.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