Steve wrote:
Mark Morgan Lloyd wrote
Otherwise we also rely on external tools (mostly the GNU linker)
here. So as a first step you'd choose the approach of using an
external assembler and linker, because simply calling a third party
utility is easier than completely implementing an internal assembler
and linker.
With the caveat here that as I understand it experienced IBM programmers
avoid the GNU assembler like the plague, since it doesn't have anything
like the sort of macro facilities they're used to. By implication, that
would imply that they prefer to avoid the GNU linker and related tools
as well.
There is a problem inherent in this discussion; zArch is not one
environment!
It's one architecture supporting multiple operating systems, much like the
386/486/586/686 etc supports Linux or DOS or OS/2 or Windows (all 37
versions)
etc. zArch has MVS (in all it's varieties) VM, DOS, LINUX, MFT, MVT,
MUSIC/SP,
and loads of other more niche stuff. In addition to all this, later
versions of
MVS supply a POSIX compliant shell called OMVS. GNU anything is
available in
hardly any of these environments even if we can handle the brain-dead
assembler.
GCC plus the basic utilities are definitely ported to VM/370 and
MUSIC/SP (i.e. I've run them), and as far as I know to the others. In
all cases possibly subject to memory restrictions, i.e. might need more
than the standard 16Mb supplied by older operating systems for large
jobs (that's where the /380 patch to Hercules comes into it).
Just to name a few: you'll need to get parameter passing for functions
correctly
Which leads to another issue: the 370 is a register-based system without
a stack as understood today. Parameters are mostly passed in registers,
but this is largely hidden since supervisor calls etc. are usually
hidden in macros.
I am an MVS person so I can't speak for the other lot but parameter passing
is mostly done in storage. The standard linkage conventions used allow for
two 32-bit signed or unsigned integers (64-bit in later models). Anything
else is passed in a storage area pointed to by register 1. Where this
storage
area comes from is complex and variable. My guess that the other IBM
systems
have similar models. A further guess is that Linux based code allocates an
area of storage, points a register at it, write a couple of mickey mouse
macros and bingo, a stack.
Don't guess, see http://wiki.lazarus.freepascal.org/ZSeries which apart
from anything else has links to the original GCC implementation notes
which in part determine what architecture variants Linux supports.
However there also appear to be later ports which support older CPU
variants, see link below.
My own feeling is that it would be best to start targeting a late-model
390, which does have a stack etc., and to use the standard GNU assembler
and linker (as and ld)/initially/ targeting Linux. Any other
combination (i.e. a proprietary assembler etc. with an antique MVS as
target) is going to cause nothing but grief, since it makes it very
difficult for developers skilled with FPC but not with IBM mainframes to
give any practical help.
Late-model 390's have a stack, but not as you know it. It's not something
you can go around lobbing arbitrary data at. It is reserved for data saved
during subroutine linkage using the appropriate hardware instruction
(Branch
and Stack). This includes various register sets, PSW status info etc)
and an
additional two 32-bit signed or unsigned integers (64-bit in later models).
The examples at
http://wiki.lazarus.freepascal.org/Assembler_and_ABI_Resources#zSeries_.28S.2F390.29
show a significant simplification of the code when a newer architecture
is specified.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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