The history of calling conventions, part 2

Foreshadowing: This information will actually be useful in a
future discussion. Well, not the fine details, but you may
notice something that explains… um… it’s hard to describe.
Just wait for it.


Curiously, it is only the 8086 and x86 platforms that have
multiple calling conventions. All the others have only one!


Now we’re going deep into trivia that absolutely nobody remembers
or even cares about: The 32-bit calling conventions you don’t
see any more.

All


All of the processors listed here are RISC-style,
which means there are lots of registers, none of which
have any particular meaning. Well, aside from the zero
register which is hard-wired to zero.
(It turns out zero is a very handy number to have readily available.)
Any meanings attached to the registers are those
imposed by the calling convention.


As a throwback to the processors of old,
the “call” instruction stores the return address in a
register instead of being pushed onto the stack.
A good thing, too, since the processor doesn’t officially
know about a “stack”, it being a construction of the
calling convention.


As always, registers or stack space used to pass parameters
may be used as scratch by the called function, as can the
return value register.


You may notice that all of the RISC calling conventions
are basically the same. Once again, evidence that the 8086/x86
is the weirdo. A wildly popular weirdo, mind you.

The Alpha AXP


The Alpha AXP (“AXP” being yet another of those
faux-acronyms that officially doesn’t stand for anything)
has 32 integer registers,
one of which is hard-wired to zero.
By convention,
one of the registers is the “stack pointer”, one is the
“return address” register; and two others
have special meanings unrelated to parameter passing.


The first six parameters are passed in registers, with the
remaining parameters on the stack. If the function is
variadic, the parameters can be spilled onto the stack
so they can be accessed as an array.


Seven other registers are preserved across calls,
one is the return value,
and the remaining thirteen
are scratch.
1 zero register +
1 stack pointer +
1 return address +
2 special +
6 parameters +
7 preserved +
1 return value +
13 scratch =
32 total integer registers.


Function names on the Alpha AXP are completely undecorated.

The MIPS R4000


The first four parameters are passed in a0, a1, a2 and
a3; the remainder are spilled onto the stack.
What’s more, there are four “dead spaces” on the stack
where the four register parameters “would have been”
if they had been passed on the stack. These are for
use by the callee to spill the register parameters
back onto the stack if desired. (Particularly handy
for variadic functions.)


Function names on the MIPS are completely undecorated.

The PowerPC


The first eight parameters
are passed in registers (r3 through r10), and the return address
is managed manually.


I forget what happens to parameters nine and up…


Function names on the PowerPC are decorated by prepending two periods.


Postclaimer:
I haven’t had personal experience with the MIPS or PPC
processors, so my discussion of those processors may be
a tad off, but the basic idea I think is sound.
[The Old New Thing]

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