Re: [9fans] assembly syntax in plan 9

[Alexander Kapshuk <alexander.kapshuk@gmail.com>]

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On Mon, Jan 16, 2012 at 3:08 PM, Charles Forsyth
<charles.forsyth@gmail.com>wrote:

>
> 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
>>
>>
>
thanks;

i'll look into that;

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