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 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 those 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.
It is possible to simulate a 32- or 64-bit system using the Hercules emulator, and IBM make 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 by GCC Pascal and possibly by Pascal-XSC.