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gnu assembly pointer access

Started by 666_1337 Mar 15, 2003 at 12:56 PM 1 reply 450+ views

Original Post

Mar 15, 2003 at 12:56 PM

hi, i''m writing a makefile-generator that uses the cpuid-instruction to get information on which extentions are supported on the platform executing it. i''ve got a struct with all the features in the there-written order in the memory, so getting through the struct with an incrementing pointer (and an incrementing bitmask) would do it.

  
struct _pfeautures
   {
    int fpu,
	vme,
        de,
	pse,
	tsc,
	msr,
	pae,
	mce,
	cx8,
	apic,
	nothing_1,
	sep,
	mtrr,
	pge,
	mca,
	cmov,
	pat,
	pse36,
	psn,
	clfsh,
	nothing_2,
	ds,
	acpi,
	mmx,
	fxsr,
	sse,
	sse2,
	ss,
	htt,
	tm,
	nothing_3,
	sbf;
   };

however, the problem is that i cannot access a memory location with anything like "mov %%eax, %%ecx+%0". somebody knows a solution? how can i access the memory address of "%0"? ... in gnu''s assembly syntax

George2

Mar 15, 2003 at 1:09 PM

It''s called AT&T assembly syntax and a quick search on google found this

from the link :
Intex Syntax

mov eax,[ebx]
mov eax,[ebx+3]

AT&T Syntax

movl (%ebx),%eax
movl 3(%ebx),%eax

666_1337

Author

666_1337

Mar 16, 2003 at 7:30 AM

i think you misunderstood me.

    
asm("mov $0x01, %%eax\n"
    "cpuid\n"
    ... //analyze %eax

    "mov $0x01, %%eax\n" //store the bitmask that will be shifted in %eax

    "mov $0x20, %%ecx\n" //nothing important in %ecx. i'll use it as counter.

    "movd %%ebx, %%mm0\n" //make a backup of %ebx in the mmx register

    "start: movd %%mm0, %%ebx\n" //restore %ebx

    "and %%eax, %%ebx\n" //apply the bitmask on %ebx

    "shl $0x01, %%eax\n" //shift the bitmask

    "mov %%eax, %12\n" //%12 is the variable "fpu"

    increment_pointer_to_%12 //this is what i need

    "dec %%ecx\n" // and

    "jnz start\n" // loop

    :..., cpu_info.cpu_features.fpu; //this is the lowest address

    :...
   );

i need to be able to determine the memory address of the 12th parameter for incrementing.

[edited by - 666_1337 on March 16, 2003 8:33:23 AM]

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