIC character set used by older IBM mainframe operating systems. Also refer to Qemu and other emulators#Debian zSeries Guest using Hercules, without VM for discussion of running zSeries Linux on a PC using the Hercules emulator.
Some of the problems with e.g. limitations on inline literals can potentially be gotten around by careful use of registers in the code. (You can get around most problems if you have spare registers). There are trade-offs: basically registers 3-12 are available for any use; each register can address up to 4K of code or data; you can use multiple registers to index to the particular portion provided you don't need to manipulate any chunk of data exceeding 4K at a time. One possibility is to use some registers for code and some for data, by moving up from 3 for either code or for data, and moving down from 12 for the other. If it's not too big in both directions, you should be okay.
Nevertheless, one possible use of doing a cross-compiler using Free Pascal for the 370/390/zSystem is to provide a reference on how to do so for other potential architectures. As noted above, Paul Robinson is doing a cross-compiler for the zSeries using Free Pascal and is explaining how he's going about it as he does so. An introduction begins with Part 1 and goes from there.