Re: [Tack-devel] back-end tables, assemblers, and front-ends

[David G. <dg...@co...>]

Gregory T. (tim) Kelly wrote:
[...]
> STMFD R12<,{R13,R14}
> MOV R13,R12

I think this is using a radically different ABI than most systems use. Th=
is
looks like it's using R12 as the stack pointer and R13 as the frame point=
er.
This means that after this:

[...]
> SUB R12,R12,#8
> STMFD R12<,{R7,R6}

=2E..it's indexing four locals off R12 (two uninitialised variables and t=
he old
values of r6 and r7, and this:

[...]
> LDR R7,[R13,#8]

=2E..is accessing function parameters, which seem to be passed on the sta=
ck. The
reason why you're not seeing the arguments in r0-r4 is that they're just =
not
there!

The stack layout they're using seems to look like this (assuming sp is as=
 it
is after the function prologue):

sp+0C max
sp+08 min
sp+04 old fp  \ return frame
sp+00 old pc  /
fp+0C unused local
fp+08 unused local
fp+04 old r6
fp+00 old r7

This is a completely alien ABI to the entire rest of the ARM world, inclu=
ding
RiscOS, which the port was done *for*... I think they can get away with i=
t
because RiscOS executables are standalone binary blobs that communicate w=
ith
the outside world via system calls, which switch stacks. So binary
compatibility is unnecessary.

It does seem to be credited to "an anonymous student at the Vrije Univers=
iteit"...

--=20
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|  dg...@co...    | scintillating, many fauceted scarlet emerald..."
| (dg...@ta...) | --- Jim Theis, _The Eye of Argon_ (spelling
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