reg_round.s

讲述linux的初始化过程

	orl	%edx,%edx
	jnz	LDo_64_round_up
	jmp	L_Re_normalise

LRound_nearest_64:
	cmpl	$0x80000000,%edx
	jc	LCheck_truncate_64

	jne	LDo_64_round_up

	/* Now test for round-to-even */
	testb	$1,%bl
	jz	LCheck_truncate_64

LDo_64_round_up:
	movb	LOST_UP,FPU_bits_lost
	addl	$1,%ebx
	adcl	$0,%eax

LCheck_Round_Overflow:
	jnc	L_Re_normalise

	/* Overflow, adjust the result (significand to 1.0) */
	rcrl	$1,%eax
	rcrl	$1,%ebx
	incw	EXP(%edi)
	jmp	L_Re_normalise

LCheck_truncate_64:
	orl	%edx,%edx
	jz	L_Re_normalise

LTruncate_64:
	movb	LOST_DOWN,FPU_bits_lost

L_Re_normalise:
	testb	$0xff,FPU_denormal
	jnz	Normalise_result

L_Normalised:
	movl	TAG_Valid,%edx

L_deNormalised:
	cmpb	LOST_UP,FPU_bits_lost
	je	L_precision_lost_up

	cmpb	LOST_DOWN,FPU_bits_lost
	je	L_precision_lost_down

L_no_precision_loss:
	/* store the result */

L_Store_significand:
	movl	%eax,SIGH(%edi)
	movl	%ebx,SIGL(%edi)

	cmpw	EXP_OVER,EXP(%edi)
	jge	L_overflow

	movl	%edx,%eax

	/* Convert the exponent to 80x87 form. */
	addw	EXTENDED_Ebias,EXP(%edi)
	andw	$0x7fff,EXP(%edi)

fpu_reg_round_signed_special_exit:

	cmpb	SIGN_POS,PARAM5
	je	fpu_reg_round_special_exit

	orw	$0x8000,EXP(%edi)	/* Negative sign for the result. */

fpu_reg_round_special_exit:

#ifndef NON_REENTRANT_FPU
	popl	%ebx		/* adjust the stack pointer */
#endif NON_REENTRANT_FPU

fpu_Arith_exit:
	popl	%ebx
	popl	%edi
	popl	%esi
	leave
	ret


/*
 * Set the FPU status flags to represent precision loss due to
 * round-up.
 */
L_precision_lost_up:
	push	%edx
	push	%eax
	call	SYMBOL_NAME(set_precision_flag_up)
	popl	%eax
	popl	%edx
	jmp	L_no_precision_loss

/*
 * Set the FPU status flags to represent precision loss due to
 * truncation.
 */
L_precision_lost_down:
	push	%edx
	push	%eax
	call	SYMBOL_NAME(set_precision_flag_down)
	popl	%eax
	popl	%edx
	jmp	L_no_precision_loss


/*
 * The number is a denormal (which might get rounded up to a normal)
 * Shift the number right the required number of bits, which will
 * have to be undone later...
 */
L_Make_denorm:
	/* The action to be taken depends upon whether the underflow
	   exception is masked */
	testb	CW_Underflow,%cl		/* Underflow mask. */
	jz	Unmasked_underflow		/* Do not make a denormal. */

	movb	DENORMAL,FPU_denormal

	pushl	%ecx		/* Save */
	movw	EXP_UNDER+1,%cx
	subw	EXP(%edi),%cx

	cmpw	$64,%cx	/* shrd only works for 0..31 bits */
	jnc	Denorm_shift_more_than_63

	cmpw	$32,%cx	/* shrd only works for 0..31 bits */
	jnc	Denorm_shift_more_than_32

/*
 * We got here without jumps by assuming that the most common requirement
 *   is for a small de-normalising shift.
 * Shift by [1..31] bits
 */
	addw	%cx,EXP(%edi)
	orl	%edx,%edx	/* extension */
	setne	%ch		/* Save whether %edx is non-zero */
	xorl	%edx,%edx
	shrd	%cl,%ebx,%edx
	shrd	%cl,%eax,%ebx
	shr	%cl,%eax
	orb	%ch,%dl
	popl	%ecx
	jmp	Denorm_done

/* Shift by [32..63] bits */
Denorm_shift_more_than_32:
	addw	%cx,EXP(%edi)
	subb	$32,%cl
	orl	%edx,%edx
	setne	%ch
	orb	%ch,%bl
	xorl	%edx,%edx
	shrd	%cl,%ebx,%edx
	shrd	%cl,%eax,%ebx
	shr	%cl,%eax
	orl	%edx,%edx		/* test these 32 bits */
	setne	%cl
	orb	%ch,%bl
	orb	%cl,%bl
	movl	%ebx,%edx
	movl	%eax,%ebx
	xorl	%eax,%eax
	popl	%ecx
	jmp	Denorm_done

/* Shift by [64..) bits */
Denorm_shift_more_than_63:
	cmpw	$64,%cx
	jne	Denorm_shift_more_than_64

/* Exactly 64 bit shift */
	addw	%cx,EXP(%edi)
	xorl	%ecx,%ecx
	orl	%edx,%edx
	setne	%cl
	orl	%ebx,%ebx
	setne	%ch
	orb	%ch,%cl
	orb	%cl,%al
	movl	%eax,%edx
	xorl	%eax,%eax
	xorl	%ebx,%ebx
	popl	%ecx
	jmp	Denorm_done

Denorm_shift_more_than_64:
	movw	EXP_UNDER+1,EXP(%edi)
/* This is easy, %eax must be non-zero, so.. */
	movl	$1,%edx
	xorl	%eax,%eax
	xorl	%ebx,%ebx
	popl	%ecx
	jmp	Denorm_done


Unmasked_underflow:
	movb	UNMASKED_UNDERFLOW,FPU_denormal
	jmp	Denorm_done


/* Undo the de-normalisation. */
Normalise_result:
	cmpb	UNMASKED_UNDERFLOW,FPU_denormal
	je	Signal_underflow

/* The number must be a denormal if we got here. */
#ifdef PARANOID
	/* But check it... just in case. */
	cmpw	EXP_UNDER+1,EXP(%edi)
	jne	L_norm_bugged
#endif PARANOID

#ifdef PECULIAR_486
	/*
	 * This implements a special feature of 80486 behaviour.
	 * Underflow will be signalled even if the number is
	 * not a denormal after rounding.
	 * This difference occurs only for masked underflow, and not
	 * in the unmasked case.
	 * Actual 80486 behaviour differs from this in some circumstances.
	 */
	orl	%eax,%eax		/* ms bits */
	js	LPseudoDenormal		/* Will be masked underflow */
#else
	orl	%eax,%eax		/* ms bits */
	js	L_Normalised		/* No longer a denormal */
#endif PECULIAR_486

	jnz	LDenormal_adj_exponent

	orl	%ebx,%ebx
	jz	L_underflow_to_zero	/* The contents are zero */

LDenormal_adj_exponent:
	decw	EXP(%edi)

LPseudoDenormal:
	testb	$0xff,FPU_bits_lost	/* bits lost == underflow */
	movl	TAG_Special,%edx
	jz	L_deNormalised

	/* There must be a masked underflow */
	push	%eax
	pushl	EX_Underflow
	call	EXCEPTION
	popl	%eax
	popl	%eax
	movl	TAG_Special,%edx
	jmp	L_deNormalised


/*
 * The operations resulted in a number too small to represent.
 * Masked response.
 */
L_underflow_to_zero:
	push	%eax
	call	SYMBOL_NAME(set_precision_flag_down)
	popl	%eax

	push	%eax
	pushl	EX_Underflow
	call	EXCEPTION
	popl	%eax
	popl	%eax

/* Reduce the exponent to EXP_UNDER */
	movw	EXP_UNDER,EXP(%edi)
	movl	TAG_Zero,%edx
	jmp	L_Store_significand


/* The operations resulted in a number too large to represent. */
L_overflow:
	addw	EXTENDED_Ebias,EXP(%edi)	/* Set for unmasked response. */
	push	%edi
	call	SYMBOL_NAME(arith_overflow)
	pop	%edi
	jmp	fpu_reg_round_signed_special_exit


Signal_underflow:
	/* The number may have been changed to a non-denormal */
	/* by the rounding operations. */
	cmpw	EXP_UNDER,EXP(%edi)
	jle	Do_unmasked_underflow

	jmp	L_Normalised

Do_unmasked_underflow:
	/* Increase the exponent by the magic number */
	addw	$(3*(1<<13)),EXP(%edi)
	push	%eax
	pushl	EX_Underflow
	call	EXCEPTION
	popl	%eax
	popl	%eax
	jmp	L_Normalised


#ifdef PARANOID
#ifdef PECULIAR_486
L_bugged_denorm_486:
	pushl	EX_INTERNAL|0x236
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit
#else
L_bugged_denorm:
	pushl	EX_INTERNAL|0x230
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit
#endif PECULIAR_486

L_bugged_round24:
	pushl	EX_INTERNAL|0x231
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit

L_bugged_round53:
	pushl	EX_INTERNAL|0x232
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit

L_bugged_round64:
	pushl	EX_INTERNAL|0x233
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit

L_norm_bugged:
	pushl	EX_INTERNAL|0x234
	call	EXCEPTION
	popl	%ebx
	jmp	L_exception_exit

L_entry_bugged:
	pushl	EX_INTERNAL|0x235
	call	EXCEPTION
	popl	%ebx
L_exception_exit:
	mov	$-1,%eax
	jmp	fpu_reg_round_special_exit
#endif PARANOID