Re: [9fans] assembly syntax in plan 9

[Charles Forsyth <charles.forsyth@gmail.com>]

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You should read /sys/doc/asm.pdf first.
careful: TOS is only for 68k. nothing else defines or uses it.

Plan 9 doesn't use a base pointer, because everything can be addressed
relative to the stack pointer,
and the loader keeps track of the SP level. thus FP is a virtual register,
that the loader implements
by replacing offsets relative to it by the current appropriate offset from
the hardware stack pointer register (whatever
that might be on a given platform). That's esp on the x86. the TEXT
directive specifies the space a function
requires for its stack frame, and the loader then adds appropriate code at
start and end to provide it.
0(FP) is the first argument, 4(FP) is the second, and so on. 0(SP) is the
bottom of the current frame,
and 0(SP), 4(SP) etc are referenced to build the arguments for outgoing
calls (but that space must
be accounted for in the TEXT directive).

(it's probably not very different in effect from -fno-frame-pointer or
whatever it is for gcc,
which also doesn't use ebp except that is implemented entirely by the
compiler.)

On 16 January 2012 12:30, Alexander Kapshuk <alexander.kapshuk@gmail.com>wrote:

> i have a question about putting things on the stack for x86 arch under
> plan 9...
>
> under unix/linux, when defining a function, i would:
> (1). push the address the base pointer is pointing to prior to this
> function being called, onto the stack; e.g. pushl %ebp
> (2). then i would have the base pointer point to the current stack
> pointer; e.g. movl %esp, %ebp
> (3). then i would allocate space on the stack for local variables, if any;
> e.g. subl $n, %esp;
> (4). then follows the function body;
> to return from the function i would:
> (1). restore the stack pointer; e.g. movl %ebp, %esp;
> (2). restore the base pointer, e.g. popl %ebp;
> (3). then return to the calling function;
>
> i searched the 9fans archives for posts on assembly programming under plan
> 9; found some bits and pieces; e.g. in one of the posts it was mentioned
> that BP is a general purpose register, not the base pointer; and that FP is
> what ebp is under unix/linux;
>
> in the paper for the plan 9 assembler, it says that there are three
> registers available to manipulate the stack, FP, SP, and TOS; would the
> following comparison stand true then?
> plan9    unix/linux
> -------     -------------
> FP        EBP
> SP        -4(%EBP)...-n(%EBP) /* local variables */
> TOS     ESP
>
> thanks;
>
> sasha kapshuk
>
>

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