Is it possible to branch to an absolute address ?

Is it possible to branch to an absolute address ?

[Amarnath]

HI,

I have a query in my mind. Please check the following sample assembly file:

----------- Snip ---------------

	.file	1 "TestingBranch.c"
	.set	mips16

gcc2_compiled.:
__gnu_compiled_c:
	.text
	.align	2
	.globl	func

	.align	2
	.globl	main
	.ent	main
main:
	.frame	$17,16,$31		# vars= 8, regs= 2/0, args= 0,
extra= 0
	.mask	0x80020000,-4
	.fmask	0x00000000,0

      b           0x4000            # >>> branch with absolute operand
      move   	$sp,$17	
$L2:
	.end  	main

	.ent  	func
	func:
	.frame	$17,16,$31		# vars= 8, regs= 1/0,
args= 0, extra= 0
	.mask	0x00020000,-8
	.fmask	0x00000000,0
	
	$L1:
		
	.end	func

-------------- End Snip ----------------

- In the above, why GCC was not assembling the branch instruction with
absolute value as operand in it?

- Is there any restriction in branching towards absolute value in ELF?

- Is it possible in ELF to emit relocation information without any symbol
information associated with it?


Please help me in the above.

Thank you.

Regards,
Amarnath M

Re: Is it possible to branch to an absolute address ?

[Thiemo Seufer]

Amarnath wrote:
> HI,
> 
> I have a query in my mind. Please check the following sample assembly file:
> 
> ----------- Snip ---------------
> 
> 	.file	1 "TestingBranch.c"
> 	.set	mips16
> 
> gcc2_compiled.:
> __gnu_compiled_c:
> 	.text
> 	.align	2
> 	.globl	func
> 
> 	.align	2
> 	.globl	main
> 	.ent	main
> main:
> 	.frame	$17,16,$31		# vars= 8, regs= 2/0, args= 0,
> extra= 0
> 	.mask	0x80020000,-4
> 	.fmask	0x00000000,0
> 
>       b           0x4000            # >>> branch with absolute operand
>       move   	$sp,$17	
> $L2:
> 	.end  	main
> 
> 	.ent  	func
> 	func:
> 	.frame	$17,16,$31		# vars= 8, regs= 1/0,
> args= 0, extra= 0
> 	.mask	0x00020000,-8
> 	.fmask	0x00000000,0
> 	
> 	$L1:
> 		
> 	.end	func
> 
> -------------- End Snip ----------------
>
> - In the above, why GCC was not assembling the branch instruction with
> absolute value as operand in it?

Can you also provide the C source code?  I fail to see how C code could
result in such assembly.

> - Is there any restriction in branching towards absolute value in ELF?

The MIPS architecture defines branches as PC relative. For an absolute
address you need a jump.


Thiemo

Re: Is it possible to branch to an absolute address ?

[Eric Christopher]

>
> - In the above, why GCC was not assembling the branch instruction with
> absolute value as operand in it?

You asked these questions in private mail yesterday. Please go read  
my response and look at the ISA manual.

-eric

RE: Is it possible to branch to an absolute address ?

[Amarnath]

> -----Original Message-----
> From: Eric Christopher [mailto:echristo@apple.com]
> Sent: Saturday, July 08, 2006 4:56 AM
> To: amarnath@acmet.com
> Cc: binutils@sourceware.org
> Subject: Re: Is it possible to branch to an absolute address ?
> 
> >
> > - In the above, why GCC was not assembling the branch instruction with
> > absolute value as operand in it?
> 
> You asked these questions in private mail yesterday. Please go read
> my response and look at the ISA manual.
> 
> -eric


Ok, Thank you. I am checking it.