.file "reg_u_sub.S"
/*---------------------------------------------------------------------------+
| reg_u_sub.S |
| |
| Core floating point subtraction routine. |
| |
| Copyright (C) 1992,1993,1995,1997 |
| W. Metzenthen, 22 Parker St, Ormond, Vic 3163, Australia |
| E-mail billm@suburbia.net |
| |
| Call from C as: |
| int FPU_u_sub(FPU_REG *arg1, FPU_REG *arg2, FPU_REG *answ, |
| int control_w) |
| Return value is the tag of the answer, or-ed with FPU_Exception if |
| one was raised, or -1 on internal error. |
| |
+---------------------------------------------------------------------------*/
/*
| Kernel subtraction routine FPU_u_sub(reg *arg1, reg *arg2, reg *answ).
| Takes two valid reg f.p. numbers (TAG_Valid), which are
| treated as unsigned numbers,
| and returns their difference as a TAG_Valid or TAG_Zero f.p.
| number.
| The first number (arg1) must be the larger.
| The returned number is normalized.
| Basic checks are performed if PARANOID is defined.
*/
#include "exception.h"
#include "fpu_emu.h"
#include "control_w.h"
.text
ENTRY(FPU_u_sub)
pushl %ebp
movl %esp,%ebp
pushl %esi
pushl %edi
pushl %ebx
movl PARAM1,%esi /* source 1 */
movl PARAM2,%edi /* source 2 */
movl PARAM6,%ecx
subl PARAM7,%ecx /* exp1 - exp2 */
#ifdef PARANOID
/* source 2 is always smaller than source 1 */
js L_bugged_1
testl $0x80000000,SIGH(%edi) /* The args are assumed to be be normalized */
je L_bugged_2
testl $0x80000000,SIGH(%esi)
je L_bugged_2
#endif /* PARANOID */
/*--------------------------------------+
| Form a register holding the |
| smaller number |
+--------------------------------------*/
movl SIGH(%edi),%eax /* register ms word */
movl SIGL(%edi),%ebx /* register ls word */
movl PARAM3,%edi /* destination */
movl PARAM6,%edx
movw %dx,EXP(%edi) /* Copy exponent to destination */
xorl %edx,%edx /* register extension */
/*--------------------------------------+
| Shift the temporary register |
| right the required number of |
| places. |
+--------------------------------------*/
cmpw $32,%cx /* shrd only works for 0..31 bits */
jnc L_more_than_31
/* less than 32 bits */
shrd %cl,%ebx,%edx
shrd %cl,%eax,%ebx
shr %cl,%eax
jmp L_shift_done
L_more_than_31:
cmpw $64,%cx
jnc L_more_than_63
subb $32,%cl
jz L_exactly_32
shrd %cl,%eax,%edx
shr %cl,%eax
orl %ebx,%ebx
jz L_more_31_no_low /* none of the lowest bits is set */
orl $1,%edx /* record the fact in the extension */
L_more_31_no_low:
movl %eax,%ebx
xorl %eax,%eax
jmp L_shift_done
L_exactly_32:
movl %ebx,%edx
movl %eax,%ebx
xorl %eax,%eax
jmp L_shift_done
L_more_than_63:
cmpw $65,%cx
jnc L_more_than_64
/* Shift right by 64 bits */
movl %eax,%edx
orl %ebx,%ebx
jz L_more_63_no_low
orl $1,%edx
jmp L_more_63_no_low
L_more_than_64:
jne L_more_than_65
/* Shift right by 65 bits */
/* Carry is clear if we get here */
movl %eax,%edx
rcrl %edx
jnc L_shift_65_nc
orl $1,%edx
jmp L_more_63_no_low
L_shift_65_nc:
orl %ebx,%ebx
jz L_more_63_no_low
orl $1,%edx
jmp L_more_63_no_low
L_more_than_65:
movl $1,%edx /* The shifted nr always at least one '1' */
L_more_63_no_low:
xorl %ebx,%ebx
xorl %eax,%eax
L_shift_done:
L_subtr:
/*------------------------------+
| Do the subtraction |
+------------------------------*/
xorl %ecx,%ecx
subl %edx,%ecx
movl %ecx,%edx
movl SIGL(%esi),%ecx
sbbl %ebx,%ecx
movl %ecx,%ebx
movl SIGH(%esi),%ecx
sbbl %eax,%ecx
movl %ecx,%eax
#ifdef PARANOID
/* We can never get a borrow */
jc L_bugged
#endif /* PARANOID */
/*--------------------------------------+
| Normalize the result |
+--------------------------------------*/
testl $0x80000000,%eax
jnz L_round /* no shifting needed */
orl %eax,%eax
jnz L_shift_1 /* shift left 1 - 31 bits */
orl %ebx,%ebx
jnz L_shift_32 /* shift left 32 - 63 bits */
/*
* A rare case, the only one which is non-zero if we got here
* is: 1000000 .... 0000
* -0111111 .... 1111 1
* --------------------
* 0000000 .... 0000 1
*/
cmpl $0x80000000,%edx
jnz L_must_be_zero
/* Shift left 64 bits */
subw $64,EXP(%edi)
xchg %edx,%eax
jmp fpu_reg_round
L_must_be_zero:
#ifdef PARANOID
orl %edx,%edx
jnz L_bugged_3
#endif /* PARANOID */
/* The result is zero */
movw $0,EXP(%edi) /* exponent */
movl $0,SIGL(%edi)
movl $0,SIGH(%edi)
movl TAG_Zero,%eax
jmp L_exit
L_shift_32:
movl %ebx,%eax
movl %edx,%ebx
movl $0,%edx
subw $32,EXP(%edi) /* Can get underflow here */
/* We need to shift left by 1 - 31 bits */
L_shift_1:
bsrl %eax,%ecx /* get the required shift in %ecx */
subl $31,%ecx
negl %ecx
shld %cl,%ebx,%eax
shld %cl,%edx,%ebx
shl %cl,%edx
subw %cx,EXP(%edi) /* Can get underflow here */
L_round:
jmp fpu_reg_round /* Round the result */
#ifdef PARANOID
L_bugged_1:
pushl EX_INTERNAL|0x206
call EXCEPTION
pop %ebx
jmp L_error_exit
L_bugged_2:
pushl EX_INTERNAL|0x209
call EXCEPTION
pop %ebx
jmp L_error_exit
L_bugged_3:
pushl EX_INTERNAL|0x210
call EXCEPTION
pop %ebx
jmp L_error_exit
L_bugged_4:
pushl EX_INTERNAL|0x211
call EXCEPTION
pop %ebx
jmp L_error_exit
L_bugged:
pushl EX_INTERNAL|0x212
call EXCEPTION
pop %ebx
jmp L_error_exit
L_error_exit:
movl $-1,%eax
#endif /* PARANOID */
L_exit:
popl %ebx
popl %edi
popl %esi
leave
ret
